Classes Of Young Stellar Objects Research Articles

The environment around young eruptive stars. SPHERE/IRDIS polarimetric imaging of seven protostars

Eruptive stars are a class of young stellar objects that show an abrupt increase in luminosity. These burst-like episodes are thought to dominate the stellar accretion process during the class 0 to class I stage. We present an overview of a survey of seven episodically accreting protostars carried out to study their potentially complex circumstellar surroundings. The observations were performed with the instrument SPHERE, mounted at the Very Large Telescope. SPHERE is equipped with an extreme adaptive optics system that allows high-contrast imaging. We observed the eruptive stars in the $H$ band with the near-infrared imager IRDIS and used the polarimeter to extract the polarized light scattered from the stars' surroundings. We produced polarized light images for three FUor objects, Z CMa, V960 Mon, and FU Ori, and four EXor objects, XZ Tau, UZ Tau, NY Ori, and EX Lup. We calculated the intrinsic polarization fraction for all the observed stars. In all systems we registered scattered light from around the primary star. FU Ori and V960 Mon are surrounded by complex structures, including spiral-like features. In Z CMa, we detected a point source 0 to the northeast of the primary. Based on the astrometric measurements from archival Keck/NIRC2 data, we find this source to be a third member of the system. Furthermore, Z CMa displays an outflow that extends for thousands of au. Unlike the other EXor objects in our sample, XZ Tau shows bright, extended scattered light structures that are also associated with an outflow on a scale of hundreds of au. The other EXors show relatively faint disk-like structures in the immediate vicinity of the coronagraph. Each object shows a unique environment, but we classified the seven objects into three categories: systems with illuminated outflows, asymmetric arms, and faint disks. Asymmetric arms were only found around FUor objects, while faint disks seem to predominantly occur around EXors. Importantly, for Z CMa the detection of the faint extended structure calls into question previous interpretations of the system's dynamic state. The streamer that was associated with a fly-by object turned out to be part of a huge outflow extending 6000 au.

CHARACTERIZING THE POPULATION OF BRIGHT INFRARED SOURCES IN THE SMALL MAGELLANIC CLOUD

ABSTRACT We have used the Infrared Spectrograph (IRS) on the Spitzer Space Telescope to observe stars in the Small Magellanic Cloud (SMC) selected from the Point Source Catalog of the Midcourse Space Experiment (MSX). We concentrate on the dust properties of the oxygen-rich evolved stars. The dust composition has smaller contributions from alumina compared to the Galaxy. This difference may arise from the lower metallicity in the SMC, but it could be a selection effect, as the SMC sample includes more stars that are brighter and thus more massive. The distribution of the SMC stars along the silicate sequence looks more like the Galactic sample of red supergiants than asymptotic giant branch stars (AGBs). While many of the SMC stars are definitively on the AGB, several also show evidence of hot bottom burning. Three of the supergiants show PAH emission at 11.3 μm. Two other sources show mixed chemistry, with both carbon-rich and oxygen-rich spectral features. One, MSX SMC 134, may be the first confirmed silicate/carbon star in the SMC. The other, MSX SMC 049, is a candidate post-AGB star. MSX SMC 145, previously considered a candidate OH/IR star, is actually an AGB star with a background galaxy at z = 0.16 along the same line of sight. We consider the overall characteristics of all the MSX sources, the most infrared-bright objects in the SMC, in light of the higher sensitivity and resolution of Spitzer, and compare them with the object types expected from the original selection criteria. This population represents what will be seen in more distant galaxies by the upcoming James Webb Space Telescope (JWST). Color–color diagrams generated from the IRS spectra and the mid-infrared filters on JWST show how one can separate evolved stars from young stellar objects (YSOs) and distinguish among different classes of YSOs.

EVOLUTION OF MASS OUTFLOW IN PROTOSTARS

ABSTRACT We have surveyed 84 Class 0, Class I, and flat-spectrum protostars in mid-infrared [Si ii], [Fe ii], and [S i] line emission, and 11 of these in far-infrared [O i] emission. We use the results to derive their mass outflow rates, . Thereby we observe a strong correlation of with bolometric luminosity, and with the inferred mass accretion rates of the central objects, , which continues through the Class 0 range the trend observed in Class II young stellar objects. Along this trend from large to small mass flow rates, the different classes of young stellar objects lie in the sequence Class 0–Class I/flat-spectrum–Class II, indicating that the trend is an evolutionary sequence in which and decrease together with increasing age, while maintaining rough proportionality. The survey results include two that are key tests of magnetocentrifugal outflow-acceleration mechanisms: the distribution of the outflow/accretion branching ratio , and limits on the distribution of outflow speeds. Neither rules out any of the three leading outflow-acceleration, angular-momentum-ejection mechanisms, but they provide some evidence that disk winds and accretion-powered stellar winds (APSWs) operate in many protostars. An upper edge observed in the branching-ratio distribution is consistent with the upper bound of b = 0.6 found in models of APSWs, and a large fraction (31%) of the sample have a branching ratio sufficiently small that only disk winds, launched on scales as large as several au, have been demonstrated to account for them.

