Evolutionary models for low-mass stars and brown dwarfs: uncertainties and limits at very young ages

We analyse pre-Main Sequence evolutionary tracks for low mass stars with masses m \le 1.4 \msol based on the Baraffe et al. (1998) input physics. We also extend the recent Chabrier et al. (2000) evolutionary models based on dusty atmosphere to young brown dwarfs down to one mass of Jupiter. We analyse current theoretical uncertainties due to molecular line lists, convection and initial conditions. Simple tests on initial conditions show the high uncertainties of models at ages \simle 1 Myr. We find a significant sensitivity of atmosphere profiles to the treatment of convection at low gravity and \te < 4000 K, whereas it vanishes as gravity increases. This effect adds another source of uncertainty on evolutionary tracks at very early phases. We show that at low surface gravity (\log g \simle 3.5,) the common picture of vertical Hayashi lines with constant \te is oversimplified. The effect of a variation of initial deuterium abundance is studied. We compare our models with evolutionary tracks available in the literature and discuss the main differences. We finally analyse to which extent current observations of young systems provide a good test for pre-Main Sequence tracks.Comment: 12 pages, Latex file, uses aa.cls, accepted for publication in A&

Sub-arcsec imaging of the AB Aur molecular disk and envelope at millimeter wavelengths: a non Keplerian disk

We present sub-arcsecond images of AB Auriga obtained with the IRAM Plateau de Bure interferometer in the isotopologues of CO, and in continuum at 3 and 1.3 mm. Instead of being centrally peaked, the continuum emission is dominated by a bright, asymmetric (spiral-like) feature at about 140 AU from the central star. The large scale molecular structure suggests the AB Aur disk is inclined between 23 and 43 degrees, but the strong asymmetry of the continuum and molecular emission prevents an accurate determination of the inclination of the inner parts. We find significant non-Keplerian motion, with a best fit exponent for the rotation velocity law of 0.41 +/- 0.01, but no evidence for radial motions. The disk has an inner hole about 70 AU in radius. The disk is warm and shows no evidence of depletion of CO. The dust properties suggest the dust is less evolved than in typical T Tauri disks. Both the spiral-like feature and the departure from purely Keplerian motions indicates the AB Aur disk is not in quasi-equilibrium. Disk self-gravity is insufficient to create the perturbation. This behavior may be related either to an early phase of star formation in which the Keplerian regime is not yet fully established and/or to a disturbance of yet unknown origin. An alternate, but unproven, possibility is that of a low mass companion located about 40 AU from AB Aur.Comment: 10 pages, 5 figures, accepted for publication in Astronomy & Astrophysic

A detailed study of the accretion disk surrounding the high-mass protostar NGC 7538S

We present deep high angular resolution observations of the high-mass protostar NGC 7538S, which is in the center of a cold dense cloud core with a radius of 0.5 pc and a mass of ~2,000 Msun. These observations show that NGC 7538S is embedded in a compact elliptical core with a mass of 85 - 115 Msun. The star is surrounded by a rotating accretion disk, which powers a very young, hot molecular outflow approximately perpendicular to the rotating accretion disk. The accretion rate is very high, ~ 1.4 - 2.8 10^-3 Msun yr^-1. Evidence for rotation of the disk surrounding the star is seen in all largely optically thin molecular tracers, H13CN J = 1-0, HN13C J = 1-0, H13CO+ J = 1-0, and DCN J = 3-2. Many molecules appear to be affected by the hot molecular outflow, including DCN and H13CO+. The emission from CH3CN, which has often been used to trace disk rotation in young high-mass stars, is dominated by the outflow, especially at higher K-levels. Our new high-angular resolution observations show that the rotationally supported part of the disk is smaller than we previously estimated. The enclosed mass of the inner, rotationally supported part of the disk (D ~ 5", i.e 14,000 AU) is ~ 14 - 24 Msun.Comment: Accepted by ApJ; 20 pages, 20 figure

Variable Accretion Rates and Fluffy First Stars

We combine the output of hydrodynamical simulations of Population III star cluster formation with stellar evolution models, and calculate the evolution of protostars experiencing variable mass accretion rates due to interactions within a massive disk. We find that the primordial protostars are extended 'fluffy' objects for the bulk of their pre-main-sequence lifetimes. Accretion luminosity feedback from such objects is high, but as shown in previous work, has a minimal effect on the star cluster. The extended radii of the protostars, combined with the observation of close encounters in the simulations, suggests that mergers will occur in such systems. Furthermore, mass transfer between close protostellar binaries with extended radii could lead to massive tight binaries, which are a possible progenitor of gamma ray bursts.Comment: 7 pages, 6 figures, 2 tables. To be published in MNRA

