Taurus-Auriga Dark Clouds Research Articles

THE CALIFORNIA MOLECULAR CLOUD

We present an analysis of wide-field infrared extinction maps of a region in Perseus just north of the Taurus-Auriga dark cloud complex. From this analysis we have identified a massive, nearby, but previously unrecognized, giant molecular cloud (GMC). From comparison of foreground star counts with Galactic models we derive a distance of 450 +/- 23 parsecs to the cloud. At this distance the cloud extends over roughly 80 pc and has a mass of approximately 10^5 solar masses, rivaling the Orion (A) Molecular Cloud as the largest and most massive GMC in the solar neighborhood. Although surprisingly similar in mass and size to the more famous Orion Molecular Cloud (OMC) the newly recognized cloud displays significantly less star formation activity with more than an order of magnitude fewer young stellar objects than found in the OMC, suggesting that both the level of star formation and perhaps the star formation rate in this cloud are an order of magnitude or more lower than in the OMC. Analysis of extinction maps of both clouds shows that the new cloud contains only 10% the amount of high extinction (A_K > 1.0 mag) material as is found in the OMC. This, in turn, suggests that the level of star formation activity and perhaps the star formation rate in these two clouds may be directly proportional to the total amount of high extinction material and presumably high density gas within them and that there might be a density threshold for star formation on the order of a few times 10^4 hydrogen molecules per cc.

A Spectroscopic Survey of Subarcsecond Binaries in the Taurus‐Auriga Dark Cloud with theHubble Space Telescope

We report the results of a spectroscopic survey of 20 close T Tauri binaries in the Taurus-Auriga dark cloud where the separations between primaries and their secondaries are less than the typical size of a circumstellar disk around a young star. Analysis of low-resolution and medium-resolution STIS spectra yields the stellar luminosities, reddenings, ages, masses, mass accretion rates, IR excesses, and emission line luminosities for each star in each pair. We examine the ability of IR color excesses, H-alpha equivalent widths, [O I] emission, and veiling to distinguish between weak emission and classical T Tauri stars. Four pairs have one cTTs and one wTTs; the cTTs is the primary in three of these systems. This frequency of mixed pairs among the close T Tauri binaries is similar to the frequency of mixed pairs in wider young binaries. Extinctions within pairs are usually similar; however, the secondary is more heavily reddened than the primary in some systems, where it may be viewed through the primary's disk. Mass accretion rates of primaries and secondaries are strongly correlated, and H-alpha luminosities, IR excesses, and ages also correlate within pairs. Primaries tend to have somewhat larger accretion rates than their secondaries do, and are typically slightly older than their secondaries according to three different sets of modern pre-main-sequence evolutionary tracks. Age differences for XZ Tau and FS Tau, systems embedded in reflection nebulae, are striking; the secondary in each pair is less massive but more luminous than the primary. The stellar masses of the UY Aur and GG Tau binaries measured from their rotating molecular disks are about 30% larger than the masses inferred from the spectra and evolutionary tracks.

Extended Near-Infrared Emission from Candidate Protostars in the Taurus-Auriga Molecular Cloud

We describe near-IR imaging data for a sample of 23 Class I sources in the Taurus-Auriga dark clouds. Combining our data with previous photometry, we detect brightness variations of � 0.1–0.5 mag in many sources. The near-IR morphologies are consistent with millimeter continuum measurements. Most (� 60%) of the sample are true protostars; the rest may be objects in transition between Class I and Class II, T Tauri stars with edge-on disks, or heavily reddened T Tauri stars.

Binarity in the Orion Trapezium Cluster

We summarise the results of recent optical and near-infrared imaging studies of the binary fraction among young low-mass stars in the dense Orion Trapezium Cluster. Over the separation range ∼ 30–500 AU and within the observational errors, there appears to be no excess of binary systems in the cluster relative to the main sequence field star population. Over the separation range ∼ 1000–5000 AU, the cluster is deficient in binaries relative to the field. Both results are in contrast to those found for the more distributed population of young stars in the Taurus-Auriga dark clouds, which is overabundant in binaries by roughly a factor of two. We briefly discuss possible origins for this difference and observational tests which may distinguish between them, and the implications these results have for our understanding of the typical environment where most young stars are born.

