Cluster Formation in Protostellar Outflow-Driven Turbulence

Most, perhaps all, stars go through a phase of vigorous outflow during formation. We examine, through 3D MHD simulation, the effects of protostellar outflows on cluster formation. We find that the initial turbulence in the cluster-forming region is quickly replaced by motions generated by outflows. The protostellar outflow-driven turbulence (``protostellar turbulence'' for short) can keep the region close to a virial equilibrium long after the initial turbulence has decayed away. We argue that there exist two types of turbulence in star-forming clouds: a primordial (or ``interstellar'') turbulence and a protostellar turbulence, with the former transformed into the latter mostly in embedded clusters such as NGC 1333. Since the majority of stars are thought to form in clusters, an implication is that the stellar initial mass function is determined to a large extent by the stars themselves, through outflows which individually limit the mass accretion onto forming stars and collectively shape the environments (density structure and velocity field) in which most cluster members form. We speculate that massive cluster-forming clumps supported by protostellar turbulence gradually evolve towards a highly centrally condensed ``pivotal'' state, culminating in rapid formation of massive stars in the densest part through accretion.Comment: 11 pages (aastex format), 2 figures submitted to ApJ

Multiple protostellar systems. II. A high resolution near-infrared imaging survey in nearby star-forming regions

(abridged) Our project endeavors to obtain a robust view of multiplicity among embedded Class I and Flat Spectrum protostars in a wide array of nearby molecular clouds to disentangle ``universal'' from cloud-dependent processes. We have used near-infrared adaptive optics observations at the VLT through the H, Ks and L' filters to search for tight companions to 45 Class I and Flat Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga, Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with published high-resolution surveys of 13 additional objects in Taurus and Ophiuchus. We found multiplicity rates of 32+/-6% and 47+/-8% over the 45-1400 AU and 14-1400 AU separation ranges, respectively. These rates are in excellent agreement with those previously found among T Tauri stars in Taurus and Ophiuchus, and represent an excess of a factor ~1.7 over the multiplicity rate of solar-type field stars. We found no non-hierarchical triple systems, nor any quadruple or higher-order systems. No significant cloud-to-cloud difference has been found, except for the fact that all companions to low-mass Orion protostars are found within 100 AU of their primaries whereas companions found in other clouds span the whole range probed here. Based on this survey, we conclude that core fragmentation always yields a high initial multiplicity rate, even in giant molecular clouds such as the Orion cloud or in clustered stellar populations as in Serpens, in contrast with predictions of numerical simulations. The lower multiplicity rate observed in clustered Class II and Class III populations can be accounted for by a universal set of properties for young systems and subsequent ejections through close encounters with unrelated cluster members.Comment: 15 pages, 6 figures, accepted for publication in Astronomy & Astrophysic

A multi-wavelength census of star formation activity in the young embedded cluster around Serpens/G3-G6

Aims. The aim of this paper is to characterise the star formation activity in the poorly studied embedded cluster Serpens/G3-G6, located ~ 45' (3 pc) to the south of the Serpens Cloud Core, and to determine the luminosity and mass functions of its population of Young Stellar Objects (YSOs). Methods. Multi-wavelength broadband photometry was obtained to sample the near and mid-IR spectral energy distributions to separate YSOs from field stars and classify the YSO evolutionary stage. ISOCAM mapping in the two filters LW2 (5-8.5 um) and LW3 (12-18 um) of a 19' x 16' field was combined with JHKs data from 2MASS, Ks data from Arnica/NOT, and L' data from SIRCA/NOT. Continuum emission at 1.3 mm (IRAM) and 3.6 cm (VLA) was mapped to study the cloud structure and the coldest/youngest sources. Deep narrow band imaging at the 2.12 um S(1) line of H2 from NOTCam/NOT was obtained to search for signs of bipolar outflows. Results. We have strong evidence for a stellar population of 31 Class II sources, 5 flat-spectrum sources, 5 Class I sources, and two Class 0 sources. Our method does not sample the Class III sources. The cloud is composed of two main dense clumps aligned along a ridge over ~ 0.5 pc plus a starless core coinciding with absorption features seen in the ISOCAM maps. We find two S-shaped bipolar collimated flows embedded in the NE clump, and propose the two driving sources to be a Class 0 candidate (MMS3) and a double Class I (MMS2). For the Class II population we find a best age of ~ 2 Myr and compatibility with recent Initial Mass Functions (IMFs) by comparing the observed Class II luminosity function (LF), which is complete to 0.08 L_sun, to various model LFs with different star formation scenarios and input IMFs.Comment: 18 pages, 16 figures, 3 online tables, accepted by A&

