A new sample of OH/IR stars in the Galactic center

Transcription

1 ASTRONOMY & ASTROPHYSICS SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 128, (1998) FEBRUARY II 1998, PAGE35 A new smple of OH/IR strs in the Glctic center L.O. Sjouwermn 1,2, H.J. vn Lngevelde 3,A.Winnberg 1, nd H.J. Hbing 2 1 Onsl Rymdobservtorium, S Onsl, Sweden 2 Sterrewcht Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlnds 3 Joint Institute for VLBI in Europe, Rdiosterrenwcht Dwingeloo, P.O. Box 2, 7990 AA Dwingeloo, The Netherlnds Received April 14, ccepted June 4, 1997 Abstrct. Two independent, lrgely overlpping 1612 MHz dt sets were serched for OH/IR strs in the Glctic center. One set, tken with the Very Lrge Arry in the period 1988 to 1991, consists of 17 epochs monitoring dt of Vn Lngevelde et l. (1993). The other set ws observed in 1994, using the Austrli Telescope Compct Arry. This rticle describes the dt reduction procedures s well s different wy of serching imge cubes for nrrow line sources, nd lists 1 totl of 155 double pek OH mser detections within 18 or 40 projected prsecs of Sgittrius A*, the compct rdio continuum source in the Glctic nucleus. Presented re 65 formerly unseen double peked 1612 MHz emitters, of which 52 re OH/IR strs. Also given re 3 single pek sources, which we believe to be msers of OH/IR strs. Aprt from being less bright in their 1612 MHz OH mser line, the previously unknown OH/IR strs do not seem to be different from the previously known popultion of OH/IR strs in the Glctic center. We find tht the OH/IR str OH mser luminosity distribution peks t L OH photons per second. Further physicl nd kinemticl nlysis of the new smple will be presented in dditionl ppers. Key words: msers ctlogs surveys strs: AGB nd post-agb Glxy: center rdio lines: strs 1. Introduction When str of low to intermedite min sequence mss (1 7 M ) hs reched the stge where its centrl prt is built up of degenerte nucleus of crbon nd oxygen, Send offprint requests to: L.O. Sjouwermn (Onsl); e-mil: [email protected] 1 Tbles 2 nd 3 re lso vilble in electronic form t CDS vi nonymous ftp to cdsrc.u-strsbg.fr ( ), or vi the WWW t Figures 4 nd 5 re only published electroniclly nd obtinble from it strts scending the symptotic gint brnch (AGB) of the Hertzsprung-Russell digrm. While burning hydrogen nd helium in shell round the nucleus it will expnd its envelope. The str pulstes nd strts losing mss t high rte (10 7 to 10 4 M yr 1 ). The str will be obscured in the visible wvelengths by its dense shell of circumstellr mtter. In this slowly expnding nd cooling shell molecules form nd with regulrity conditions re met, such tht the molecules in the circumstellr shell support mser. Since the discovery of the double peked profile of the hydroxyl mser stellite line t 1612 MHz in lte-type infrred strs, the so-clled OH/IR strs hve been studied to investigte stellr evolution nd Glctic dynmics. The chrcteristic double spectrl feture origintes from hydroxyl (OH) msers in the expnding circumstellr shells of oxygen rich AGB strs. It is esy to recognise the object s n OH/IR str; one obtins the position nd line-of-sight velocity of the str s well s the shell expnsion velocity directly from the double peked spectrum. As the vrible stellr rdition t visible nd nerinfrred wvelengths is bsorbed, the only mens of investigting the properties of the underlying str is by observing rdition tht is re-emitted by the shell: infrred nd sub-mm emission of the dust nd gs, in which the molecules cn show mser emission (OH, H 2 O nd SiO). Reviews on the str nd its envelope cn be found in Iben & Renzini (1983) nd in Hbing (1996). Aprt from studying the underlying str, one cn lso mke use of OH/IR strs to probe the Glxy for its structure, evolution nd dynmics. For exmple, Whitelock et l. (1991) derived period-luminosity reltionship similr to the Mir vribles nd Blommert et l. (1994) compred the OH/IR strs in the center, bulge nd outer prt of our Glxy. Good reviews bout using OH/IR strs s tools, re from Hbing (1993) nd Dejonghe (1993). The Glctic center Lindqvist et l. (1992; herefter LWHM) surveyed the Glctic Center (GC) region for OH/IR strs with the

2 36 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Very Lrge Arry (VLA) in 1984 nd The OH/IR strs in the GC re one of the few stellr smples in the GC tht cn be directly observed despite the enormous visul extinction (A V > 25 mg) in front of the GC. The totl of 150 OH/IR strs found (see ppendix) re ll t bout the sme distnce nd thus cn be studied s smple without ssumptions on their individul distnces (e.g. Bud et l. 1981; Jones et l. 1994; Blum et l. 1996). In Lindqvist et l. (1992b) the strs re investigted for their sptil nd kinemtic properties nd used s trcers of the centrl potentil nd mss distribution. It is shown tht the distribution of the GC smple generlly depends on the shell expnsion velocity t 1612 MHz nd tht the surfce density increses strongly towrds Sgittrius A* (Sgr A*). Sevenster et l. (1995) hve shown tht the OH/IR strs in the GC consist of globl Glctic component nd seprte, strongly rotting disk of younger OH/IR strs, possibly formed t distinct event. Unfortuntely the number of OH/IR strs known is too low to do conclusive dynmicl study, especilly within bout pc of Sgr A*. Lindqvist et l. (1997) show tht there should be mny more OH/IR strs in the GC with pprent wek OH msers. The reson why they hve not been found in the LWHM survey is minly mtter of sensitivity. Also the OH msers vry in luminosity, s they re indirectly pumped by the vrible stellr rdition. To find ll strs one should preferbly observe nd serch the sme region for more thn one epoch. By monitoring the vribility of some of the LWHM OH sources with the VLA, Vn Lngevelde et l. (1993; herefter vljghw, or the monitor ) hve tried to mesure phse-lg distnces to these OH/IR strs in order to get direct estimte of the distnce to the GC. However, highly scttering interstellr medium ws discovered in the direction of the GC, mking it impossible to chieve their primry gol (Vn Lngevelde & Dimond 1991). Nevertheless, 20 observtions of the GC region hd been done, ech in sensitivity comprble to the originl LWHM survey observtions. A chep wy of finding fint OH/IR strs is by nlysing the conctented dt set. In tht wy the serch cn be done in highsensitivity dt cube, nd one is ble to detect OH/IR strs which were in minimum of their OH mser luminosity t the time of the LWHM survey. By verging mny different epochs tken over time longer thn the typicl periods of the strs, the detection becomes effectively function of the time verged flux density in the spectrl peks. With 20 epochs, the most sensitive wy to find strs is by using the conctented dt rther thn to serch ll epochs seprtely (under the ssumption tht the OH/IR strs vry typiclly fctor of two during the monitor). We intend to use the new, extended smple of OH/IR strs for testing the loction of, nd probing the potentil in the very center of our Glxy, nd, secondly, to study the smple of OH/IR strs in the GC compred to ll other known smples of OH/IR strs. To overcome symmetry problems, introduced by the prticulr pointing of the VLA dt (optimised for the monitoring progrm; see below), we used the Austrli Telescope Compct Arry (ATCA). The bndwidth for the ATCA observtions hs been chosen to include n equl sensitive serch for high-velocity 2 OH/IR strs (velocity up to 600 km s 1 ), which might dd importnt clues for future dynmicl modelling. Outline of this pper In this pper we describe the dt reduction procedure of the VLA monitor dt set nd of the ATCA observtions in Sect. 2. Section 3 presents list of both known nd suspected OH/IR strs in the GC. Here, when we refer to n OH/IR str, we ctully refer to circumstellr OH mser emission. The lrge interstellr visul extinction prevents us to mke cler distinction between opticlly thick circumstellr shells, s for genuine OH/IR strs, nd opticlly thin circumstellr shells s for the evolutionry closely relted Mir vribles. We my even hve picked up n individul supergint; however, see Blum et l. (1996) for recent evidence tht the number of supergints in the GC is low. Section 3 lso includes n error budget for our mesurements. In Sect. 4, we comment on some of the detections. We discuss the survey sensitivity nd nomlous fetures in some OH/IR str spectr. Briefly, we compre the previously unknown OH/IR strs with the known OH/IR strs nd derive the OH luminosity distribution. From this we conclude in Sect. 5, tht the centrl strs of the new detections re not different from the AGB strs tht constitute the known popultion in the GC. We do not ttempt to mesure the periods of the individul detections s the objects re too fint to be detected in ech epoch seprtely. Also, detiled discussion of the sptil, kinemtic nd physicl properties of the new, extended smple of OH/IR strs in the GC, s well s the issue whether Sgr A* is the dynmicl center of the OH/IR str smple, is deferred to dditionl ppers. A preliminry result on the survey cn be found in Sjouwermn & Vn Lngevelde (1996). 2. Dt hndling nd imge nlysis Tble 1 summrises the chrcteristics of the three different imge cubes mde; Figure 1 depicts n overview of the sky re surveyed, together with the sensitivity contours thereof. 2 Stellr velocities in this pper refer to line-of-sight velocities s mesured with respect to the locl stndrd of rest reference frme (LSR).