Imaging polarimetry of class I young stellar objects

We present near-infrared imaging polarimetry of three class I young stellar objects in the Taurus–Auriga dark cloud. We use Monte Carlo simulations to analyse the flux distributions and polarization patterns of these three sources and five others from an earlier paper. In addition, we present high-resolution polarimetry of HL Tau using the shift and add technique. Most young stellar objects in the sample display sharp, unresolved, peaks in the scattered light distribution. This is most simply explained by a strong concentration of matter in the centre, which we model by applying the ρ ∝ r−1.5 power law throughout the envelope. In terms of the Ulrich/Terebey, Shu and Cassen solution for the late stages of contraction of an initially spherical non-magnetic cloud, this corresponds to rc < 10 au. However, this almost spherically symmetric density distribution is inconsistent with observations of flattened, disc-like structures, so we conclude that this solution is not appropriate and different initial conditions apply. The multiple-scattering models with spherical grains do not reproduce some features of the observed polarization patterns, in particular the broad regions of aligned vectors seen in some sources. We interpret this as evidence for elongated aligned grains. The weak wavelength dependence of nebular morphology shows that the dust grains in circumstellar envelopes obey a much shallower extinction law than interstellar grains in the near-infrared, which we describe by the opacity ratio κ(J/K) = 1.8 ± 0.3, compared to the interstellar value of 3.25. We place an upper limit on albedo of ω < 0.6 from 1.25 to 2.2 μm and we find 0.1 < ω < 0.6 at 2.2 μm. Analysis of the quadrupolar source IRAS 04302+2247 indicates that the scattering function is not highly forward-throwing, which we express by the constraint g = 〈 cos(τ) 〉 < 0.4. With the addition of two more observables derived from the observed degrees of linear and circular polarization, we identify five empirical constraints on the properties of circumstellar dust. After calculating these observables for grain mixtures with a wide range of refractive indices and a variety of size distributions, we find that highly metallic spherical grains or highly elongated grains can satisfy these constraints and the size distribution extends to a a 0.35 μm. Amorphous carbon is the most plausible absorptive component, since graphite appears to be ruled out by its strong dispersion in the near-infrared.

Optical Spectroscopy of Embedded Young Stars in the Taurus-Auriga Molecular Cloud

This paper describes the first optical spectroscopic survey of Class I sources (also known as embedded sources and protostars) in the Taurus-Auriga dark cloud. We detect 10 of the 24 known Class I sources in the cloud at 5500–9000 A. All detected Class I sources have strong Hα emission; most also have strong [O I] and [S II] emission. These data—together with high-quality optical spectra of T Tauri stars in the Taurus-Auriga cloud—demonstrate that forbidden emission lines are stronger and more common in Class I sources than in T Tauri stars. Our results also provide a clear discriminant in the frequency of forbidden line emission between weak-emission and classical T Tauri stars. In addition to strong emission lines, three Class I sources have prominent TiO absorption bands. The M-type central stars of these sources mingle with optically visible T Tauri stars in the H-R diagram and lie somewhat below both the birth line for spherical accretion and the deuterium burning sequence for disk accretion.

Search for young low-mass stars in a ROSAT selected sample south of the Taurus-Auriga molecular clouds

We present results of intermediate resolution spectroscopy of 131 optical counterparts to 115 ROSAT All-Sky Survey X-ray sources south of the Taurus-Auriga dark cloud complex. These objects have been selected as candidate young stars from a total of 1084 ROSAT sources in a square degree area. We identify 30 objects as low-mass PMS stars on the basis of the Li i doublet in their spectrum, a signature of their young age. All these stars have a spectral type later than F7 and show spectral characteristics typical of weak-line and post-T Tauri stars. The presence of young objects several parsecs away from the regions of ongoing star formation is discussed in the light of the current models of T Tauri dispersal.