The Detection and Characterization of cm 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. Followup 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 cm continuum emission with the VLA. The emission is characterized by a quiescent, unresolved 90 uJy 6 cm source within 0.2" of the Spitzer source. The spectral index of the quiescent component is $\alpha = 0.37\pm 0.34$ between 6 cm and 3.6 cm. A factor of two increase in 6 cm emission was detected during one epoch and circular polarization was marginally detected at the $5\sigma$ level with Stokes {V/I} $= 48 \pm 16$% . We have searched for 22 GHz H2O maser emission toward L1014-IRS, but no masers were detected during 7 epochs of observations between June 2004 and December 2006. L1014-IRS appears to be a low-mass, accreting protostar which exhibits cm emission from a thermal jet or a wind, with a variable non-thermal emission component. The quiescent cm radio emission is noticeably above the correlation of 3.6 cm and 6 cm luminosity versus bolometric luminosity, indicating more radio emission than expected. We characterize the cm 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.Comment: 14 pages. Accepted for publication in Ap

Star formation triggered by the Galactic HII region RCW 120: First results from the Herschel Space Observatory

By means of different physical mechanisms, the expansion of HII regions can promote the formation of new stars of all masses. RCW 120 is a nearby Galactic HII region where triggered star formation occurs. This region is well-studied - there being a wealth of existing data - and is nearby. However, it is surrounded by dense regions for which far infrared data is essential to obtain an unbiased view of the star formation process and in particular to establish whether very young protostars are present. We attempt to identify all Young Stellar Objects (YSOs), especially those previously undetected at shorter wavelengths, to derive their physical properties and obtain insight into the star formation history in this region. We use Herschel-PACS and -SPIRE images to determine the distribution of YSOs observed in the field. We use a spectral energy distribution fitting tool to derive the YSOs physical properties. Herschel-PACS and -SPIRE images confirm the existence of a young source and allow us to determine its nature as a high-mass (8-10 MSun) Class 0 object (whose emission is dominated by a massive envelope) towards the massive condensation 1 observed at (sub)-millimeter wavelengths. This source was not detected at 24 micron and only barely seen in the MISPGAL 70 micron data. Several other red sources are detected at Herschel wavelengths and coincide with the peaks of the millimeter condensations. SED fitting results for the brightest Herschel sources indicate that, apart from the massive Class 0 that forms in condensation 1, young low mass stars are forming around RCW 120. The YSOs observed on the borders of RCW 120 are younger than its ionizing star, which has an age of about 2.5 Myr.Comment: 5 pqges, 3 figures, accepted by A&A (Special issue on the Herschel first results

Asymptotic behavior of the density of states on a random lattice

We study the diffusion of a particle on a random lattice with fluctuating local connectivity of average value q. This model is a basic description of relaxation processes in random media with geometrical defects. We analyze here the asymptotic behavior of the eigenvalue distribution for the Laplacian operator. We found that the localized states outside the mobility band and observed by Biroli and Monasson (1999, J. Phys. A: Math. Gen. 32 L255), in a previous numerical analysis, are described by saddle point solutions that breaks the rotational symmetry of the main action in the real space. The density of states is characterized asymptotically by a series of peaks with periodicity 1/q.Comment: 11 pages, 2 figure