CSI 2264: CHARACTERIZING YOUNG STARS IN NGC 2264 WITH SHORT-DURATION PERIODIC FLUX DIPS IN THEIR LIGHT CURVES

We identify nine young stellar objects (YSOs) in the NGC 2264 star-forming region with optical {\em CoRoT} light curves exhibiting short-duration, shallow, periodic flux dips. All of these stars have infrared (IR) excesses that are consistent with their having inner disk walls near the Keplerian co-rotation radius. The repeating photometric dips have FWHM generally less than one day, depths almost always less than 15%, and periods (3<P<11 days) consistent with dust near the Keplerian co-rotation period. The flux dips vary considerably in their depth from epoch to epoch, but usually persist for several weeks and, in two cases, were present in data collected on successive years. For several of these stars, we also measure the photospheric rotation period and find that the rotation and dip periods are the same, as predicted by standard "disk-locking" models. We attribute these flux dips to clumps of material in or near the inner disk wall, passing through our line of sight to the stellar photosphere. In some cases, these dips are also present in simultaneous {\em Spitzer} IRAC light curves at 3.6 and 4.5 microns. We characterize the properties of these dips, and compare the stars with light curves exhibiting this behavior to other classes of YSO in NGC 2264. A number of physical mechanisms could locally increase the dust scale height near the inner disk wall, and we discuss several of those mechanisms; the most plausible mechanisms are either a disk warp due to interaction with the stellar magnetic field or dust entrained in funnel-flow accretion columns arising near the inner disk wall.

Can we expect the massive discs around young stellar objects class O-I to be a birthplace of planetesimal?

AbstractPossibility of large bodies formation in massive discs of young stellar objects (YSO) class O-I was investigated. On the stage of YSO O-I the whole of factors: chemical composition of gas and solids, chemical catalytic reactions, the disc self-gravitation, the increased ratio of solids to gas surface density, adiabatic gas cooling provides favorable conditions for gravitational instabilities development. We simulated 3D dynamics of gas and dust under self-consistent gravitational field and reproduced the formation and evolution of the disc around the protostar. We found that for stars of Solar mass there are regimes when the disc of variable mass is unstable for the development of fast gravitational instabilities.

POISSON project

Characterising stellar and circumstellar properties of embedded young stellar objects (YSOs) is mandatory for understanding the early stages of the stellar evolution. This task requires the combination of both spectroscopy and photometry, covering the widest possible wavelength range, to disentangle the various protostellar components and activities. As part of the POISSON project, we present a multi-wavelength spectroscopic and photometric investigation of embedded YSOs in L1641, aimed to derive the stellar parameters and evolutionary stages and to infer their accretion properties. Our database includes low-resolution optical-IR spectra from the NTT and Spitzer (0.6-40 um) and photometric data covering a spectral range from 0.4 to 1100 um, which allow us to construct the YSOs spectral energy distributions (SEDs) and to infer the main stellar parameters. The SED analysis allows us to group our 27 YSOs into nine Class I, eleven Flat, and seven Class II objects. However, on the basis of the derived stellar properties, only six Class I YSOs have an age of ~10^5 yr, while the others are older 5x10^5-10^6 yr), and, among the Flat sources, three out of eleven are more evolved objects (5x10^6-10^7 yr), indicating that geometrical effects can significantly modify the SED shapes. Inferred mass accretion rates (Macc) show a wide range of values (3.6x10^-9 to 1.2x10^-5 M_sun yr^-1), which reflects the age spread observed in our sample. Average values of mass accretion rates, extinction, and spectral indices decrease with the YSO class. The youngest YSOs have the highest Macc, whereas the oldest YSOs do not show any detectable jet activity in either images and spectra. We also observe a clear correlation among the YSO Macc, M*, and age, consistent with mass accretion evolution in viscous disc models.