Butterfly star in Taurus: structures of young stellar objects

We present high-resolution, near-infrared images of a sample of five low-mass young stellar objects in the Taurus-Auriga dark cloud, mostly observed with the ‘shift and add’ technique, together with deep polarimetric images. These systems are at the ‘Class I’ phase of evolution, prior to the T Tauri phase. Monte Carlo models tailored to the individual sources support the view that the flux distribution of the circumstellar nebulosity arises from an envelope with a bipolar cavity, but show that dust in outflows can also be prominent in the near-infrared. The source IRAS 04302 + 2247 is analysed in detail, since it displays a unique quadrupolar morphology, which is explained in terms of a system oriented precisely edge-on with a perpendicular outflow. The masses of the envelopes derived from the models are in good agreement with estimates obtained from submillimetre thermal fluxes and estimates derived from CS(J=2-l) emission. Constraints are placed on envelope and cavity structure, indicating higher density near the equatorial plane, smooth, continuous density profiles and physically thin inner discs. Three of the five systems show evidence for two bipolar cavity/outflow systems, perhaps indicating that these are close binaries. The very high (up to 80 per cent) degrees of polarization observed in the envelopes show that the scattering is dominated by small, interstellar-type dust grains, and the morphologies and high polarizations of the cores show that the protostars themselves are usually obscured at this phase of evolution, even at 3.7 μm.

Is HL Tauri and FU Orionis system in quiescence?

A recent Nobeyama map of HL Tau reveals that gas is infalling in a flattened region approximately 1400 AU around the central star. The apparent motion of the gas provides the necessary condition for the formation of a Keplerian disk with a radius comparable to the size of the primordial solar nebula. The inferred mass infall rate onto the disk is approximately equal to 5 x 10(exp -6) solar mass/yr, which greatly exceeds the maximum estimate of the accretion rate onto the central star (approximately 7 x 10(exp -7) solar mass/yr). Consequently, mass must currently be accumulating in the disk. The estimated age and disk mass of HL Tau suggest that the accumulated matter has been flushed repeatedly on a timescale less than 10(exp 4) yr. Based on the similarites between their evolution patterns, we propose that HL Tau is an FU Orionis system in quiescence. In addition to HL Tau, 14 out of 86 pre-main-sequence stars in the Taurus-Auriga dark clouds have infrared luminosities much greater than their otherwise normal extinction-corrected stellar luminosities. These sources also tend to have flat spectra which may be due to the reprocessing of radiation by dusty, flattened, collapsing envelopes with infall rates a few 10(exp -6) solar mass/yr. Such rates are much larger than estimated central accretion rates for these systems, which suggests that mass must also be accumulating in these disks. If these sources are FU Orionis stars in quiescence, similar to HL Tau, their age and relative abundance imply that the FU Orionis phase occurs over a timescale of approixmately 10(exp 5) yr, and the quiescent phase between each outburst lasts approximately 10(exp 3) =10(exp 4) yr. These inferred properties are compatible with the scenario that FU Orionis outbursts are regulated by a thermal instability in the inner region of the disk.

Additional constraints on circumstellar disks in the Trapezium Cluster

We discuss the new constraints on the population of compact ionized sources in the Trapezium Cluster thought to arise from the ionization by the central OB stars of circumstellar disks around low-mass pre-main sequence stars. We present new Hubble Space Telescope (HST) Planetary Camera observations of two of these candidate disk sources, resolving extended nebulosity around them. One source shows a small-scale (greater than or approximately = 100 AU) bow-shock structure, previously seen on larger scales by O'Dell et al. We show that the circumstellar disk model is the most likely one for the majority of sources, although is remains plausible that some of the larger objects could be equilibrium globules. We combine the most complete censuses of compact radio sources and stars in the core region to derive the fraction of the stellar population that may be associated with a circumstellar disk. Our estimate of 25%-75% is comparable to that found for pre-main sequence (PMS) stars in the Taurus-Auriga dark clouds, indicating that the dense cluster environment of the Trapezium has not drastically reduced the frequency of disks seen around PMS stars.

