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Register a new userEvolutionary Signatures in the Formation of Low-Mass Protostars. II. Towards Reconciling Models and Observations
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A long-standing problem in low-mass star formation is the luminosity problem, whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. Motivated by this luminosity problem, we present a set of evolutionary models describing the collapse of low-mass, dense cores into protostars, using the Young & Evans (2005) model as our starting point. We calculate the radiative transfer of the collapsing cores throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures to directly compare to observations. We incorporate several modifications and additions to the original Young & Evans model in an effort to better match observations with model predictions. We find that scattering, 2-D geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. A cycle of episodic mass accretion, however, can resolve this problem and bring the model predictions into better agreement with observations. Standard assumptions about the interplay between mass accretion and mass loss in our model give star formation efficiencies consistent with recent observations that compare the core mass function (CMF) and stellar initial mass function (IMF). The combination of outflow cavities and episodic mass accretion reduce the connection between observational Class and physical Stage to the point where neither of the two common observational signatures (bolometric temperature and ratio of bolometric to submillimeter luminosity) can be considered reliable indicators of physical Stage.
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We present an evolutionary picture of a forming star. We assume a singular, isothermal sphere as the initial state of the core that undergoes collapse as described by citet{shu77}. We include the evolution of a first hydrostatic core at early times and allow a disk to grow as predicted by citet{adams86}. We use a 1-dimensional radiative transfer code to calculate the spectral energy distribution for the evolving protostar from the beginning of collapse to the point when all envelope material has accreted onto the star+disk system. Then, we calculate various observational signatures ($T_{bol}$, $L_{bol}/L_{smm}$, and infrared colors) as a function of time. As defined by the bolometric temperature criterion, the Class 0 stage should be very short, while the Class I stage persists for much of the protostars early life. We present physical distinctions among the classes of forming stars and calculate the observational signatures for these classes. Finally, we present models of infrared color-magnitude diagrams, as observed by the Spitzer Space Telescope, that should be strong discriminators in determining the stage of evolution for a protostar.
We aim at studying with high angular resolution a dense core associated with a low-luminosity IRAS source, IRAS 00213+6530, in order to investigate whether low mass star formation is really taking place in isolation. We performed observations at 1.2mm with the IRAM 30m telescope, VLA observations at 6cm, 3.6cm, 1.3cm, 7mm, and H2O maser and NH3 lines, and observations with the NASA 70m antenna in CCS and H2O maser. The cm and mm continuum emission, together with the near infrared data from the 2MASS allowed us to identify 3 YSOs, IRS1, VLA8A, and VLA8B, with different radio and infrared properties, and which seem to be in different evolutionary stages. The NH3 emission consists of three clouds. Two of these, MM1 and MM2, are associated with dust emission, while the southern cloud is only detected in NH3. The YSOs are embedded in MM1, where we found evidence of line broadening and temperature enhancements. On the other hand, the southern cloud and MM2 appear to be quiescent and starless. We modeled the radial intensity profile at 1.2mm of MM1. The model fits reasonably well the data, but it underestimates the intensity at small projected distances from the 1.2mm peak, probably due to the presence of multiple YSOs embedded in the envelope. There is a differentiation in the relative NH3 abundance with low values, ~2x10^-8, toward MM1, and high values, up to 10^-6, toward the southern cloud and MM2, suggesting that these clouds could be in a young evolutionary stage. IRAS 00213+6530 is harboring a multiple system of low-mass protostars, indicating that star formation in this cloud is taking place in groups, rather than in isolation. The low-mass YSOs found in IRAS 00213+6530 are in different evolutionary stages suggesting that star formation is taking place in different episodes.
I briefly review recent observations of regions forming low mass stars. The discussion is cast in the form of seven questions that have been partially answered, or at least illuminated, by new data. These are the following: where do stars form in molecular clouds; what determines the IMF; how long do the steps of the process take; how efficient is star formation; do any theories explain the data; how are the star and disk built over time; and what chemical changes accompany star and planet formation. I close with a summary and list of open questions.
We report our current SMA and ALMA studies of disk and planet formation around protostars. We have revealed that $r gtrsim$100 AU scale disks in Keplerian rotation are ubiquitous around Class I sources. These Class I Keplerian disks are often embedded in rotating and infalling protostellar envelopes. The infalling speeds of the protostellar envelopes are typically $sim$ 3-times smaller than the free-fall velocities, and the rotational profiles follow the $r^{-1}$ profile, that is, rotation with the conserved specific angular momentum. Our latest high-resolution ($sim$0$farcs$5) ALMA studies, as well as the other studies in the literature, have unveiled that $r sim$100-AU scale Keplerian disks are also present in several Class 0 protostars, while in the other Class 0 sources the inferred upper limits of the Keplerian disks are very small ($r lessim$20 AU). Our recent data analyses of the ALMA long baseline data of the Class I-II source HL Tau have revealed gaps in molecular gas as well as in dust in the surrounding disk, suggesting the presence of sub-Jovian planets in the disk. These results imply that disk and planet formation should be completed in the protostellar stage.
We present a comparison of the observed evolving galaxy stellar mass functions with the predictions of eight semi-analytic models and one halo occupation distribution model. While most models are able to fit the data at low redshift, some of them struggle to simultaneously fit observations at high redshift. We separate the galaxies into passive and star-forming classes and find that several of the models produce too many low-mass star-forming galaxies at high redshift compared to observations, in some cases by nearly a factor of 10 in the redshift range $2.5 < z < 3.0$. We also find important differences in the implied mass of the dark matter haloes the galaxies inhabit, by comparing with halo masses inferred from observations. Galaxies at high redshift in the models are in lower mass haloes than suggested by observations, and the star formation efficiency in low-mass haloes is higher than observed. We conclude that many of the models require a physical prescription that acts to dissociate the growth of low-mass galaxies from the growth of their dark matter haloes at high redshift.
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