Chadwick H. Young

Evolutionary Signatures in the Formation of Low-Mass Protostars. II. Toward Reconciling Models and Observations

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. We use as our starting point the evolutionary model following the inside-out collapse of a singular isothermal sphere as presented by Young & Evans. We calculate the radiative transfer of the collapsing core throughout the full duration of the collapse in two dimensions. From the resulting spectral energy distributions, we calculate standard observational signatures (L bol, T bol, L bol/L smm) 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 include (1) the opacity from scattering in the radiative transfer, (2) a circumstellar disk directly in the two-dimensional radiative transfer, (3) a two-dimensional envelope structure, taking into account the effects of rotation, (4) mass-loss and the opening of outflow cavities, and (5) a simple treatment of episodic mass accretion. We find that scattering, two-dimensional geometry, mass-loss, and outflow cavities all affect the model predictions, as expected, but none resolve the luminosity problem. On the other hand, we find that a cycle of episodic mass accretion similar to that predicted by recent theoretical work 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 and stellar initial mass function. Finally, the combination of outflow cavities and episodic mass accretion reduces the connection between observational class and physical stage to the point where neither of the two commonly used observational signatures (T bol and L bol/L smm) can be considered reliable indicators of physical stage.

The Spitzer c2d Survey of Nearby Dense Cores. II. Discovery of a Low-Luminosity Object in the “Evolved Starless Core” L1521F

We present Spitzer Space Telescope observations of the evolved starless core L1521F that reveal the presence of a very low luminosity object (L 5μm), but only in scattered light at shorter infrared wavelengths, showing a bipolar nebula oriented east-west that is probably tracing an outflow cavity. The nebula strongly suggests that L1521F-IRS is embedded in the L1521F core. Thus, L1521F-IRS is similar to the recently discovered L1014-IRS and the previously known IRAM 04191 in its substellar luminosity and dense core environment. However, these objects differ significantly in their core density, core chemistry, and outflow properties, and some may be destined to be brown dwarfs rather than stars.

The Spitzer c2d Survey of Nearby Dense Cores. I. First Direct Detection of the Embedded Source in IRAM 04191+1522

We report the first detections of the Class 0 protostellar source IRAM 04191+1522 at wavelengths shortward of 60 μm with the Spitzer Space Telescope. We see extended emission in the Spitzer images that suggests the presence of an outflow cavity in the circumstellar envelope. We combine the Spitzer observations with existing data to form a complete data set ranging from 3.6 to 1300 μm and use these data to construct radiative transfer models of the source. We conclude that the internal luminosity of IRAM 04191+1522, defined to be the sum of the luminosity from the internal sources (a star and a disk), is L_(int) = 0.08 ± 0.04 L_⊙, placing it among the lowest luminosity protostars known. Although it was discovered before the launch of the Spitzer Space Telescope, IRAM 04191+1522 falls within a new class of very low luminosity objects being discovered by Spitzer. Unlike the two other well-studied objects in this class, which are associated either with weak, compact outflows or no outflows at all, IRAM 04191+1522 has a well-defined molecular outflow with properties consistent with those expected based on relations derived from higher luminosity (L_(int) ≥ 1 L_⊙) protostars. We discuss the difficulties in understanding IRAM 04191+1522 in the context of the standard model of star formation and suggest a possible explanation for the very low luminosity of this source.

The Spitzer c2d Survey of Nearby Dense Cores. IV. Revealing the Embedded Cluster in B59

Infrared images of the dark cloud core B59 were obtained with the Spitzer Space Telescope as part of the Cores to Disks Legacy Science project. Photometry from 3.6-70 μm indicates at least 20 candidate low-mass young stars near the core, more than doubling the previously known population. Out of this group, 13 are located within ~0.1 pc in projection of the molecular gas peak, where a new embedded source is detected. Spectral energy distributions span the range from small excesses above photospheric levels to rising in the mid-infrared. One other embedded object, probably associated with the millimeter source B59-MMS1, with a bolometric luminosity L_(bol) ~ 2 L_⊙, has extended structure at 3.6 and 4.5 μm, possibly tracing the edges of an outflow cavity. The measured extinction through the central part of the core is A_V ≳ 45 mag. The B59 core is producing young stars with a high efficiency.

The Detection and Characterization of Centimeter Radio Continuum Emission from the Low-Mass Protostar L1014-IRS

Observations by the Cores to Disk Legacy Team with the Spitzer Space Telescope have identified a low-luminosity, mid-infrared source within the dense core, Lynds 1014, which was previously thought to harbor no internal source. Follow-up near-infrared and submillimeter interferometric observations have confirmed the protostellar nature of this source by detecting scattered light from an outflow cavity and a weak molecular outflow. In this paper, we report the detection of centimeter continuum emission with the VLA. The emission is characterized by a quiescent, unresolved 90 μJy 6 cm source within 0.2 of the Spitzer source. The spectral index of the quiescent component is α = 0.37 ± 0.34 between 6 and 3.6 cm. A factor of 2 increase in 6 cm emission was detected during one epoch and circular polarization was marginally detected at the 5 σ level with Stokes V/I = 48% ± 16%. We have searched for 22 GHz H2O maser emission toward L1014-IRS, but no masers were detected during seven epochs of observations between 2004 June and 2006 December. L1014-IRS appears to be a low-mass, accreting protostar, which exhibits centimeter emission from a thermal jet or a wind, with a variable nonthermal emission component. The quiescent cm radio emission is noticeably above the correlation of 3.6 and 6 cm luminosity versus bolometric luminosity, indicating more radio emission than expected. In this paper, we characterize the centimeter continuum emission in terms of observations of other low-mass protostars, including updated correlations of centimeter continuum emission with bolometric luminosity and outflow force, and discuss the implications of recent larger distance estimates on the physical attributes of the protostar and dense molecular core.