The Highly Eccentric Pre-Main Sequence Spectroscopic Binary RX J0529.3+1210

The young system RX J0529.3+1210 was initially identified as a single-lined spectroscopic binary. Using high-resolution infrared spectra, acquired with NIRSPEC on Keck II, we measured radial velocities for the secondary. The method of using the infrared regime to convert single-lined spectra into double-lined spectra, and derive the mass ratio for the binary system, has been successfully used for a number of young, low-mass binaries. For RX J0529.3+1210, a long- period(462 days) and highly eccentric(0.88) binary system, we determine the mass ratio to be 0.78+/-0.05 using the infrared double-lined velocity data alone, and 0.73+/-0.23 combining visible light and infrared data in a full orbital solution. The large uncertainty in the latter is the result of the sparse sampling in the infrared and the high eccentricity: the stars do not have a large velocity separation during most of their ~1.3 year orbit. A mass ratio close to unity, consistent with the high end of the one sigma uncertainty for this mass ratio value, is inconsistent with the lack of a visible light detection of the secondary component. We outline several scenarios for a color difference in the two stars, such as one heavily spotted component, higher order multiplicity, or a unique evolutionary stage, favoring detection of only the primary star in visible light, even in a mass ratio ~1 system. However, the evidence points to a lower ratio. Although RX J0529.3+1210 exhibits no excess at near-infrared wavelengths, a small 24 micron excess is detected, consistent with circumbinary dust. The properties of this binary and its membership in Lambda Ori versus a new nearby stellar moving group at ~90 pc are discussed. We speculate on the origin of this unusual system and on the impact of such high eccentricity on the potential for planet formation.Comment: 4 Figure

Star Formation in Space and Time: Taurus-Auriga

To understand the formation of stellar groups, one must first document carefully the birth pattern within real clusters and associations. In this study of Taurus-Auriga, we combine pre-main-sequence ages from our own evolutionary tracks with stellar positions from observational surveys. Aided by the extensive, millimeter data on the molecular clouds, we develop a picture of the region's history. Star formation began, at a relatively low level and in a spatially diffuse manner, at least 10 Myr in the past. Within the last few million years, new stars have been produced at an accelerating rate, almost exclusively within a confined group of striated cloud filaments. The gas both inside and around the filaments appears to be in force balance. Thus, the appearance of the filaments is due to global, quasi-static contraction of the parent cloud material. Gravity drives this contraction and shock dissipation mediates it, but the internal motion of the gas does not appear to be turbulent. The accelerating nature of recent star formation means that the condensation of cloud cores is a threshold phenomenon, requiring a minimum background density. Other, nearby cloud regions, including Lupus and Chamaeleon, contain some locales that have attained this density, and others that have not. In the latter, we find extensive and sometimes massive molecular gas that is still devoid of young stars.Comment: 19 pages, 7 figures, to be published in ApJ - December 20, 200

Spitzer View of Young Massive Stars in the LMC HII Complex N44

The HII complex N44 in the Large Magellanic Cloud (LMC) provides an excellent site to perform a detailed study of star formation in a mild starburst, as it hosts three regions of star formation at different evolutionary stages and it is not as complicated and confusing as the 30 Doradus giant HII region. We have obtained Spitzer Space Telescope observations and complementary ground-based 4m uBVIJK observations of N44 to identify candidate massive young stellar objects (YSOs). We further classify the YSOs into Types I, II, and III, according to their spectral energy distributions (SEDs). In our sample of 60 YSO candidates, ~65% of them are resolved into multiple components or extended sources in high-resolution ground-based images. We have modeled the SEDs of 36 YSOs that appear single or dominant within a group. We find good fits for Types I and I/II YSOs,but Types II and II/III YSOs show deviations between their observed SEDs and models that do not include PAH emission. We have also found that some Type III YSOs have central holes in their disk components. YSO counterparts are found in four ultracompact HII regions and their stellar masses determined from SED model fits agree well with those estimated from the ionization requirements of the HII regions. The distribution of YSOs is compared with those of the underlying stellar population and interstellar gas conditions to illustrate a correlation between the current formation of O-type stars and previous formation of massive stars. Evidence of triggered star formation is also presented.Comment: 63 pages, 15 figures, accepted for publication for ApJ; some figures in jpeg format to meet the size limit; preprint with high resolution images can be found at http://www.astro.virginia.edu/~cc5ye/n44yso.pd