3 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 37 Tble MHz survey imge cubes Survey Imge cube center (epoch) Size Resolution Chnnel width RMS Velocity rnge km s 1 khz mjy km s 1 VLA 17 h 42 m 12 ṣ (B1950) b VLA 17 h 42 m 29 ṣ (B1950) c ATCA 17 h 45 m 40 ṣ (J2000) The VLA dt cten The monitor dt ws in 1992 the most extended, uniform 1612 MHz spectrl line dt set of the GC known by the uthors. We did not consider extending the conctention with observtions by others (Hbing et l. 1983; LWHM), becuse those observtions were tken in period when the VLA ws limited in its spectrl line cpbilities. The observtions nd dt reduction of the monitor dt used re described in detil in vljghw. We chose to strt from their clibrted dt sets, minly becuse bd visibilities nd interference from the Russin GLONASS stellite positioning system hd lredy been removed. In summry, the 4 GByte of clibrted visibility dt sets consist of 20 epochs of two-hour VLA observtions, tken in different rry configurtions in the period from Jnury 1988 to Jnury Unfortuntely for the current project, the sky position is not exctly centered on Sgr A*, nd the velocity coverge is limited to only 110 to +190 km s 1. The dt hve been reduced, clibrted nd nlysed using the NRAO AIPS reduction pckge (versions from 15JAN88, up to 15JUL94), which we continued to use on the conctented dt set. Being forced to use different AIPS versions over the yers introduced some problems with the dt tbles during the conctention process. We will comment on tht below. We re confident tht the finl results re not different from wht could hve been obtined if we hd strted with the rw visibility dt nd performed the conctention in one AIPS version only Initil VLA dt set selection The following steps were tken for ech individul dt set to ensure the homogeneity of the sets, before conctenting them to one visibility file for finl processing. Ech of the clibrted monitor dt sets ws checked for consistency by fully imging couple of known strong OH sources. Becuse of different problems, three epochs hd to be regrded lost for our project. We did not process the rw dt, s few missing epochs would not mke significnt difference in the noise sttistics or detection probbilities. Also, due to inconsistent removl of interference in severl epochs, we did not use ny of the visibility dt which could hve been ffected. Therefore, ll dt for which the bselines were shorter thn 3 kλ ws disregrded. If necessry, the u, v, w vectors were reclculted to hve the coordintes (B1950) RA 17 h 42 m 12 ṣ 600 nd DEC s phse center. The lrgest effect of using different AIPS versions could be seen in some tbles contining dditionl informtion to the dt. We minly hd to del with the flgging tbles s the tble formt hd chnged. It ws impossible to restore the originl flgging, so for ech combintion of bseline nd time in flgging tble, ll visibility informtion ws mrked s bd dt. Whenever pplying the tsk SPLIT, the sky frequency (different for ech epoch) nd velocity informtion in ech heder nd ntenn file got reset nd hd to be repired by hnd. As we will not use frequencies (but velocities insted) nd the velocity consistency hd lredy been checked with spectr nd mps, we took n rbitrry file heder nd used it to set ll the relevnt vlues to mtch. The sme ws done for the frequency informtion in the ntenn file. By doing this, one introduces n error less thn 0.05%, in ppernce comprble to rdil bndwidth smering, but in net effect negligible. At this time the dt sets were lso converted from circulr polristions to Stokes I to reduce the file sizes with fctor of two immeditely. As the bsolute flux clibrtion of ech dt set hd been done crefully by vljghw for the flux monitoring progrm, we hve not performed ny further bndpss or mplitude correction, visibility phse reclcultion or flgging opertion on the individul monitor dt sets Conctented VLA dt set processing Alldtsetswereddedinsimilrmnnertocrete one conctented visibility dt set of bout 35 observing hours. Ech epoch ws put in different sub-rry without rescling the visibility weights. Due to different clibrtion pths, the individul weights of the visibility points differed per monitor dt set. To get ech epoch contribute eqully, we treted ll visibility weights to be unity. Although formlly one hs to ccount for the epochs tht re tken in one polristion only (two dt sets before August 1988), we hve not looked into this mtter fter converting ech dt set to Stokes I. To subtrct confusing continuum emission, bseline ws fitted to ech visibility spectrum, using three regions

4 38 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Fig. 1. Composite survey re. The filled circle represents the position of Sgr A*, s well s the center for both the high resolution VLA nd the ATCA imge cubes. The tringle represents the imge cube center for the conctented VLA monitor dt. Long-dshed squres re the 37 VLA nd ATCA surveys; we filtered out the corners for which the distnce to the field (pointing) center ws lrger thn 24. The short-dshed squre encloses the high resolution, 17 VLA imge. Drwn in solid lines re contours of equl detection probbility; lines connecting points with equl sensitivity, ccording to the most sensitive imge cube, fter correcting for the primry bem ttenution (thus cting s primry bem response for this combined survey). Contours drwn re, from inside out, 1.005, 1.1, 1.25, 1.5, 2, 4, 8 nd 16 times 25 mjy. Note tht the VLA bem degrdes fster becuse of lrger dish size; the VLA hs 25 meter, the ATCA hs 22 meter dishes. Approximte J2000 corner coordintes re: upper right 17 h 44 m 05 s to lower left 17 h 47 m 06 s in the totl spectrl bnd which seemed to be void of line sources (Vn Lngevelde & Cotton 1990; Cornwell et l. 1992). Severl chnnels of ech region were verged, nd interpolted to represent the continuum emission. This visibility model for the continuum emission ws subtrcted from ll chnnels of the conctented VLA dt set. On the fly, ll visibility points exceeding twice the expected flux in the chnnel with the strongest source, were clipped. For epoch lignment we used single self-cl itertion on the visibility phses. It corrects for reltive systemtic (tmospheric) effects in the different monitor dt sets. We selected the red shifted pek of OH ; single chnnel with only one strong, 6 Jy pek, nd close to the field center. From this clibrted visibility dt set we mde two nturlly weighted imge cubes nd number of clen boxes ; strong sources outside the min imge cube were mpped in smll fields to limit their side-lobe interference. The first imge cube is full primry bem, low sptil, but full frequency resolution imge cube (survey in Tble 1). The next is smll field, high sptil resolution imge cube ( b ) of the sme VLA dt set, for which we verged two chnnels. The ltter ws centered on Sgr A* nd the not fully removed extended continuum emission of the Sgr A complex. The low resolution imge ws chosen to mtch the sptil resolution of the ATCA. The lrge pixel size resulted in hving only the shorter bselines (bout one hlf of the visibilities) contribute to the imge, wheres for the high resolution imge cube, bout 90% of the visibilities could be used. Using n even higher resolution, to llow ll visibilities to be used, would result in huge mps or lterntively too mny clen boxes (more thn 15). It would thus require n extr pss of subtrction of sources, without substntil decrese in noise level. The numbers used were trde-off between resolution, noise level, sky coverge, execution time nd disk usge. For reference, to mp the entire region t full sptil nd spectrl resolution would require t lest 34 Gb of disk spce (with our choices less thn 4), nd becuse the execution time is strongly dependent on the disk I/O speed, even with modern worksttions it would tke months to execute (compred to hndful of dys). Effectively, only the bselines between 4 nd 32.2 kλ nd 4 nd kλ were used for the low nd high sptil resolution imges, respectively. Fitted RMS noise levels per chnnel were mesured to be 7.2 nd 3.7 mjy on verge, respectively ATCA observtions nd dt reduction The ATCA ws used on 8, 10, 11 nd 13 July To reduce brodbnd interference by GLONASS nd to filter out s much s possible of the extended continuum emission in the GC region, the longest bseline configurtion ws selected for ll the observtions. The GC ws observed t centrl frequency of MHz, covering 8 MHz bndwidth over 1024 frequency chnnels. The integrtion time ws bout 12 hours for ech of the observing dys.