Results from DROXO

X-rays from very young stars are powerful probes to investigate the mechanisms at work in the very first stages of the star formation and the origin of X-ray emission in very young stars. We present results from a 500 ks long observation of the Rho Ophiuchi cloud with a XMM-Newton large program named DROXO, aiming at studying the X-ray emission of deeply embedded Young Stellar Objects (YSOs). The data acquired during the DROXO program were reduced with SAS software, and filtered in time and energy to improve the signal to noise of detected sources; light curves and spectra were obtained. We detected 111 sources, 61 of them associated with rho Ophiuchi YSOs as identified from infrared observations with ISOCAM. Specifically, we detected 9 out of 11 Class I, 31 out of 48 Class II and 15 out 16 Class III objects. Six objects out of 21 classified Class III candidates are also detected. At the same time we suggest that 15 Class III candidates that remain undetected at log Lx < 28.3 are not related to the cloud population. The global detection rate is ~64%. We have achieved a flux sensitivity of ~5 x 10^{-15} erg s^{-1} cm^{-2}. The Lx to L_bol ratio shows saturation at a value of ~ -3.5 for stars with T_eff <= 5000 K or 0.7 M_sun as observed in the Orion Nebula. The plasma temperatures and the spectrum absorption show a decline with YSO class, with Class I YSOs being hotter and more absorbed than Class II and III YSOs. In one star (GY 266) with infrared counterpart in 2MASS and Spitzer catalogs we have detected a soft excess in the X-ray spectrum which is best fitted by a cold thermal component less absorbed than the main thermal component of the plasma. Such a soft component hints to the presence of plasma heated by shocks due to jets outside the dense circumstellar material.

A study of the Galactic star forming region IRAS 02593+6016/S 201 in infrared and radio wavelengths

We present infrared and radio continuum observations of the S 201 star forming region. A massive star cluster is seen, which contains different classes of young stellar objects. The near-infrared colour-colour and colour-magnitude diagrams are studied to determine the nature of these sources. We have discovered knots of molecular hydrogen emission at 2.122 μ m in the central region of S 201. These knots are clearly seen along the diffuse emission to the north–west and are probably obscured Herbig-Haro objects. High sensitivity and high resolution radio continuum images from GMRT observations at 610 and 1280 MHz show an arc-shaped structure due to the interaction between the HII region and the adjacent molecular cloud. The ionization front at the interface between the HII region and the molecular cloud is clearly seen comparing the radio, molecular hydrogen and Br γ images. The emission from the carriers of Unidentified Infrared Bands in the mid-infrared 6–9 μ m (possibly due to PAHs) as extracted from the Midcourse Space Experiment survey (at 8, 12, 14 and 21 μ m) is compared with the radio emission. The HIRES processed IRAS maps at 12, 25, 60 and 100 μ m have also been used for comparison. The spatial distribution of the temperature and the optical depth of the warm dust component around the S 201 region has been generated from the mid-infrared images.

A study of the Galactic star forming region IRAS 02593+6016/S 201 in infrared and radio wavelengths

We present infrared and radio continuum observations of the S 201 star forming region. A massive star cluster is seen, which contains different classes of young stellar objects. The near-infrared colour-colour and colour-magnitude diagrams are studied to determine the nature of these sources. We have discovered knots of molecular hydrogen emission at 2.122 μ m in the central region of S 201. These knots are clearly seen along the diffuse emission to the north–west and are probably obscured Herbig-Haro objects. High sensitivity and high resolution radio continuum images from GMRT observations at 610 and 1280 MHz show an arc-shaped structure due to the interaction between the HII region and the adjacent molecular cloud. The ionization front at the interface between the HII region and the molecular cloud is clearly seen comparing the radio, molecular hydrogen and Br γ images. The emission from the carriers of Unidentified Infrared Bands in the mid-infrared 6–9 μ m (possibly due to PAHs) as extracted from the Midcourse Space Experiment survey (at 8, 12, 14 and 21 μ m) is compared with the radio emission. The HIRES processed IRAS maps at 12, 25, 60 and 100 μ m have also been used for comparison. The spatial distribution of the temperature and the optical depth of the warm dust component around the S 201 region has been generated from the mid-infrared images.

Evolution of FU Orionis Outbursts in Protostellar Disks

Applying a fully two-dimensional axisymmetric radiation hydrodynamical scheme, the accretion of matter from a protostellar disk onto the central star is investigated. We follow the evolution of the disk in the vicinity of the protostar during the early epoch of star formation, when the mass inflow rate through the disk is 10-5 M☉ yr-1. This mass transfer rate is appropriate for a class of young stellar objects known as FU Orionis stars. These calculations are carried out for about 5000 Keplerian orbital periods near the stellar surface, which is equivalent to 3000 days. For this mass accretion rate the disk is thermally unstable. We examine the evolution of the disk as it evolves from a quiescent, geometrically thin low state to an outburst state with a higher mass-inflow rate and a geometrically thick disk. During the onset of the instability, a heating front moves slowly outward, starting from the inner edge of the disk near the stellar radius; in its wake, the temperature at the disk midplane undergoes a transition from about 104 K to a few times 105 K. This transition describes the onset of an outburst of an FU Ori system. A model light curve and blackbody spectra are computed and compared to the outburst of the FU Ori.