Giant Molecular Outflows Powered by Protostars in L1448

We present sensitive, large-scale maps of the CO J=1-0 emission of the L1448 dark cloud. These maps were acquired using the On-The-Fly capability of the NRAO 12-meter telescope. CO outflow activity is seen in L1448 on parsec-scales for the first time. Careful comparison of the spatial and velocity distribution of our high-velocity CO maps with previously published optical and near-infrared images and spectra has led to the identification of six distinct CO outflows. We show the direct link between the heretofore unknown, giant, highly-collimated, protostellar molecular outflows and their previously discovered, distant optical manifestations. The outflows traced by our CO mapping generally reach the projected cloud boundaries. Integrated intensity maps over narrow velocity intervals indicate there is significant overlap of blue- and red-shifted gas, suggesting the outflows are highly inclined with respect to the line-of-sight, although the individual outflow position angles are significantly different. The velocity channel maps also show that the outflows dominate the CO line cores as well as the high-velocity wings. The magnitude of the combined flow momenta, as well as the combined kinetic energy of the flows, are sufficient to disperse the 50 solar mass NH3 cores in which the protostars are currently forming, although some question remains as to the exact processes involved in redirecting the directionality of the outflow momenta to effect the complete dispersal of the parent cloud.Comment: 11 pages, 9 figures, to be published in the Astronomical Journa

Infall, Outflow, Rotation, and Turbulent Motions of Dense Gas within NGC 1333 IRAS 4

Millimeter wavelength observations are presented of NGC 1333 IRAS 4, a group of highly-embedded young stellar objects in Perseus, that reveal motions of infall, outflow, rotation, and turbulence in the dense gas around its two brightest continuum objects, 4A and 4B. These data have finest angular resolution of approximately 2" (0.0034 pc) and finest velocity resolution of 0.13 km/s. Infall motions are seen from inverse P-Cygni profiles observed in H2CO 3_12-2_11 toward both objects, but also in CS 3-2 and N2H+ 1-0 toward 4A, providing the least ambiguous evidence for such motions toward low-mass protostellar objects. Outflow motions are probed by bright line wings of H2CO 3_12-2_11 and CS 3-2 observed at positions offset from 4A and 4B, likely tracing dense cavity walls. Rotational motions of dense gas are traced by a systematic variation of the N2H+ line velocities, and such variations are found around 4A but not around 4B. Turbulent motions appear reduced with scale, given N2H+ line widths around both 4A and 4B that are narrower by factors of 2 or 3 than those seen from single-dish observations. Minimum observed line widths of approximately 0.2 km/s provide a new low, upper bound to the velocity dispersion of the parent core to IRAS 4, and demonstrate that turbulence within regions of clustered star formation can be reduced significantly. A third continuum object in the region, 4B', shows no detectable line emission in any of the observed molecular species.Comment: LateX, 51 pages, 9 figures, accepted by Ap

Molecular hydrogen jets and outflows in the Serpens south filamentary cloud

We aimed to map the jets and outflows from the Serpens South star forming region and find an empirical relationship between the magnetic field and outflow orientation. Near-infrared H2 v=1-0 S(1) 2.122{\mu}m -line imaging of the \sim 30'-long filamentary shaped Serpens South star forming region was carried out. K s broadband imaging of the same region was used for continuum subraction. Candidate driving sources of the mapped jets/outflows are identified from the list of known protostars and young stars in this region, which was derived from studies using recent Spitzer and Herschel telescope observations. 14 Molecular Hydrogen emission-line objects(MHOs) are identified using our continuum-subtracted images. They are found to constitute ten individual flows. Out of these, nine flows are located in the lower-half(southern) part of the Serpens South filament, and one flow is located at the northern tip of the filament. Four flows are driven by well-identified Class 0 protostars, while the remaining six flows are driven by candidate protostars mostly in the Class I stage, based on the Spitzer and Herschel observations. The orientation of the outflows is systematically perpendicular to the direction of the near-infrared polarization vector, recently published in the literature. No significant correlation was observed between the orientation of the flows and the axis of the filamentary cloud.Comment: Accepted by A&A for publication. 7 pages, 5 figure