5 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 39 To circumvent GLONASS interference in the bndpss clibrtion, the primry flux clibrtor B ws observed three times per observing run; typiclly every 6 hours. The phse clibrtor B ws observed roughly every 40 minutes. Once per observing run we lso observed the VLA primry flux clibrtor B to check the consistency of the flux density mesurements between the ATCA nd VLA. Actully, for July 10th, we used B for bndpss clibrtion, becuse interference ffected ll B observtions of tht dy. After removing bd visibility dt points nd pplying the bndpss clibrtion, the sky-frequencies were converted to LSR velocities. Following the stndrd phse clibrtion, the extended continuum emission from compct HII regions nd the Sgr A complex ws subtrcted with simple two region bseline interpoltion in the visibility domin. Excessive mplitude visibilities were clipped before pplying one self-clibrtion itertion solution of the visibility phses, on the sme pek nd in similr mnner s to the conctented VLA dt set. Agin we used clen boxes for removing side-lobes of strong sources outside the min imge cube. To void remining effects of interference by GLONASS nd the subtrcted continuum emission of the Sgr A complex in the imge cube, we only used bselines exceeding 4 kλ. Becuse of the lrge mount of disk spce required (5 MB per chnnel), full resolution imges were mde by the hundred for chnnels 69 to 964 ( 600 to 700 km s 1 ). The RMS noise level is bout 4.5 mjy on verge. Becuse we used self-cl to lign different observing dys fter we hd removed ll continuum emission from the dt, ll positions of the OH/IR strs chnged with respect to Sgr A*. Therefore, the chnnel with the mser line we used for self-cl, ws mpped from the unsubtrcted clibrted ATCA dt set. In this mp, the positions of Sgr A* nd the line were mesured, fter which the mser positions with respect to Sgr A* could be determined. The sme ws done for one of the A-rry VLA monitor dt sets. The positionl offsets with respect to Sgr A* in Tble 2 link both VLA nd ATCA observtions together Imge cube nlysis In order to serch for discrete line sources, ech of the three dimensionl imge cubes ws projected into one two dimensionl imge in the following wy: for ech pixel in the sky plne, the mximum intensity over the whole frequency/velocity xis ws stored in new, two dimensionl sky imge. We shll refer to this imge s the mxmp. In tht wy one gets n overll view of the sky loction of intensity mxim, lthough without directly knowing the velocity corresponding to the peks. The mxmp resembles continuum imge (but recll tht we lredy hve filtered out the continuum emission), but with much higher signl to noise rtio thn if we would hve verged the pixel intensities over ll chnnels. Alterntively, one cn mke totl intensity mp (or zeroth order mp ); n imge for which the flux densities tht re higher thn the threshold re integrted over the feture. However, s the totl intensity mp tkes into ccount every pixel bove certin threshold nd thus indeed would be very useful when looking for double peked fetures it is lso very sensitive to brod line sources nywhere in the spectrum. Becuse we hve to del with the remins of the extended continuum emission of the Sgr A complex, we observe vst mount of brod line sources (see lso Fig. 8). These sources cuse severe problems in serching the GC re; from non-liner spectrl slopes to different noise nd detection sttistics. With this in mind, we prefer to use the mxmp insted of the totl intensity mp concept. The finl results in both methods do not differ much, but the serch is much more strightforwrd in the mxmp. Resulting were three mxmps tht ll hve the sme noise level for the detection of sources s the originl imge cubes, but now consisting of pixels contining sources nd the high-end prt of the noise distribution of the originl mp (i.e. only positive vlues round nd bove the RMS noise level; comprble to Ryleigh noise distribution with constnt offset). Now, insted of serching the originl imge cubes in ech chnnel seprtely, nd finding ech source in severl chnnels, one obtins the sme detections by serching the (one chnnel) mxmp only. The mxmps were serched for pixels with intensities over 40 mjy (5.5σ), 25 mjy (6.7σ) nd 25 mjy (5.5σ) for the VLA 37,VLA17 nd the ATCA 37 imge cubes, respectively. Pixels were then grouped in islnds nd for ech islnd, the spectrum in the originl imge cube ws tken t the position with the highest pixel vlue. The mjor dvntge is tht one would only get one spectrum for ech islnd (e.g pixels mke 200 islnds, thus only 200 spectr), risking tht one could miss second object tht is sptilly djcent to the object whose spectrum is drwn. Chnces of missing str re higher close to stronger sources (hving lrger islnds, bem re) nd close to the remnnts of improperly removed continuum fetures of the Sgr A complex (nonliner spectrl slopes). Therefore we checked individul pixels of the locl mxim in the confused re of the centrl 5 by hnd. We think we might hve missed only couple of objects in our totl nlysis. The expecttion vlue for two sources to be positionlly coincident is less thn 1% over the dt set, but on the other hnd we cnnot correct for sources submerged in res of bsorption, or the bowl of negtive emission cused by the extended bckground. The spectr were inspected for second pek, which we required to be t lest 4.5σ (respectively 32.4, 16.6 nd 20.2 mjy observed flux density) nd within the intervl of 5 80 km s 1 of the first pek.

6 40 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Fig. 2. Sky distribution. Closed symbols re new detections, open symbols the previously known OH/IR strs re-detected in this survey. The offsets refer to Sgr A* ( l, b in Tble 2). Note tht the overll concentrtion of the new detections towrd the center is prtly due to the survey bem response (Fig. 1); the concentrtion in the very center is however rel 3. Results Figures 2 nd 3 summrise our results, where it should be noted tht the stronger (previously known) sources cn be detected further out from the pointing center becuse of the drop in sensitivity. Tbles 2 nd 3 (lso vilble electroniclly through CDS) list ll detections within the surveyed re, lso when it concerns previously known source. Figures 4 nd 5 (only vilble electroniclly through CDS) disply the 1612 MHz OH mser spectr of ll sources. Single pek or suspected double pek detections with pek flux exceeding 8σ (36 mjy before correction in the ATCA dt), re lso listed. However, we do not list ny of the obvious single peks in the region where confusion with the extended continuum emission occurs. These detections re probbly not of stellr origin. This mens we hve excluded ll single pek detections within the box defined by lɛ[359.92, ], bɛ[ 0.08, 0.03], even when the pek flux exceeds 36 mjy. Actully, some of the double pek detections in this region might be debtble for being stellr sources. Such cses re indicted in Tble 3. In cse of filure detecting the second pek, we usully took the velocity of the second pek from our VLA/ATCA dt, or else from LWHM. For ech detection, t the red nd blue shifted peks, fluxes nd velocities were determined. The position, together with the forml errors were mesured with the Fig. 3. Velocity distribution. Symbols s in Fig. 2. The dotted lines outline the limited coverge of the monitor; the solid lines the velocity intervl covered by LWHM AIPS fitting progrm IMFIT, in the chnnel with the highest pek flux. For the VLA dt, J2000 positions nd Glctic coordintes (l, b) were clculted from the B1950 positions nd then truncted ccording to the IAU convention. The Glctic coordintes from the ATCA dt were clculted fter the inverse trnsformtion from J2000 to B1950. All positionl nd kinemtic dt of the detections re given in Tble 2. Tble 3 lists corresponding physicl dt to ech entry in Tble 2. Where pproprite, we first list the VLA (, or b ) nd secondly the ATCA ( c ) dt. If seen in both the low nd high resolution VLA imge cubes, we used the high resolution ( b ) result for the position (Tble 2) nd the low resolution ( ) result for the velocity nd flux informtion (Tbles 2 nd 3 nd Fig. 4); the other result ws used s consistency check in such cses. As the fluxes mesured for the VLA dt re verges over 17 observtions nd depend on the resolution, we show VLA nd ATCA spectr seprtely in Fig. 4 nd in Fig. 5. There re considerble difficulties when reching such low noise levels in the GC re; residuls of the extended OH bsorption, continuum subtrction, nd very strong mser sources cuse poor bselines in both the VLA nd ATCA dt. Therefore, t the position of the str in the imge cube nd voiding the region of the stellr mser emission, liner bseline ws fitted to, nd subtrcted from the spectrum.

