Taurus-Auriga Molecular Cloud Research Articles

The Pan-STARRS1 Proper-motion Survey for Young Brown Dwarfs in Nearby Star-forming Regions. I. Taurus Discoveries and a Reddening-free Classification Method for Ultracool Dwarfs

Abstract We are conducting a proper-motion survey for young brown dwarfs in the Taurus-Auriga molecular cloud based on the Pan-STARRS1 3π Survey. Our search uses multi-band photometry and astrometry to select candidates, and is wider (370 deg2) and deeper (down to ≈3 M Jup) than previous searches. We present here our search methods and spectroscopic follow-up of our high-priority candidates. Since extinction complicates spectral classification, we have developed a new approach using low-resolution (R ≈ 100) near-infrared spectra to quantify reddening-free spectral types, extinctions, and gravity classifications for mid-M to late-L ultracool dwarfs (≲100–3 M Jup in Taurus). We have discovered 25 low-gravity (vl-g) and the first 11 intermediate-gravity (int-g) substellar (M6–L1) members of Taurus, constituting the largest single increase of Taurus brown dwarfs to date. We have also discovered 1 new Pleiades member and 13 new members of the Perseus OB2 association, including a candidate very wide separation (58 kau) binary. We homogeneously reclassify the spectral types and extinctions of all previously known Taurus brown dwarfs. Altogether our discoveries have thus far increased the substellar census in Taurus by ≈40% and added three more L-type members (≲5–10 M Jup). Most notably, our discoveries reveal an older (>10 Myr) low-mass population in Taurus, in accord with recent studies of the higher-mass stellar members. The mass function appears to differ between the younger and older Taurus populations, possibly due to incompleteness of the older stellar members or different star formation processes.

Protoplanetary Disks in ρ Ophiuchus as Seen from ALMA

Abstract We present a high angular resolution ( ), high-sensitivity ( mJy) survey of the 870 μm continuum emission from the circumstellar material around 49 pre-main-sequence stars in the ρ Ophiuchus molecular cloud. Because most millimeter instruments have resided in the northern hemisphere, this represents the largest high-resolution, millimeter-wave survey of the circumstellar disk content of this cloud. Our survey of 49 systems comprises 63 stars; we detect disks associated with 29 single sources, 11 binaries, 3 triple systems, and 4 transition disks. We present flux and radius distributions for these systems; in particular, this is the first presentation of a reasonably complete probability distribution of disk radii at millimeter wavelengths. We also compare the flux distribution of these protoplanetary disks with that of the disk population of the Taurus–Auriga molecular cloud. We find that disks in binaries are both significantly smaller and have much less flux than their counterparts around isolated stars. We compute truncation calculations on our binary sources and find that these disks are too small to have been affected by tidal truncation and posit some explanations for this. Lastly, our survey found three candidate gapped disks, one of which is a newly identified transition disk with no signature of a dip in infrared excess in extant observations.

B- AND A-TYPE STARS IN THE TAURUS-AURIGA STAR-FORMING REGION

We describe the results of a search for early-type stars associated with the Taurus-Auriga molecular cloud complex, a diffuse nearby star-forming region noted as lacking young stars of intermediate and high mass. We investigate several sets of possible O, B and early A spectral class members. The first is a group of stars for which mid-infrared images show bright nebulae, all of which can be associated with stars of spectral type B. The second group consists of early-type stars compiled from (i) literature listings in SIMBAD; (ii) B stars with infrared excesses selected from the Spitzer Space Telescope survey of the Taurus cloud; (iii) magnitude- and color-selected point sources from the 2MASS; and (iv) spectroscopically identified early-type stars from the SDSS coverage of the Taurus region. We evaluated stars for membership in the Taurus-Auriga star formation region based on criteria involving: spectroscopic and parallactic distances, proper motions and radial velocities, and infrared excesses or line emission indicative of stellar youth. For selected objects, we also model the scattered and emitted radiation from reflection nebulosity and compare the results with the observed spectral energy distributions to further test the plausibility of physical association of the B stars with the Taurus cloud. This investigation newly identifies as probable Taurus members three B-type stars: HR 1445 (HD 28929), tau Tau (HD 29763), 72 Tau (HD 28149), and two A-type stars: HD 31305 and HD 26212, thus doubling the number of stars A5 or earlier associated with the Taurus clouds. Several additional early-type sources including HD 29659 and HD 283815 meet some, but not all, of the membership criteria and therefore are plausible, though not secure, members.