A study of the stellar population in the Chamaeleon dark clouds

The properties of the stellar population in the Chamaeleon dark clouds are discussed. Spectral energy distributions, based on the extant photometric and spectroscopic data base and IRAS fluxes measured from coadded data taken at the position of each star, and spectral types allow placement of the stars in an H-R diagram. The age and mass distributions and the luminosity function for the Chamaeleon stars are compared to those in the Taurus-Auriga dark clouds and are found to be similar. A small subsample (eight of 36) of the Chamaeleon stars show unusual spectral energy distributions which seem best interpreted as arising from circumstellar disks whose inner regions (R(in)) is less than 30-50 stellar radii) area devoid of material. The X-ray properties of this sample of premain-sequence objects are compared to those of other premain-sequence samples, as well as to the Hyades and the Pleiades main-sequence stars.

Optical excess emission in T Tauri stars

We present a set of simultaneous high-resolution spectroscopic and spectrophotometric observations of 22 K7-M1 T Tauri stars in the Taurus-Auriga dark cloud. Analysis of the high-resolution data makes it possible to separate the optical excess (veiling) emission from the photospheric fluxes in these stars. The amount of optical excess emission at 5500 A ranges from undetectable (≤10% of the photosphere) to as much as 10 times the photospheric flux in the most extreme objects

A survey for circumstellar disks around young stellar objects

view Abstract Citations (1591) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Survey for Circumstellar Disks around Young Stellar Objects Beckwith, Steven V. W. ; Sargent, Anneila I. ; Chini, Rolf S. ; Guesten, Rolf Abstract Continuum observations at 1.3 mm of 86 pre-main sequence stars in the Taurus-Auriga dark clouds show that 42% have detectable emission from small particles. The detected fraction is only slightly smaller for the weak-line and "naked" T Tauri stars than for classical T Tauris, indicating that the former stars often have circumstellar material. In both categories, the column densities of particles are too large to be compatible with spherical distributions of circumstellar matter -- the optical extinctions would be too large; the particles are almost certainly in spatially thin, circumstellar disks. Models of the spectral energy distributions from 10 to 1300 μm indicate that for the most part the disks are transparent at 1.3 mm, although the innermost (≪1 AU) regions are opaque even at millimeter wavelenths. The aggregate particle masses are between 10-5 and 10-2 M⊙. The disk mass does not decrease with increasing stellar age up to at least 107 years among the stars detected at 1.3 mm. There is some evidence for temperature evolution, in the sense that older disks are colder and less luminous. There is little correlation between disk mass and Hα equivalent width among the detected stars, suggesting that the Hα line is not by itself indicative of disk mass. Spectral indices for several sources between 1.3 and 2.7 mm suggest that the particle emissivities ɛ are weaker functions of frequency ν than is the usual case of interstellar grains. Particle growth via adhesion in the dense disks might explain this result. The typical disk has an angular momentum comparable to that generally accepted for the early solar nebula, but very little stored energy, almost five orders of magnitude smaller than that of the central star. Our results demonstrate that disks more massive than the minimum mass of the proto-solar system commonly accompany the birth of solar-mass stars and suggest that planetary systems are common in the Galaxy. Publication: The Astronomical Journal Pub Date: March 1990 DOI: 10.1086/115385 Bibcode: 1990AJ.....99..924B Keywords: Molecular Clouds; Pre-Main Sequence Stars; Sky Surveys (Astronomy); Stellar Envelopes; T Tauri Stars; Auriga Constellation; Computational Astrophysics; Emission Spectra; H Alpha Line; Milky Way Galaxy; Stellar Evolution; Taurus Constellation; Astrophysics; STARS: PRE-MAIN-SEQUENCE; STARS: CIRCUMSTELLAR SHELLS full text sources ADS | data products SIMBAD (90)

A two micron polarization survey of T Tauri stars

Results are presented of a K-band (2.2 microns) polarization survey of 39 T Tauri stars (TTSs) and, in addition, of eight other young stellar objects and 11 background field stars in the Taurus-Auriga dark cloud complex. It was found that about 60 percent of the TTSs are linearly polarized, but with relatively small degrees of polarization up to about 3 percent. A comparison of the results of this survey with those of the IR polarization survey of the low-luminosity IRAS sources in the same region suggests an evolutionary sequence in the IR polarization of young stellar objects. 79 refs.