7 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 41 Tble 2. OH/IR strs in the Glctic center: positionl nd kinemtic dt OH Nme R. A. nd Declintion δ mx V blue V red V str l b Ref. J km s 1 km s 1 km s 1 c (13) c (14) c (17) c (18) c (19) (21) c (21) c (22) (23) c (23) c (24) (25) c (25) (26) c (26) c (27) c (29) (31) c (31) c (32) c (32) (33) c (33) (34) c (34) (35) c (35) c (36)

8 42 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Tble 2. continued OH Nme R.A. nd Declintion δ mx V blue V red V str l b Ref. J km s 1 km s 1 km s (37) c (37) b (38) c (38) c b * (40) c (40) c (41) b (42) c (42) c (43) (5) c (5) b (44) c (44) (45) c (45) b (47) c (47) (46) c (46) b c c b (48) c (48) b (6), 6 c (6), 6 b (50) c (50) c c

9 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 43 Tble 2. continued OH Nme R.A. nd Declintion δ mx V blue V red V str l b Ref. J km s 1 km s 1 km s 1 b b c b c (51) c (51) b b (52) c (52) b (53) c (53) (55) c (55) (56) c (56) b c c (57) c (58) b , * c , * b (59) c (59) c c b (60) c (60) c c b (61) c (61)

10 44 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Tble 2. continued OH Nme R.A. nd Declintion δ mx V blue V red V str l b Ref. J km s 1 km s 1 km s 1 c b b (7) b c b (62) c (62) b (63) c (63) c * c * c (64) b c c b b (65) c (65) c b * b b (66) c (66) c b (67) c (67) b (68) c (68)

11 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 45 Tble 2. continued OH Nme R.A. nd Declintion δ mx V blue V red V str l b Ref. J km s 1 km s 1 km s 1 b c c c b b c c b * b (69), * b c b (70) c (70) b (8) c (8) (71) c (71) c b (72) c (72) b c b (73) c (73) b * c * b * c * b (74) c (74)

12 46 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Tble 2. continued OH Nme R.A. nd Declintion δ mx V blue V red V str l b Ref. J km s 1 km s 1 km s 1 b c b (75) c (75) (76) b (77) c (77) b * c * c b c (78) c (78) (79) c (79) b c b ? c ? (80) c (80) b c b (81) c (81) b c b c

13 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 47 Tble 2. continued OH Nme R.A. nd Declintion δ mx V blue V red V str l b Ref. J km s 1 km s 1 km s c b (82) c (82) b (83) c (83) c * b c (84) c (84) (85) c (85) b (86) c (86) b (87) c (87) (88) c (88) c b (90) c (90) c c (92) c (92) (91) c (91)

14 48 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Tble 2. continued OH Nme R.A. nd Declintion δ mx V blue V red V str l b Ref. J km s 1 km s 1 km s (93) c (93) (94) c (94) (95) c (95) (96) (97) c (97) (98) c (98) (99) c (99) (100) c (100) (101) c (103) (105) c (105) c (106) c (107) c (108) c (111) c (111) c (112) c Lindqvist et l. (1992) (number) nd references therein 4 Levine et l. (1995) 1 Lindqvist et l. (1992), single pek (number) 5 Yusef-Zdeh & Mehringer (1995) 2 Bud et l. (1975) 6 I.S. Glss, pers. comm. (Glss et l. 1996) 3 Vn Lngevelde et l. (1992) * See remrks in min text.

15 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 49 Tble 3. OH/IR strs in the Glctic center: physicl dt OH Nme PBF S blue I blue ɛ S red I red ɛ V exp L OH Deconvolved Note 1 b mjy % mjy % km s c c c c LWHM c c c ATCA c c c c c LWHM c c c c c c c c

16 50 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Tble 3. continued OH Nme PBF S blue I blue ɛ S red I red ɛ V exp L OH Deconvolved Note 1 b mjy % mjy % km s c c c c c c c LWHM c c c c c ATCA c c c c c c c

17 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 51 Tble 3. continued OH Nme PBF S blue I blue ɛ S red I red ɛ V exp L OH Deconvolved Note 1 b mjy % mjy % km s c c c b c c c c ATCA c c c c ATCA c c c ATCA c c MC? c MC ATCA c

18 52 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Tble 3. continued OH Nme PBF S blue I blue ɛ S red I red ɛ V exp L OH Deconvolved Note 1 b mjy % mjy % km s c MC b ATCA c ATCA c c c MC? c MC? c MC c MC c b MC c c b c c MC? c c

19 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 53 Tble 3. continued OH Nme PBF S blue I blue ɛ S red I red ɛ V exp L OH Deconvolved Note 1 b mjy % mjy % km s c c c MC b MC c MC c MC b MC c MC c c MC c MC c MC c c c c c c

20 54 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Tble 3. continued OH Nme PBF S blue I blue ɛ S red I red ɛ V exp L OH Deconvolved Note 1 b mjy % mjy % km s c c c c VLA c c c c ATCA c VLA c c c c b c b c

21 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 55 Tble 3. continued OH Nme PBF S blue I blue ɛ S red I red ɛ V exp L OH Deconvolved Note 1 b mjy % mjy % km s c c c c b c c c c c c c c c c c c

22 56 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Tble 3. continued OH Nme PBF S blue I blue ɛ S red I red ɛ V exp L OH Deconvolved Note 1 b mjy % mjy % km s c c c c c c c c ATCA c c c c c c c c ) Units in Jy km s 1 LWHM Pek velocity secondry in Tble 2 from Lindqvist et l. (1992) b) Units in LOG (photons per second) VLA Pek velocity secondry in Tble 2 from our VLA dt MC Moleculr cloud complex(es) ATCA Pek velocity secondry in Tble 2 from our ATCA dt.

23 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 57 In Tble 2 for ech entry we list in order: the survey code, the given nme ccording to its Glctic coordintes, the mesured R.A. nd Declintion in J2000 with the mximum error in either R.A. or Declintion, the blue nd red shifted velocities, the derived stellr velocity nd the Glctic coordinte offsets with respect to Sgr A*. The stellr velocity is tken s the men of the velocities of both intensity mxim, wheres the Glctic coordinte offsets (i.e. for flt sky, or cos b 1) is defined with respect to Sgr A*: l = l OH l Sgr A, b = b OH b Sgr A. In Tble 3 we repet the survey code nd source nme, give the primry bem ttenution fctor, the pek flux density nd (sptilly nd spectrlly) integrted flux together with the estimted reltive integrted flux error for the blue shifted side, s well s for the red shifted side of the stellr velocity. We lso list the the shell expnsion velocity, nd the OH mser luminosity (for n ssumed isotropic rdition field nd distnce of 8 kpc to the GC; Reid 1993). If the source ppers to be extended, we determined n pproximte deconvolved ellipticl Gussin for the source (the mjor xis, minor xis nd position ngle). The ngulr brodening of sources is probbly cused by instrumentl effects, time verging of the visibilities or the extended bckground, but we cnnot exclude tht n extreme cse of interstellr scttering of individul sources lso plys role (Vn Lngevelde & Dimond 1991; Vn Lngevelde et l. 1992b; Fril et l. 1994). The primry bem ttenution fctor ws clculted for both the VLA nd ATCA with polynomil, given internlly in AIPS. The expnsion velocity is hlf of the velocity seprtion between the mxim t both sides; however, it is not lwys the full extent of the feture. The blue nd red shifted integrted fluxes re clculted by integrting flux densities over the chnnels from the stellr velocity to the first negtive flux density outside the mximum The errors Both VLA nd ATCA dt sets show positionl offsets when compred with the positions mesured by LWHM nd vljghw. The smll difference between the LWHM nd vljghw dt is due to using phse clibrtors with different positionl ccurcies (B nd B , respectively); the LWHM nd vljghw positions however re consistent with ech other. The internl lignment of the VLA dt introduced systemtic positionl shift, s did the self-cl itertion of the ATCA dt. Hence, there is significnt offset of few rc-seconds between the positions mesured in our VLA nd ATCA imge cubes. Our ATCA positions however re roughly consistent with the LWHM nd vljghw dt. We therefore ttribute the systemtic difference between our VLA nd ATCA positions to the self-cl itertion performed to lign the monitor dt. Furthermore, smll errors re Fig. 6. Sky sensitivity coverge. Shown is the frction of the surveyed sky re for which we could hve detected source of given flux density. All sources with pek flux density over 530 mjy must hve been detected; for homogeneous distribution one reds detection rte of 80% of the 100 mjy sources nd only bout 20% of the sources round 30 mjy introduced by trnsforming the coordintes to (nd from) epoch J2000. We stress tht our bsolute positions re not expected to be ccurte t the one rc-second level. By mesuring the offsets with respect to Sgr A* (ssuming no mesurble reltive proper motion of the OH/IR strs with respect to Sgr A* during the monitor), however we hve tken out the reltive differences between our VLA nd ATCA observtions, nd from there they cn be linked to other dt sets. The positionl error quoted, for given source in dt set, is the lrger of the forml errors in R.A. nd Declintion s derived by IMFIT in the chnnel with the highest pek flux density of the source. The mesured positions in individul chnnels re consistent with the position of the source. Flux density errors re lso tken from IMFIT. This reltive error cn be used to derive the error in the OH mser luminosity. Becuse the spectrl fetures re unresolved, we cn only quote the velocity resolution s estimte of the error in velocity: 1.14 (or 2.27) km s 1 for the VLA nd 1.45 km s 1 for the ATCA observtions. Similrly, the flux densities from the VLA dt set depend on the spectrl resolution used nd on the effect of verging flux densities over severl epochs. OH/IR str OH flux densities nd luminosities re vrible, up to fctor two (e.g. Hrvey et l. 1974; Hermn & Hbing