The submillimetre colour of young stellar objects

A survey is presented of the submillimetre continuum spectra from dust around 73 young stellar objects (YSOs) associated with Herbig–Haro objects and molecular outflows. Spectra at millimetre to submillimetre wavelengths taken with similar beamsizes have been obtained for 42 of the objects, and the remainder either have been detected or have upper limits at 800 μm. The data are combined with far-infrared IRAS fluxes, and then compared with published submillimetre data from other types of YSO; this results in a data set of ∼150 submillimetre spectra. We have then employed colour–colour diagrams to investigate the differences in submillimetre continuum spectra between classes of YSOs. The majority of targets are optically obscured YSOs (generally Lada Class I); all are found to have a submillimetre spectral index (αmm) of ∼3.5. None of these objects has αmm ≤ 3.0. This is significantly different from published optically visible T Tauri stars (Class II–III), many of which have αmm ≤ 2.5. The few main-sequence stars so far observed in the submillimetre generally also have a relatively low value of αmm. We find that objects which might be classified as T Tauri stars, but which are not directly visible, have higher values of αmm. Assuming isothermal optically thin dust emission, this implies a dust emissivity index β of ∼1.5 for embedded objects, and ∼0.5 for visible objects. The reduced αmm in low optical extinction sources can be attributed to either grain growth or the formation of a compact optically thick circumstellar disc, and we discuss these two options in the light of the new data. If, as the results suggest, large fractal grains are present in the more evolved systems, then these will have higher millimetre mass opacity, implying that the determination of the circumstellar mass evolution from submillimetre continuum fluxes must take into account the evolution of the dust itself. High-mass embedded YSOs also have spectra very similar to their low-mass counterparts, but with higher dust temperature (again, under the isothermal optically thin assumption). Also, most Class 0 YSOs have high values of αmm, although three have flatter spectra more typical of fractal dust, which is difficult to explain under simple grain coagulation models. The submillimetre continuum flux is also compared with the intensity of two molecular lines: C18O J = 2–1 and H2CO 303 − 202. The integrated emission from the higher density tracer, H2CO, is well correlated with continuum flux. The C18O line shows a less significant correlation, particularly at low masses, suggesting that this may not be a good molecule to search for low-mass YSOs. By comparing T Tauri stars and younger YSOs with the same continuum flux, we find evidence that T Tauri stars are unusually weak in C18O emission, and we give some possible explanations.

Multi-epoch radio continuum surveys of the Rho Ophiuchi dark cloud

view Abstract Citations (61) References (26) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Multi-Epoch Radio Continuum Surveys of the Rho Ophiuchi Dark Cloud Stine, P. C. ; Feigelson, E. D. ; Andre, P. ; Montmerle, T. Abstract Radio-continuum surveys of 4 sq. deg of the ρ Ophiuchi cloud were conducted on four occasions at 1.4 GHz with the Very Large Array to study the variability of radio counterparts of over one hundred pre-main-sequence stars detected at X-ray, infrared, and optical wavelengths. These observations are supplemented with 5 GHz observations of the dense core region of the cloud and pointed observations of selected sources. This work significantly increases the sensitivity of a previous study by André, Montmerle, and Feigelson (1987), in which eight definite young stellar radio sources were uncovered. In this study, only one new reliable stellar source was found, located near the cloud core. The overwhelming majority (≡90%) of young stellar objects over a large area remain undetected to low flux-density levels (≡1 mJy). The failure to detect most PMS stars in ρ Ophiuchi and other star-formation regions may be explained by a sufficiently dense layer of circumstellar ionized material, such as a weak stellar wind, that absorbs nonthermal surface radio emission but is not itself detectable in radio. It seems that the stellar radio sources in the ρ Ophiuchi cloud are the prototypes of a new, rare class of young stellar objects. These sources are characterized by an absence of dense circumstellar ionized material and by extended magnetic structures with sizes ⪆3 stellar radii. Publication: The Astronomical Journal Pub Date: October 1988 DOI: 10.1086/114889 Bibcode: 1988AJ.....96.1394S Keywords: Early Stars; Ophiuchi Clouds; Pre-Main Sequence Stars; Sky Surveys (Astronomy); Stellar Flares; Very Large Array (Vla); Radio Emission; Stellar Activity; Stellar Mass; X Ray Sources; Astrophysics; STARS: PRE-MAIN-SEQUENCE full text sources ADS | data products SIMBAD (220)