Hubble Space Telescope NICMOS Polarization Observations of Three Edge-on Massive YSOs

Massive young stellar objects (YSOs), like low-mass YSOs, appear to be surrounded by optically thick envelopes and/or disks and have regions, often bipolar, that are seen in polarized scattered light at near-infrared wavelengths. We are using the 0.2'' spatial resolution of NICMOS on Hubble Space Telescope to examine the structure of the disks and outflow regions of massive YSOs in star-forming regions within a few kpc of the Sun. Here we report on 2 micron polarimetry of NGC 6334 V and S255 IRS1. NGC 6334 V consists of a double-lobed bright reflection nebula seen against a dark region, probably an optically thick molecular cloud. Our polarization measurements show that the illuminating star lies ~ 2'' south of the line connecting the two lobes; we do not detect this star at 2 micron, but there are a small radio source and a mid-infrared source at this location. S255 IRS1 consists of two YSOs (NIRS1 and NIRS3) with overlapping scattered light lobes and luminosities corresponding to early B stars. Included in IRS1 is a cluster of stars from whose polarization we determine the local magnetic field direction. Neither YSO has its scattered light lobes aligned with this magnetic field. The line connecting the scattered light lobes of NIRS1 is twisted symmetrically around the star; the best explanation is that the star is part of a close binary and the outflow axis of NIRS1 is precessing as a result of non-coplanar disk and orbit. The star NIRS3 is also offset from the line connecting its two scattered light lobes. We suggest that all three YSOs show evidence of episodic ejection of material as they accrete from dense, optically thick envelopes.Comment: 39 pages, 7 figures, 4 tables To be published in The Astrophysical Journa

The Effects of Accretion Luminosity upon Fragmentation in the Early Universe

We introduce a prescription for the luminosity from accreting protostars into smoothed particle hydrodynamics simulation, and apply the method to simulations of five primordial minihalos generated from cosmological initial conditions. We find that accretion luminosity delays fragmentation within the halos, but does not prevent it. In halos that slowly form a low number of protostars, the accretion luminosity can reduce the number of fragments that are formed before the protostars start ionising their surroundings. However, halos that rapidly form many protostars become dominated by dynamical processes, and the effect of accretion luminosity becomes negligible. Generally the fragmentation found in the halos is highly dependent on the initial conditions. Accretion luminosity does not substantially affect the accretion rates experienced by the protostars, and is far less important than dynamical interactions, which can lead to ejections that effectively terminate the accretion. We find that the accretion rates onto the inner regions of the disks (20 AU) around the protostars are highly variable, in contrast to the constant or smoothly decreasing accretion rates currently used in models of the pre-main sequence evolution of Population III stars.Comment: 12 pages, 10 figures and 3 tables. Accepted by MNRA

The BLAST Survey of the Vela Molecular Cloud: Physical Properties of the Dense Cores in Vela-D

The Balloon-borne Large-Aperture Submillimeter Telescope (BLAST) carried out a 250, 350 and 500 micron survey of the galactic plane encompassing the Vela Molecular Ridge, with the primary goal of identifying the coldest dense cores possibly associated with the earliest stages of star formation. Here we present the results from observations of the Vela-D region, covering about 4 square degrees, in which we find 141 BLAST cores. We exploit existing data taken with the Spitzer MIPS, IRAC and SEST-SIMBA instruments to constrain their (single-temperature) spectral energy distributions, assuming a dust emissivity index beta = 2.0. This combination of data allows us to determine the temperature, luminosity and mass of each BLAST core, and also enables us to separate starless from proto-stellar sources. We also analyze the effects that the uncertainties on the derived physical parameters of the individual sources have on the overall physical properties of starless and proto-stellar cores, and we find that there appear to be a smooth transition from the pre- to the proto-stellar phase. In particular, for proto-stellar cores we find a correlation between the MIPS24 flux, associated with the central protostar, and the temperature of the dust envelope. We also find that the core mass function of the Vela-D cores has a slope consistent with other similar (sub)millimeter surveys.Comment: Accepted for publication in the Astrophysical Journal. Data and maps are available at http://blastexperiment.info