24 58 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center Fig. 7. Number distribution of pek flux densities. The dshed line gives the distribution empiriclly corrected for incompleteness; it is however n overestimte (see text). For exmple, the dotted line shows the effect on the completeness correction, when the strs re sptilly distributed ccording to n rbitrrily scled surfce distribution Σ(R) (R + R c) ; Vn Lngevelde et l. 1990). The mesured ATCA fluxes re therefore snpshots of the flux vribility; the VLA fluxes pproximte the verge fluxes better, becuse of verging out the mplitude vritions over the monitor period. Estimting the vribility effect would require detiled knowledge of the OH period nd mplitude distributions nd is therefore not ttempted. 4. Discussion 4.1. Completeness nd sensitivity Figure 1 shows pproximte contours of the best sensitivity chieved in the survey. We use it to mke n empiricl estimte of the (in)completeness of the survey. The number of sources detected, s function of highest pek flux density, is corrected by dividing ech source by the frction of the surveyed sky re in which it could hve been found. Becuse the VLA nd ATCA dt sets hve comprble spectrl resolutions, we neglect the influence of chnnel width on the detection probbility. The frction s function of flux density cn be found grphiclly in Fig. 6. It lso shows tht ll sources with pek flux density brighter thn 530 mjy must hve been detected (frction = 100%, the empiricl completeness limit of the survey; or more formlly 390 mjy for 99%). If one would confine the re surveyed to the size of the fields used in Lindqvist et l. (1992, 1997), i.e. 32 squred, the completeness limit would be pproximtely 65 mjy. In Fig. 7 we disply the number of sources with respect to the highest pek flux density, where we hve given preference to the multi-epoch VLA over the one-epoch ATCA dt. Becuse of the vrible nture of the OH/IR strs, the distribution is somewht brodened. By verging the flux densities in the conctented monitor dt set, the effect should be smller thn in single epoch observtion. However, the exct mount of the brodening is very difficult to clculte, in prticulr with the mixed observtions in this survey. Strictly, this completeness correction is for homogeneous number density distribution. As the OH/IR str distribution is concentrted towrd Sgr A*, nd thus towrd the survey center, we expect (nd find) the lrgest number of fint strsin the prt ofthe survey tht is most sensitive. Correcting prts of the survey tht re less sensitive for these fint strs, just by liner function bsed on the sensitivity or geometry of the re, would then ssume tht pointing the telescopes fr wy from Sgr A*, the detection probbility of n OH/IR str is equl to the detection probbility when pointing t Sgr A*. This would imply n overestimtion of the completeness correction. We hve tried to show this effect in Fig. 7, by lso clculting completeness correction for n ssumed number density proportionl to r 2, corresponding to surfce distribution Σ(R) R 1 (Lindqvist et l. 1992b). It is evident tht the mgnitude of the effect depends on the ctul concentrtion of strs; it cnnot be extrcted directly from our dt Extended emission nd bsorption complex Becuse of confusion t the loction of the Sgr A complex, the survey is in prctice less sensitive for detections thn the ctul instrumentl response. As n exmple of the residul line emission, we show the centrl prt of the mxmp of the ATCA survey in Fig. 8. Note the wy the known OH/IR strs stnd out with respect to the residuls. Recll tht in this region lrge continuum emission complex hs lredy been subtrcted. Most of the remining line emission is locted s seprte spots in ring-like shpe t the inside of the supernov remnnt tht forms the estern prt of the Sgr A complex (e.g. Ekers et l. 1983; Pedlr et l. 1989). The few bsorption mesurements done in our dt set, hve not been explicit bout the line-of-sight loction of the spots with respect to the bsorption nd emission lines of the supernov remnnt. The ring-like shpe cnnot be seen in individul or severl consecutive frequency chnnels. The mjority of the emission fetures hve velocity between 25 nd 70 km s 1, plcing them t, or close to either the +50 km s 1 moleculr cloud complex behind, or the stremer moleculr cloud complex G in front of Sgr A* (Zylk et l. 1990). This emission my be

25 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 59 DECLINATION (J2000) Sgr A complex; ATCA "mxmp" RIGHT ASCENSION (J2000) Fig. 8. ATCA mxmp of the Sgr A complex. Contours re t 0.030, 0.050, 0.070, 0.100, 0.200, 0.500, 1, 2 nd 5 Jy. Sgr A* is locted in the middle of the right side of the imge. The ring-like shpe is due to Sgr A Est nd is not to be confused with the circumnucler disk indictive for shock fronts, where, on the ner nd fr side of the expnding supernov shell, the shell nd the moleculr cloud complexes collide. This interprettion would support the recent results of the observtion of shock-excited 1720 MHz OH msers by Yusef-Zdeh et l. (1996). In this region, the bselines ginst which we try to find double peked OH sources re mostly irregulr nd mke serching difficult. However, becuse the velocity chrcteristic is obvious, it is reltively esy to recognise these sources; listing ll of them s single sometimes double pek detections, on the other hnd, hs not been the purpose of this survey. Besides OH emission, the presence of OH molecules lso give rise to res of 1612 MHz bsorption, which re not visible in Fig. 8 becuse of the mxmp procedure followed. We hve similr minmps with the detection of 1612 MHz OH bsorption. Aprt from the supernov remnnt nd other distinct regions, it is seen most pronounced in the core of the G moleculr cloud. The bsorption t +24 km s 1 coincides with the densest NH 3 concentrtion in the GC, tht is known to host n ultrcompct HII region, nd is very close to two H 2 O msers (e.g. Güsten & Downes 1983; Okumur et l. 1989). With projected distnce of only 10 prsec from Sgr A*, it is perfect cndidte cloud to investigte present-dy str formtion in the GC Remrks on individul sources Current ctlogs on OH/IR strs in the GC cn be found in the ppendix. When reference dt is tken from LWHM, Te Lintel Hekkert et l. (1989; herefter TLH) nd Vn Lngevelde et l. (1992), we (re)confirm 87 of the 89 sources tht should hve been visible in the ATCA dt set. In the VLA dt set, 67 of the 71 sources re found. An dditionl 3 sources, tht re not (but should hve been) seen in the VLA dt, re confirmed in our ATCA imge cube. We lso confirm 4 out of 5 single pek detections of LWHM; 3 of which turn out to hve definite double peked nture. Unconfirmed from previous surveys in the OH 1612 MHz mser line remin 4 sources: OH , OH plus the single pek OH from LWHM, nd the double peked source OH from TLH (see the Appendix for two more non-confirmed TLH sources outside this survey). Summrising the new detections, we find 65 previously unknown double peked nd 3 single peked OH 1612 MHz msering sources in the VLA nd ATCA surveys. We count totl of 52 previously unknown OH/IR strs. Bsed on more creful exmintion of sptil extension nd the spectrl shpes, we suspect 13 detections to be moleculr clouds resembling OH/IR str spectr. We confirm tht ll three previously known high-velocity OH/IR strs, s well s one newly found high-velocity source, re blue shifted with n bsolute line-of-sight velocity exceeding 250 km s 1. Becuse of being detectble in only one survey, we were not ble to confirm 15 VLA nd 11 ATCA suspected OH/IR strs ourselves. Some of these sources hve, however, been detected in the infrred. Non-detections We did not confirm OH (TLH number 175); neither did LWHM. We should hve seen it in the ATCA dt, even when the rdition would hve been in its minimum. Considering the flux density nd spectrum given by Hbing et l. (1983), we conclude tht OH is not rel OH/IR str. Yet, one or couple of flse detections do not lter the dynmicl conclusions discussed in the Hbing et l. (1983) pper, or in ny following rticle using this dt point. OH : LWHM report highest flux density of 0.09 Jy, which is, djusted for the primry bem ttenution, just bove our detection limit. We my hve been unlucky to observe with the ATCA close to the minimum of its rdition. The source lies outside our VLA survey. OH : As opposed to the other single peks found by LWHM, for which we found secondry pek in both surveys, this single pek hs not been detected here. No counterprts were found in the literture in ny other mser line or in the infrred. OH : This source lies close to the edge of the survey. LWHM report highest pek flux density of

