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Register a new userCurrent Star Formation in the Perseus Molecular Cloud: Constraints from Unbiased Submillimeter and Mid-Infrared Surveys
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Jes K. J{\\o}rgensen
Publication date
2006
fields
Physics
and research's language is
English
Authors
Jes K. Jorgensen
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We present a census of the population of deeply embedded young stellar objects (YSOs) in the Perseus molecular cloud complex based on a combination of Spitzer Space Telescope mid-IR data from the c2d legacy team and JCMT/SCUBA submillimeter maps from the COMPLETE team. The mid-IR sources detected at 24 micron and having [3.6]-[4.5] > 1 are located close to the center of the SCUBA cores, typically within 15 of their peaks. The narrowness of the spatial distribution of mid-IR sources around the peaks of the SCUBA cores suggests that no significant dispersal of the newly formed YSOs has occurred. This argues against the suggestion that motions of protostars regulate the time scales over which significant (Bondi-Hoyle) accretion can occur. The most deeply embedded YSOs are found in regions with high extinction, AV > 5, similar to the extinction threshold observed for the SCUBA cores. All the SCUBA cores with high concentrations have embedded YSOs, but not all cores with low concentrations are starless. An unbiased sample of 49 deeply embedded YSOs is constructed. Embedded YSOs are found in 40 of the 72 SCUBA cores with only three cores harboring multiple embedded YSOs within 15. The equal number of SCUBA cores with and without embedded YSOs suggests that the time scale for the evolution through the dense prestellar stages, where the cores are recognized in the submillimeter maps and have central densities of 5e4-1e5 cm^{-3}, is similar to the time scale for the embedded protostellar stages. The current star formation efficiency of cores is estimated to be approximately 10-15%. In contrast, the star formation efficiency averaged over the cloud life time and compared to the total cloud mass is only a few percent, reflecting also the efficiency in assembling cloud material into the dense cores forming stars.
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Before having exhausted the helium in its tank on April 8, 1998, the Infrared Space Observatory had time to perform several complementary surveys at various depths and areas within selected regions of the sky. We present the results of some of the surveys done with the mid-infrared camera, ISOCAM, on-board ISO, and more specifically those surveys which were performed using the broad-band filter LW3 (12-18 microns). We show that mid-infrared allows to discriminate starburst and post-starburst galaxies among galaxies which present similar optical-UV morphological and colour signatures. Furthermore, the strong evolution found in the surveys indicates that more star formation is hidden in the infrared at higher redshifts.
We present observations performed with the Green Bank Telescope at 1.4 and 5 GHz of three strips coincident with the anomalous microwave emission features previously identified in the Perseus molecular cloud at 33 GHz with the Very Small Array. With these observations we determine the level of the low frequency (~1 - 5 GHz) emission. We do not detect any significant extended emission in these regions and we compute conservative 3sigma upper limits on the fraction of free-free emission at 33 GHz of 27%, 12%, and 18% for the three strips, indicating that the level of the emission at 1.4 and 5 GHz cannot account for the emission observed at 33 GHz. Additionally, we find that the low frequency emission is not spatially correlated with the emission observed at 33 GHz. These results indicate that the emission observed in the Perseus molecular cloud at 33 GHz, is indeed in excess over the low frequency emission, hence confirming its anomalous nature.