SHARC-II Mapping of Spitzer c2d Small Clouds and Cores

We present the results of a submillimeter survey of 53 low-mass dense cores with the Submillimeter High Angular Resolution Camera II (SHARC-II). The survey is a follow-up project to the Spitzer Legacy Program From Molecular Cores to Planet-Forming Disks, with the purpose of creating a complete data set of nearby low-mass dense cores from the infrared to the millimeter. We present maps of 52 cores at 350 μm and three cores at 450 μm, two of which were observed at both wavelengths. Of these 52 cores, 41 were detected by SHARC-II; 32 contained one submillimeter source, while 9 contained multiple sources. For each submillimeter source detected, we report various source properties including source position, fluxes in various apertures, size, aspect ratio, and position angle. For the 12 cores that were not detected we present upper limits. The sources detected by SHARC-II have, on average, smaller sizes at the 2 σ contours than those derived from longer wavelength bolometer observations. We conclude that this is not caused by a failure to integrate long enough to detect the full extent of the core; instead it arises primarily from the fact that the observations presented in this survey are insensitive to smoothly varying extended emission. We find that SHARC-II observations of low-mass cores are much better suited to distinguishing between starless and protostellar cores than observations at longer wavelengths. Very low luminosity objects, a new class of objects being discovered by the Spitzer Space Telescope in cores previously classified as starless, look very similar at 350 μm to other cores with more luminous protostars.

Submillimeter Common-User Bolometer Array Mapping of Spitzer c2d Small Clouds and Cores

We present submillimeter observations of dark clouds that are part of the Spitzer Legacy Program, From Molecular Cores to Planet-Forming Disks (c2d). We used the Submillimetre Common Users Bolometer Array to map the regions observed by Spitzer by the c2d program to create a census of dense molecular cores including data from the infrared to the submillimeter. In this paper, we – 2 – present the basic data from these observations: maps, fluxes, and source attributes. We also show data for an object just outside the Perseus cloud that was serendipitously observed in our program. We propose that this object is a newly discovered, evolved protostar. Finally, these data are publicly available atWe present submillimeter observations of dark clouds that are part of the Spitzer Legacy program From Molecular Cores to Planet-Forming Disks (c2d). We used the Submillimeter Common-User Bolometer Array to map the regions observed by Spitzer with the c2d program to create a census of dense molecular cores including data from the infrared to the submillimeter. In this paper we present the basic data from these observations: maps, fluxes, and source attributes. We also show data for an object just outside the Perseus cloud that was serendipitously observed in our program. We propose that this object is a newly discovered evolved protostar.

THE SPITZER c2d SURVEY OF LARGE, NEARBY, INTERSTELLAR CLOUDS. XII. THE PERSEUS YSO POPULATION AS OBSERVED WITH IRAC AND MIPS

The Spitzer Space Telescope mapped the Perseus molecular cloud complex with IRAC and MIPS as part of the c2d Spitzer Legacy project. This paper combines the observations from both instruments giving an overview of low-mass star formation across Perseus from 3.6 to 70 micron. We provide an updated list of young stellar objects with new classifications and source fluxes from previous works, identifying 369 YSOs in Perseus with the Spitzer dataset. By synthesizing the IRAC and MIPS maps of Perseus and building on the work of previous papers in this series (Jorgensen et al. 2006, Rebull et al. 2007), we present a current census of star formation across the cloud and within smaller regions. 67% of the YSOs are associated with the young clusters NGC 1333 and IC 348. The majority of the star formation activity in Perseus occurs in the regions around the clusters, to the eastern and western ends of the cloud complex. The middle of the cloud is nearly empty of YSOs despite containing regions of high visual extinction. The western half of Perseus contains three-quarters of the total number of embedded YSOs (Class 0+I and Flat SED sources) in the cloud and nearly as many embedded YSOs as Class II and III sources. Class II and III greatly outnumber Class 0+I objects in eastern Perseus and IC 348. These results are consistent with previous age estimates for the clusters. Across the cloud, 56% of YSOs and 91% of the Class 0+I and Flat sources are in areas where Av > 5 mag, indicating a possible extinction threshold for star formation.

THE SPITZER c2d SURVEY OF NEARBY DENSE CORES. VI. THE PROTOSTARS OF LYNDS DARK NEBULA 1221

Observations of Lynds Dark Nebula 1221 from the Spitzer Space Telescope are presented. These data show three candidate protostars toward L1221, only two of which were previously known. The infrared observations also show signatures of outflowing material, an interpretation which is also supported by radio observations with the Very Large Array. In addition, molecular line maps from the Five College Radio Astronomy Observatory are shown. One-dimensional dust continuum modeling of two of these protostars, IRS1 and IRS3, is described. These models show two distinctly different protostars forming in very similar environments. IRS1 shows a higher luminosity and a larger inner radius of the envelope than IRS3. The disparity could be caused by a difference in age or mass, orientation of outflow cavities, or the impact of a binary in the IRS1 core.