26 60 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 0.11 Jy, which mkes it only hlf of our detection threshold. The source is not covered by our ATCA survey. Detections Generlly, the overlpping sources i.e. sources tht re in the sptil nd velocity domin of both the VLA nd ATCA dt sets re detected in ech dt set. The min cuse of overlpping sources being detected in only the VLA dt is overll sensitivity (i.e. men flux density over severl epochs); the ATCA observtions were pprently done when the str ws close to its minimum, nd the OH mser flux density therefore below the detection limit. Bckwrds, sources detected only in the ATCA dt re either outside the VLA cube, or the ATCA observtions were pprently done when the str ws close to its mximum, nd the OH mser flux density therefore ws lrger, nd bove the detection limit, in contrst to the men flux density. Following we mke individul remrks on smll selection of the sources, where we use the bbrevition RR for sources in, or the pper by Rieke & Rieke (1988). OH : Single pek, but most likely of stellr origin becuse it coincides with long period K bnd vrible (I.S. Glss, pers. comm.). OH : Either low expnsion velocity source, or the red shifted pek of double peked source. Not seen in the ATCA dt, but the flux nd shpe justify its entry. Note tht lthough we give vlue for the blue shifted velocity, we cnnot distinguish between second pek nd possible effect of the end of the frequency bnd. OH : Listed s single pek in LWHM. OH : Listed s single pek in LWHM. OH : The only new source for which the bsolute stellr velocity exceeds 250 km s 1. OH : Coincides with, nd mtches the velocity of source #49 in RR. As for ll other cses where we find n infrred counterprt for our OH mser sources, we clim the OH nd infrred emission to originte from one (stellr) source. OH : Found s single pek by LWHM. Most probbly double with the second pek t +13 km s 1, but we cnnot confirm this; the source is not detected in our ATCA dt. OH : RR report source (#32) with velocity of 14 km s 1. Our velocity mesurement of +42 km s 1 is within three times their RMS of 20 km s 1. Tking into ccount the sptil extension of the emission, we re however tempted to ttribute the OH emission to moleculr clouds on the line-of-sight. If one supposes the velocity of RR is very ccurte, one cn rgue tht source #32 cn be seen in the OH spectrum with two peks of 15 mjy, seprted by bout 10 km s 1. OH : Mtches the velocity of the nerby source GCIRS 19 (Sellgren et l. 1987; source #22 in RR), but we doubt the identifiction s such. Besides, GCIRS 19 is clssified s probbly being supergint (most recently by Blum et l. 1996), for which one would expect shell expnsion velocity lrger thn the 6.5 km/s found here. OH : Although our bsolute positions re not expected to be very ccurte (see Sect. 3.1), our VLA position for this source grees within one rcsecond of the position of GCIRS 5. The velocity mesured mtches the one given in Krbbe et l. (1991) nd Hller et l. (1996). We propose this source to be the OH counterprt for GCIRS 5. Note tht the OH mser revels low shell expnsion velocity, hinting towrd low metllicity, nd low mss AGB str. OH : The clerest exmple of interesting spectrl structure seen in some (of the stronger) OH/IR strs in the GC. We discuss these peculir spectr further in Sect OH : Found s n H 2 O mser by Levine et l. (1995) nd Yusef-Zdeh & Mehringer (1995). Its nture young, mssive supergint or n evolved, intermedite mss AGB str hs been extensively discussed in Sjouwermn & Vn Lngevelde (1996). The AGB nture of this object, identified with GCIRS 24 s its infrred counterprt, hs been supported recently by Blum et l. (1996), nd ws initilly motivted by Sellgren et l. (1987). OH : A single pek detection, but probbly stellr: it hs been identified with long period K bnd vrible (I.S. Glss, pers. comm.). OH : The SIMBAD dt bse reports this position to be close to the position of IRC However, the error in the position of IRC is lrge ( 1 ) nd we therefore find it more likely tht IRC coincides with one of the other nerby luminous (previously known) OH/IR strs insted. OH : On top of the extended emission we find double peked point source. It hs most probbly been detected by LWHM s their source #69. LWHM pprently mistook pek of (the moleculr cloud) OH s their secondry pek velocity, resulting in reltively high vlue for the shell expnsion velocity. More likely thn OH , this source seems to be relted to source #38 in RR. They give velocity of 109 km s 1 with n RMS of 20 km s 1 where we mesured 89 km s 1. OH : Listed s single pek in LWHM. OH : Found s n H 2 OmserbyYusef- Zdeh & Mehringer (1995), but in fct n OH/IR str; see discussion in Sjouwermn & Vn Lngevelde (1996). OH : RR found source (#51) t 55 km s 1, three times their RMS wy from our velocity. However, we re confident tht the identifiction of the OH source with source #51 in RR fits. OH : This source lies close to IRAS Te Lintel Hekkert & Chpmn (1996) serched IRAS for OH emission, but did not find it becuse of noise level of 440 mjy. The position of OH is however more thn 20 off from

