Starless cores Shapes Density profiles Temperatures Velocity structure Magne8c fields Mass func8on. Molecular Cloud Cores

Transcription

1 Starless cores Shapes Density profiles Temperatures Velocity structure Magne8c fields Mass func8on Molecular Cloud Cores

2 Molecular cloud cores Cores are the immediate loca8ons in which stars are born, and allow measurements of the ini8al condi8ons of star forma8on. Cores can be studied with tracers of dense gas (CS, HCO +, HCN, H 2 CO, NH 3 ), emission from warm dust (far- IR con8nuum), and dust ex8nc8on Early observa8ons with low resolu8on provided only average measurements of core- wide masses and densi8es IR and radio telescopes now have sufficient resolu8on and sensi8vity for studying the internal structure of cores Modern studies of cores ooen select their targets from the database of 264 cores mapped in NH 3 from Jijina et al. 1999, which includes posi8ons, N(NH 3 ), volume densi8es, line widths, T kin, sizes, and aspect ra8os.

3

4 Starless cores Spitzer (Young et al. 2004) >50% of cores are known to contain protostars. The remainder are called starless cores. Spitzer has provided much deeper mid- IR images of cores than were available from IRAS and ISO. In a few cores that were previously believed to be starless, Spitzer has detected very faint, embedded sources that could be protostellar brown dwarfs. CO ou`low (Bourke et al. 2005)

5 Shapes of cores Most cores are not spherical, and are typically ellip8cal with aspect ra8os of 2:1. Whether cores are prolate, oblate, or triaxial remains a subject of debate, but the lack of spherical symmetry indicates an asymmetrical force or the absence of perfect equilibrium. Myers et al. 1991

6 Density profiles of cores The density as a func8on of radius in cores has been measured with three methods that trace dust: 1) Near- IR ex8nc8on maps Depends on ex8nc8on law and A V /N H 2) Submm/mm dust con8nuum maps Depends on temperature and emissivity of dust 3) Absorp8on maps of mid- IR background emission Depends on uniformity of background emission and opacity/n H The density profiles produced by these methods agree qualita8vely between different cores, but no systema8c comparison has been made for the same cores observed with mul8ple techniques.

7 Bergin & Tafalla 2007

8 Density profiles of cores Density profiles flajen near the centers of cores, changing from r - 2 at large radii to r - 1 within 5000 AU. Some physical models for cores are inconsistent with these data and can be ruled out. The origin of these profiles is not fully understood yet. Galli et al. 2002

9 Dust temperature T dust is determined by hea8ng from the interstellar field and cooling from thermal emission. Based on far- IR/submm colors, T dust varies from 20 K in clouds to 10 K in cores. But this gradient is uncertain because the dust emissivity may change with density. TMC- 1C based on 450 and 850 μm maps from SCUBA (Schnee & Goodman 2005) Dust temperature for constant emissivity Dust emissivity for constant temperature

10 Gas temperature Gas in cores is heated by cosmic rays and cooled by CO CO and other molecules begin to freeze out onto dust grains at the high densi8es in cores, which reduces the gas cooling But because of the high densi8es, the T gas and T dust are coupled, and thermal radia8on of the dust keeps the gas at a low temperature NH 3 transi8ons near 1.3 cm provide the best measurements of gas temperatures in cores, producing values near 10 K However, because of the long wavelengths of the NH 3 lines, the spa8al resolu8on of the observa8ons is low, and few measurements of the radial profiles of T gas are available.

11 Velocity structure Clouds have supersonic line widths because of turbulence, but cores have much smaller line widths that are close to thermal, indica8ng slow internal mo8ons. The line width- size rela8on for clouds does not extend to cores. Below 0.1 pc, the nonthermal component of the line width is roughly constant. Thermal pressure rather than turbulence dominates the support of the core below this coherence scale. See data for the Pipe Nebula from Muench et al. 2007, Rathborne et al. 2008, Lada et al Goodman et al. 1998

12 Velocity structure Rota8on is expected for gravita8onally collapsing clouds However, cores rotate slowly; the ra8o of the rota8onal energy to gravita8onal energy is only a few percent Rota8on is not an important source of support The slow rota8on of cores requires a mechanism for removal of angular momentum; this may occur via magne8c coupling of the contrac8ng core and the ambient medium Cores have similar angular momenta as later stages of evolu8on (envelopes and disks of protostars and wide binaries) except for the last one (i.e., stars)

13 Velocity structure One of the primary predic8ons of models of star forma8on is the collapse of cores, which can be detected through the kinema8cs of tracers of dense gas like CS, H 2 CO, and HCO +. Low- excita8on foreground gas in the outer core produces absorp8on against the blue- shioed emission from the infalling central core. The resul8ng asymmetric line profile with red absorp8on is a signature of core infall. The inward veloci8es are typically 0.1 km/s. B335 Zhou et al. 1993

14 Velocity structure Evans et al. 1999

15 Velocity structure The Bok globule B68 exhibits an asymmetric velocity field with both inward and outward mo8ons, indica8ng the presence of non- radial oscilla8ons in the outer layers of the cloud. These oscilla8ons are consistent with hydrodynamic waves (i.e., sound waves). Life8mes of such oscilla8ons are predicted to be ~1 Myr. Lada et al. 2003

16 Magne8c fields in clouds and cores The magne8c field is one of the most difficult aspects of clouds and cores to study. A large amount of effort has produced only limited results. The direc8ons of magne8c fields have been mapped in a few nearby molecular clouds with polariza8on of background starlight. The fields are highly regular on scales of 10 pc and are not strongly correlated with filaments and other large- scale structures. Taurus Goodman et al. 1990

17 Magne8c fields in clouds and cores Zeeman splivng is the only diagnos8c of magne8c field strength (line- of- sight) in molecular clouds. It is difficult to detect and is applied to only a few isolated posi8ons in clouds; large- scale maps are not possible. The best species for Zeeman are H, OH, and CN. Because their transi8ons occur at long wavelengths, the spa8al resolu8on of the measurements is low (few arcmin; 0.1 pc). These species trace low- density gas, so they are not ideal for cores. When Zeeman is detected, field strengths are typically 10 μg. Based on the limited data, magne8c fields contribute non- negligible support to cores, but aren t strong enough to stabilize against gravity (cores are modestly supercri8cal rather than subcri8cal). See Crutcher et al. 1999, 2008, 2009.

18 Mass func8ons Over the last decade, there has been a great deal of interest in measuring core mass func8ons for comparison to the stellar IMF Submm/mm con8nuum maps of dust emission in the nearest molecular clouds have been used predominantly (Moje et al. 1998, 2001; Tes8 & Sargent 1998; Johnstone et al. 2000, 2001, 2006; Reid & Wilson 2005, 2006). More recently, ex8nc8on maps have proven valuable for this purpose as well (Alves et al. 2007; Rathborne et al. 2009).

19 Mass func8ons Cores have steeper mass func8ons than clumps, and are similar to the Salpeter slope for the IMF of stars. This implies that core fragmenta8on is responsible for the shape of the IMF. However, to be confident that the mass func8ons of cores and stars are truly connected, it would be helpful to detect addi8onal characteris8cs in common, such as the flajening of the stellar IMF at low masses. Moje et al. 1998

20 The Pipe Nebula B68

21 Mass func8ons The core mass func8on measured from the ex8nc8on map of the Pipe Nebula agrees with the stellar IMF both in its high- mass slope and the presence of a flajening at low masses. The factor of 3-5 offset between the core and stellar masses can be interpreted as a star forma8on efficiency of 20-30% for cores. Alves et al. 2007, Rathborne et al. 2009