A spectroscopic survey of the Taurus-Auriga dark clouds for pre-main-sequence stars having CA II H, K emission

A large area of the Tau-Aur dark clouds was surveyed to about B = 15.0 in a search for stars having the Ca II H, K lines in emission. Essentially all the known T Tauri Stars (TTS) in the area were rediscovered (51 in all), as well as three of the four known x-ray sources; H, K emission was detected in about 20 other faint, red stars. Slit spectrograms and multicolor photometry have been obtained for most of the latter, which turn out to be low-luminosity K and M stars with weak H emission. A majority, but not all, are believed to be pre-main-sequence association members on the basis of strong Li I lambda6707, proper motions (in a few cases), radial velocity, or infrared excess. Several stars that otherwise qualify as cloud members seem to have radial velocities that are offset from -17 to -26 km s with respect to the cloud CO velocity. The location of the H, K emission stars in an H-R diagram shows them to be mixed among the conventional TTS: i.e., they do not appear systematically old, and clearly cannot be identified with a post-T Tauri population. They appear simply to be TTS that havemore » H emission too weak to have been detected in the conventional surveys. These results emphasize the point that, if initial conditions are similar and if pre-main-sequence ages as usually assigned are correct, then emission-line intensity does not decay with time in any simple, general way.« less

VLA observations of T Tauri stars. II - A luminosity-limited survey of Taurus-Auriga

view Abstract Citations (188) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS VLA observations of T Tauri stars. II. A luminosity limited survey ofTaurus-Auriga. Bieging, J. H. ; Cohen, M. ; Schwartz, P. R. Abstract The results of a 4.885-GHz VLA survey of 46 T Tau stars and Herbig-Haro objects with log L 0.2 or greater in the Taurus-Auriga dark clouds, performed in February 1982, with 40-60-min observation times, are presented in tables and contour maps and discussed. Upper limits are given for objects not detected at 4.885 GHz; additional observations at 1.465, 14.965, and 22.485 GHz are included for the objects detected; and spectral indices are derived for 10 objects. The individual objects are characterized in detail, and it is concluded that detectable radio emission is more likely for T Tau stars with associated Herbig-Haro objects than for those without. The implications of the nonthermal spectra identified in several objects for theories of theoretical models of radio emission are indicated. Publication: The Astrophysical Journal Pub Date: July 1984 DOI: 10.1086/162251 Bibcode: 1984ApJ...282..699B Keywords: Pre-Main Sequence Stars; Radio Astronomy; Stellar Luminosity; Stellar Mass Ejection; Stellar Spectrophotometry; T Tauri Stars; Astronomical Maps; Herbig-Haro Objects; Infrared Spectra; Molecular Clouds; Spectral Resolution; Astronomy full text sources ADS | data products SIMBAD (17)

An investigation of T Tauri variability

view Abstract Citations (16) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An investigation of T Tauri variability. Schmelz, J. T. Abstract Three mechanisms have been suggested to account for the photometric variability in T Tauri stars: (1) changes in the effective photospheric spectral type; (2) changes in the optical thickness of the chromosphere; and (3) changes in the optical thickness of the dust shell. The author investigates these processes with color-color diagrams and energy distribution plots of 14 stars located in the Taurus-Auriga dark cloud complex. A relationship between a strong chromosphere and chromospheric variability was found as well as a similar relationship between a thick dust shell and dust shell variability. There is some evidence that the stellar photosphere becomes more stable with increasing age during the T Tauri phase. Publication: The Astronomical Journal Pub Date: January 1984 DOI: 10.1086/113487 Bibcode: 1984AJ.....89..108S Keywords: Chromosphere; Stellar Activity; Stellar Envelopes; Stellar Spectrophotometry; T Tauri Stars; Color-Color Diagram; Optical Thickness; Photosphere; Spectral Energy Distribution; Stellar Color; Stellar Evolution; Stellar Spectra; Variability; Astrophysics; Circumstellar Shells:T Tauri Stars; Stellar Chromospheres:T Tauri Stars; T Tauri Stars:Circumstellar Shells; T Tauri Stars:Stellar Chromospheres; T Tauri Stars:Variations full text sources ADS | data products SIMBAD (15)

Proper motions of T Tauri variables and other stars associated with the Taurus-Auriga dark clouds

Proper motions of T Tauri variables and other stars associated with the Taurus-Auriga dark clouds