27 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 61 the positions given for severl possible ner-infrred counterprts for IRAS in Monetti et l. (1992). Although IRAS cn lso be interpreted s n ultr-compct HII region, the ssocition with double peked OH emission is more convincing for (post-)agb object (e.g. Volk & Cohen 1989; Monetti et l. 1994; Te Lintel Hekkert & Chpmn 1996). OH : A source nmed OH in TLH (#155), observed by Hbing et l. (1983) with the 100 m single dish telescope in Effelsberg (Bonn). We confirm the position mesured by LWHM; it differs bout one rcminute from the position quoted in TLH. OH : Becuse of its spectrl shpe nd becuse it is not sptilly resolved, we suspect this source to be the red shifted pek of double peked OH/IR source. OH : Close to, nd mybe relted to the infrred source GCS 6 in Kobyshi et l. (1983). OH nd OH : The former ws mrked by LWHM s detected by Winnberg et l. (1985); it should hve been the ltter. OH : Locted ner compct HII region (#16 in Downes et l. 1979) Comprison with known OH/IR strs The OH/IR strs found for the first time in this survey hve less luminous OH msers in their circumstellr shells compred to the previously known OH/IR strs in the GC. In this section we rgue tht the newly found strs re of similr nture to the ones previously known. This result ws used in Sjouwermn & Vn Lngevelde (1996) Sptil nd kinemtic distribution Figures 2 nd 3 show the sptil nd kinemtic distribution of the known nd previously unknown OH/IR strs detected in this survey. It is cler in Fig. 2, tht the more luminous OH msers, generlly the known OH/IR strs, cn lso be detected further out from the survey center. The fct tht there re more strs t positive ltitude offsets is n effect of the symmetry in the survey pointing. Where the survey is sensitive enough, one sees tht there is no preferred, nd no distinct loction for ech of the smples. However, we mke the observtion tht the lleged void of known OH/IR strs t smll positive longitudes nd smll positive ltitudes, does not comply with the combined smple. Furthermore, comprison of the loction of strs in Fig. 2 nd Fig. 3 between both smples lso suggests similr distribution in phse-spce Expnsion velocity distribution To rgue further tht the smples consist of the sme type of strs, we show the distributions of the shell expnsion Fig. 9. Expnsion velocity distribution. The solid line is the distribution of previously known OH/IR strs in our survey, new detections re distributed ccording to the dotted line. The striking resemblnce of both distributions is in lrge contrst to the expnsion velocity distributions of OH/IR strs with different metllicity, for exmple in the outer Glxy nd Glctic plne (dshed: Blommert et l plus Blommert et l. 1994, multiplied by three for disply purposes) velocity in Fig. 9. Both smples 3 hve shell expnsion velocities shrply peked round 19 km s 1. Expnsion velocity distributions for mny other smples of OH/IR strs cn be found in the literture, e.g. Eder et l. (1988), Te Lintel Hekkert et l. (1991) nd Wood et l. (1992). We wnt to restrict ourselves by compring the expnsion velocities of the OH/IR str in the GC with smples of the Glctic plne (Blommert et l. 1993, 1994) nd the Glctic bulge (Sevenster et l. 1997). The distribution of the OH/IR strs found in the Glctic plne is lso shown in Fig. 9. The distribution for the Glctic bulge hs its pek round 15 km s 1,ndis broder thn for the GC (see Sevenster et l. 1997). When compring the expnsion velocities of the OH/IR strs in the GC with the ones in the Glctic plne nd bulge, we rech two conclusions. First, the expnsion velocity distribution in the GC is different from the expnsion velocity distribution of ny other smple known, which we ttribute to generlly higher metllicities in the GC (see the discussions in e.g. Wood et l. 1992; Blommert et l nd Hbing 1996). Second, we note the striking 3 Note tht we do not include ll strs found in the whole GC region in our nlysis; only the strs detected in this survey.

28 62 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center flux density for which the survey is not complete. It prevents us to derive firm conclusion bout possible low luminosity cut-off in the OH mser distribution. We cn however stte, tht the distribution peks t L OH photons per second within the sttisticl errors Triple nd qudruple mser lines Fig. 10. OH luminosity distribution. The dshed curve combines the previously known OH/IR strs in our survey nd the new detections. The known OH/IR str luminosity distribution is depicted by the dotted line. The solid line outlines the mximum extent of the distribution, i.e. corrected for incompleteness ccording to homogeneous sptil distribution resemblnce of the expnsion velocity distribution of the known nd the previously unknown OH/IR strs in this survey, nd rgue for generlly identicl metllicities for both of our smples. Hence, s the shell expnsion velocity is function of metllicity nd stellr luminosity, we conclude tht the stellr luminosity distribution for both of our smples is identicl; s fr s we cn tell, the centrl strs re the sme OH mser luminosity function Becuse both the known nd previously unknown OH/IR strs re intrinsiclly identicl, we cn investigte the combined OH luminosity distribution. Figure 10 shows the result when we ssume tht ll strs re locted t distnce of 8 kpc. Tht the new detections re minly the low OH luminosity sources cn be seen redily from the difference in the totl distribution nd the known OH/IR str distribution. Also shown is the mximum extent of the distribution when it is corrected for (in)completeness. We clculted this correction by weighing ech source by the inverse of its detection probbility nd ssuming homogeneous number distribution. Agin, this correction is n overestimte. Unfortuntely the number of sources with respect to the corrections t the left-hnd side of the luminosity distribution is smll. In generl these re the sources with low pek In Sect. 4.3 we mentioned non-stndrd spectrl structure seen in OH ( ctegory 1 OH vrible in vljghw). Less cler exmples, both vrible (ct. 1, 2) nd non-, or irregulrly vrible 4 (ct. 3) sources, re OH (ct. 2), OH (ct. 1), OH (ct. 2), OH (ct. 3), OH (ct. 1), OH (not monitored), OH (ct. 1) nd OH (ct. 3). The msers in OH nd OH re qudruple peked, the rest re triple peked. In ddition to the spectr in Fig. 5, Fig. 11 shows the lower frequency resolution spectr mesured in imge cube b for four of these strs. We hve only seen this type of spectr in the conctented dt; spectr with regulr dditionl emission fetures t roughly either 15 or 50 km s 1 on the redshifted side of the redshifted pek, or bout 15 km s 1 outside both min peks. Instrumentl effects (e.g. the Gibbs phenomenon) nd side-lobe fetures of nerby sources cn be excluded. In the cse of OH we mesured consistent positionl offset of 1.34 rcsecond ( AU, or 10 times the shell rdius tht ws mesured from the phselg of the min peks by vljghw) to the southwest, from the min peks to both outer peks. The 61 km s 1 feture inside the min peks is positionlly coincident with the min peks. The outer peks of OH re displced bout 0.6 northest from the min peks, gin within the errors t mutul excluding positions. Becuse of the symmetry seen in OH , nd in OH , nd the rther remrkble velocity intervl structure in ll exmples, we tend to conclude tht the emission is from (the shell of) the OH/IR str itself; not from nother source (n OH/IR binry, or more exoticly, n OH/IR str cptured by n OH msering supergint) seen t the sme projected coordintes. Suggestions then rnge from double shell, indicting different epochs of interrupted mss-loss s seen for crbon rich AGB strs (Olofsson et l. 1996), cretion of other molecules besides CO lredy erly in the mss-loss history, n effect relted or similr to the mode switching seen in the H 2 O mser lines of OH (Engels et l. 1997), to bipolr outflow. These strs clerly mke excellent test-cses for our understnding of the mss-loss mechnism of evolved strs. 4 However, OH nd OH were found to be regulrly vrible in the infrred by Jones et l. (1994). OH ws not in their smple.

29 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center 63 Fig. 11. Spectr from imge cube b for selected sources. See Sect. 4.5 Although the both uniformly smpled VLA nd ATCA dt sets hve comprble sensitivity, the spectrl fetures re only clerly seen in the conctented VLA dt set. Therefore the fetures result from either wek emission from mny epochs, or strong emission t limited mount of individul epochs. Mking cse for the ltter, other exmples of similr, one epoch spectr in the literture cn be found, however undiscussed, in Te Lintel Hekkert et l. (1991; IRAS nd IRAS ), nd in Sevenster et l. (1997; e.g. OH , OH nd OH ). Furthermore, Eder et l. (1988) mention n identicl phenomenon in IRAS (OH ), nd suggest it to be mpped. However, OH ws found unresolved t 1612 MHz with the VLA in CD-rry by Lewis et l. (1990), nd observtions with the Europen VLBI Network hve not been published yet (M. Lindqvist, pers. comm.). Note however, tht this OH emission of OH nd OH cnnot be detected with VLBI observtions, becuse of severe scttering of the source t decimeter wvelengths (0.6 t 1612 MHz for OH ; Vn Lngevelde et l. 1992b). 5. Conclusions We detected 155 double peked 1612 MHz sources within 18 of Sgr A*. Of the 155 detections, 52 re previously unknown OH/IR strs. In ddition, 3 single pek detections re given, which re most probbly msers of OH/IR strs s well. We hve lso listed 13 double peked sources tht exceed the RMS noise levels by fctor of 8, nd tht most probbly originte in the moleculr cloud complex locted t the GC. The sky nd velocity distribution, s well s the expnsion velocity distribution of the low OH luminosity strs compre very well with the previously known OH/IR strs in the GC. We therefore conclude tht this survey reveled the low OH luminosity prt of the GC OH/IR str popultion; the dditionl sources in the new smple re intrinsiclly identicl to the AGB strs of the known smple, except for less efficient OH mser. The OH/IR str OH mser luminosity distribution peks t L OH photons per second. From this survey however, we cn not conclude with certinty tht there is low luminosity cut-off of the OH mser luminosity distribution. Acknowledgements. LOS hereby thnks ll who mde it possible to hve this unusully lrge dt set conctented nd nlysed; in prticulr everyone t Sterrewcht Leiden for voiding the locl re network during nighttime, nd the AIPS users in Onsl for flexible disk spce usge. Thnks to In Glss for providing dt before publiction nd to Michel Lindqvist for his continuous interest. For this project LOS received finncil support from Sterrewcht Leiden, Svensk Institutet nd Onsl Rymdobservtorium. HJvL cknowledges support for his reserch by the Europen Union under contrct CHGECT nd AW cknowledges support by the Swedish Nturl Science Reserch Council. In this rticle we use observtions obtined with the Austrli Telescope, which is funded by the Commonwelth of Austrli for opertion s Ntionl Fcility mnged by CSIRO, nd dt collected with the Very Lrge Arry, operted s prt of the Ntionl Rdio Astronomy Observtory by Associted Universities Inc. under coopertive greement with the Ntionl Science Foundtion. This reserch hs mde use of the SIMBAD dtbse, operted t CDS, Strsbourg, Frnce. A. Appendix: Ctlogs For the identifiction of OH/IR strs in the Glctic center (GC), the ctlog of Lindqvist et l. (1992; LWHM) is the most complete. The pre-iras ctlogue of stellr 1612 MHz mser sources, compiled from the literture by Te Lintel Hekkert et l. (1989; TLH), lists dditionl GC stellr OH mser sources tht lie outside the re or velocity coverge surveyed by LWHM. However, LWHM did not confirm two sources in the TLH ctlog: OH nd OH (TLH numbers 175 nd 207). Source OH hs been discussed in Sect For OH (TLH#207) TLH give three references: Bud et l. (1979), Bud et l. (1981) nd Olnon et l. (1981). The ltter however, use the dt of Bud et l. (1979). We noticed tht in Bud et l. (1979), compring with Bud et l. (1981), the velocity nd flux dt corresponding to OH (OH or OH ) hs