Does star formation proceed in the same way in large spirals such as the Milky Way and in smaller chemically younger galaxies? Earlier work suggests a more rapid transformation of H$_2$ into stars in these objects but (1) a doubt remains about the validity of the H$_2$ mass estimates and (2) there is currently no explanation for why star formation should be more efficient. M~33, a local group spiral with a mass $sim 10$% and a metallicity half that of the Galaxy, represents a first step towards the metal poor Dwarf Galaxies. We have searched for molecular clouds in the outer disk of M~33 and present here a set of detections of both $^{12}$CO and $^{13}$CO, including the only detections (for both lines) beyond the R$_{25}$ radius in a subsolar metallicity galaxy. The spatial resolution enables mass estimates for the clouds and thus a measure of the $N({rm H}_2) / I_{rm CO}$ ratio, which in turn enables a more reliable calculation of the H$_2$ mass. Our estimate for the outer disk of M~33 is $N({rm H}_2) / I_{rm CO(1-0)} sim 5 times 10^{20} ,{rm cm^{-2}/(K{rm km s^{-1}})}$ with an estimated uncertainty of a factor $le 2$. While the $^{12/13}$CO line ratios do not provide a reliable measure of $N({rm H}_2) / I_{rm CO}$, the values we find are slightly greater than Galactic and corroborate a somewhat higher $N({rm H}_2) / I_{rm CO}$ value. Comparing the CO observations with other tracers of the interstellar medium, no reliable means of predicting where CO would be detected was identified. In particular, CO detections were often not directly on local HI or FIR or H$alpha$ peaks, although generally in regions with FIR emission and high HI column density. The results presented here provide support for the quicker transformation of H$_2$ into stars in M~33 than in large local universe spirals.
We present a complete survey of current star formation in the Perseus molecular cloud, made at 850 and 450 micron with SCUBA at the JCMT. Covering 3 deg^2, this submillimetre continuum survey for protostellar activity is second in size only to that of rho Ophiuchus (Johnstone et al. 2004). Complete above 0.4 msun (5 sigma detection in a 14 beam), we detect a total of 91 protostars and prestellar cores. Of these, 80% lie in clusters, representative of star formation across the Galaxy. Two of the groups of cores are associated with the young stellar clusters IC348 and NGC1333, and are consistent with a steady or reduced star formation rate in the last 0.5 Myr, but not an increasing one. In Perseus, 40--60% of cores are in small clusters (< 50 msun) and isolated objects, much more than the 10% suggested from infrared studies. Complementing the dust continuum, we present a C^18O map of the whole cloud at 1 resolution. The gas and dust show filamentary structure of the dense gas on large and small scales, with the high column density filaments breaking up into clusters of cores. The filament mass per unit length is 5--11 msun per 0.1 pc. Given these filament masses, there is no requirement for substantial large scale flows along or onto the filaments in order to gather sufficient material for star formation. We find that the probability of finding a submillimetre core is a strongly increasing function of column density, as measured by C^18O integrated intensity, prob(core) proportional to I^3.0. This power law relation holds down to low column density, suggesting that there is no A_v threshold for star formation in Perseus, unless all the low-A_v submm cores can be demonstrated to be older protostars which have begun to lose their natal molecular cloud.
We present a study of hierarchical structure in the Perseus molecular cloud, from the scale of the entire cloud ($gtrsim$10 pc) to smaller clumps ($sim$1 pc), cores ($sim$0.05-0.1 pc), envelopes ($sim$300-3000 AU) and protostellar objects ($sim$15 AU). We use new observations from the Submillimeter Array (SMA) large project Mass Assembly of Stellar Systems and their Evolution with the SMA (MASSES) to probe the envelopes, and recent single-dish and interferometric observations from the literature for the remaining scales. This is the first study to analyze hierarchical structure over five scales in the same cloud complex. We compare the number of fragments with the number of Jeans masses in each scale to calculate the Jeans efficiency, or the ratio of observed to expected number of fragments. The velocity dispersion is assumed to arise either from purely thermal motions, or from combined thermal and non-thermal motions inferred from observed spectral line widths. For each scale, thermal Jeans fragmentation predicts more fragments than observed, corresponding to inefficient thermal Jeans fragmentation. For the smallest scale, thermal plus non-thermal Jeans fragmentation also predicts too many protostellar objects. However at each of the larger scales thermal plus non-thermal Jeans fragmentation predicts fewer than one fragment, corresponding to no fragmentation into envelopes, cores, and clumps. Over all scales, the results are inconsistent with complete Jeans fragmentation based on either thermal or thermal plus non-thermal motions. They are more nearly consistent with inefficient thermal Jeans fragmentation, where the thermal Jeans efficiency increases from the largest to the smallest scale.
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