30 64 L.O. Sjouwermn et l.: A new smple of OH/IR strs in the Glctic center been entered for OH (OH ) nd vice vers 5. Source OH hs been confirmed by Hbing et l. (1983) nd LWHM t the proper (i.e. Bud et l. 1981) velocities; source OH hs probbly been detected s n H 2 O mser (with velocity of 58 km s 1 ; Btchelor et l who, however, did not recognise it s stellr source), s signl ccidently picked up in single dish bem pointed towrds the HII region G (e.g. Downes et l. 1979). There re no confirmtions the other wy round. Therefore, the dt listed in Bud et l. (1979) should be red s in Bud et l. (1981). This mens therefore lso, tht the velocity nd flux dt for TLH#207 nd TLH#214 should be exchnged. Hence, it ppers tht TLH#206 nd TLH#207, respectively TLH#213 nd TLH#214, re entries for the sme sources. Most surveys for OH/IR strs fter 1983 re bsed on the IRAS Point Source Ctlog dt bse, exploiting the highly successful predictive property of the IRAS twocolour digrm for detecting OH/IR strs (Olnon et l. 1984; Vn der Veen & Hbing 1988). Becuse of confusion in the GC, these surveys omit the GC re (however, see Tylor et l for description of n IRAS Glctic Center Ctlog ). Additionlly to the dt listed in TLH nd LWHM, two other 1612 MHz surveys covering the GC hve been done. Vn Lngevelde et l. (1992) found two highvelocity OH/IR strs in the GC, nd Sevenster et l. (1997) filled the gp of the GC nd bulge, where the IRAS survey hd suffered from confusion. Becuse the ltter survey hs low sensitivity compred to LWHM, in prticulr in the GC region, no new mtches were found. We lso extended our serch for possible stellr counterprts for our new detections in other wvelength regions. Mny mtches were found in preliminry list of long period K bnd vribles, kindly provided by In Glss (for description see Glss et l. 1996). New detections were cross-correlted with the SIMBAD dt bse (< 20 rdius), nd with dt sets found in the min journls between 1979 nd However, in generl the overlp is miniml. References Btchelor R.A., Cswell J.L., Goss W.M., Hynes R.F., Knowles S.H., Wellington K.J., 1980, Aust. J. Phys. 33, 139 Bud B., Hbing H.J., Mtthews H.E., O Sullivn J.D., Winnberg A., 1975, Nt 258, 406 Bud B., Hbing H.J., Mtthews H.E., Winnberg A., 1979, A&AS 35, 179 Bud B., Hbing H.J., Mtthews H.E., Winnberg A., 1981, A&A 95, 171 Blommert J.A.D.L., vn der Veen W.E.C.J., Hbing H.J., 1993, A&A 267, 39 5 In the first chpter of Bud s thesis, preliminry version of the 1979 pper, this exchnge hd not yet occurred. Blommert J.A.D.L., vn Lngevelde H.J., Michiels W.F.P., 1994, A&A 287, 479 Blum R.D., Sellgren K., DePoy D.L., 1996, AJ 112, 1988 Cornwell T.J., Uson J.M., Hddd N., 1992, A&A 258, 583 Dejonghe H., 1993, in: Glctic bulges, Dejonghe H., Hbing H.J. (eds.). Kluwer Acdemic Publishers, Dordrecht, p. 73 Downes D., Goss W.M., Schwrz U.J., Wouterloot J.G.A., 1979, A&AS 35, 1 Eder J., Lewis B.M., Terzin Y., 1988, ApJS 66, 183 Ekers R.D., vn Gorkom J.H., Schwrz U.J., Goss W.M., 1983, A&A 122, 143 Engels D., Winnberg A., Wlmsley C.M., Brnd J., 1997, A&A (in press) Fril D.A., Dimond P.J., Cordes J.M., vn Lngevelde H.J., 1994, ApJ 427, L43 Glss I.S., Mtsumoto S., Ono T., Sekiguchi K., 1996, in: The Glctic Center, Gredel R. (ed.) PASPC 102, 312 Güsten R., Downes D., 1983, A&A 117, 343 Hbing H.J., 1993, in: Glctic bulges, Dejonghe H., Hbing H.J. (eds.). Kluwer Acdemic Publishers, Dordrecht, p. 57 Hbing H.J., 1996, A&AR 7, 97 Hbing H.J., Olnon F.M., Winnberg A., Mtthews H.E., Bud B., 1983, A&A 128, 230 Hller J.W., Rieke M.J., Rieke G.H., Tmblyn P., Close L., Meli F., 1996, ApJ 456, 194 Hrvey P.M., Bechis K.P., Wilson W.J., Bll J.A., 1974, ApJS 27, 331 Hermn J., Hbing H.J., 1985, A&AS 59, 523 Iben I., Renzini A., 1983, ARA&A 21, 271 Jones T.J., McGregor P.J., Gehrz R.D., Lwrence G.F., 1994, AJ 107, 1111 Kobyshi Y., Okud H., Sto S., Jugk J., Dyck H.M., 1983, PASJ 35, 101 Krbbe A., Genzel R., Drptz S., Rotciuc V., 1991, ApJ 382, L19 Levine D.A., Figer D.F., Morris M., McLen I.S., 1995, ApJ 447, L101 Lewis B.M., Chenglur J.N., Schmelz J., Terzin Y., 1990, MNRAS 246, 523 Lindqvist M., Winnberg A., Hbing H.J., Mtthews H.E., 1992, A&AS 92, 43 (LWHM) Lindqvist M., Hbing H.J., Winnberg A., 1992b, A&A 259, 118 Lindqvist M., Winnberg A., Hbing H.J., 1997 (in preprtion) Monetti A., Glss I.S., Moorwood A.F.M., 1992, MNRAS 258, 705 Monetti A., Glss I.S., Moorwood A.F.M., 1994, MNRAS 268, 194 Okumur S.K., Ishiguro M., Fomlont E.B., et l., 1989, ApJ 347, 240 Olnon F.M., Wlterbos R.A.M., Hbing H.J., et l., 1981, ApJ 245, L103 Olnon F.M., Bud B., Hbing H.J., de Jong T., Hrris S., Pottsch S.R., 1984, ApJ 278, L41 Olofsson H., Bergmn P., Eriksson K., Gustfsson B., 1996, A&A 311, 587 Pedlr A., Annthrmih K.R., Ekers R.D., Goss W.M., vn Gorkom J.H., Schwrz U.J., Zho J-H., 1989, ApJ 342, 769 Reid M.J., 1993, ARA&A 31, 345 Rieke G.H., Rieke M.J., 1988, ApJ 330, L33 (RR) Sellgren K., Hll D.N.B., Kleinmnn S.G., Scoville N.Z., 1987, ApJ 317, 881

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