COMBINED ANALYSIS OF IMAGES AND SPECTRAL ENERGY DISTRIBUTIONS OF TAURUS PROTOSTARS

We present an analysis of spectral energy distributions (SEDs), near- and mid-infrared images, and Spitzer spectra of eight embedded Class I/II objects in the Taurus-Auriga molecular cloud. The initial model for each source was chosen using the grid of young stellar objects (YSOs) and SED fitting tool of Robitaille et al. Then the models were refined using the radiative transfer code of Whitney et al. to fit both the spectra and the infrared images of these objects. In general, our models agree with previous published analyses. However, our combined models should provide more reliable determinations of the physical and geometrical parameters since they are derived from SEDs, including the Spitzer spectra, covering the complete spectral range; and high-resolution near-infrared and Spitzer IRAC images. The combination of SED and image modeling better constrains the different components (central source, disk, envelope) of the YSOs. Our derived luminosities are higher, on average, than previous estimates because we account for the viewing angles (usually nearly edge-on) of most of the sources. Our analysis suggests that the standard rotating collapsing protostar model with disks and bipolar cavities works well for the analyzed sample of objects in the Taurus molecular cloud.

High-Resolution Spectroscopy of T Tauri Stars

We have conducted high-resolution spectroscopic observations of a sample of both classical and weak-line T Tauri stars embedded in the Taurus-Auriga molecular cloud using the Okayama Astrophysical Observatory’s 188 cm telescope equipped with HIDES (HIgh Dispersion Echelle Spectrograph). Utilizing the ratios of equivalent widths of specific lines, we efficiently eliminated the effects of “veiling” and determined the atmospheric parameters individually for each object. Eight pairs of temperature-sensitive line-ratios were selected to derive the effective temperature with high precision and reliability. As for the surface gravity, we have developed a new method to determine the value. The method was first tested on observed spectral standards, and the derived surface gravities had a less than 5% deviation from the values in the literature. The derived surface gravities are well within the range of the assumed values for T Tauri stars.

High Spatial Resolution Near-Infrared Images of Taurus Protostars

We analyze near-infrared K- and L-band images of six embedded Class I objects in the Taurus-Auriga molecular cloud. Three of these sources were observed in both bands and three were observed only at L. The L- and K-band images were obtained with the Cryogenic Optical Bench at the KPNO 4 m telescope in the diffraction-limited mode. With the high spatial resolution L-band data, we identify IRAS 04239+2436 as a close pair with a projected separation of 0.29''. We use a radiative transfer code to model the images and derive physical and geometrical parameters for each source. Because the projected positions of the six analyzed sources on the cloud coincide with the densest zones in the complex (AV ~ 10-30 mag), we use two models for the optical properties of dust grains in the envelopes: a diffuse ISM model and a grain model with ratio of total to selective extinction RV = 4.3 corresponding to the measured R values in the densest regions of the Taurus molecular cloud. Although the two grain models have different opacities, we can match the observed images with either model by selecting the appropriate envelope infall rate. Thus we cannot distinguish between the two grain models over this narrow wavelength range (K and L bands).

Near‐Infrared Synthetic Images of Protostellar Disks and Envelopes

We present a grid of near-infrared (IR) synthetic images of pre-main-sequence stars at different stages of evolution, which we simulate by varying envelope mass, disk radius and mass, and outflow cavity shape. Our aim is to determine how variations in physical properties of young stellar objects (e.g., mass infall rate, disk size) affect their observed colors and morphology, and use this information to highlight observable differences between different evolutionary states. We show that the near-IR colors are a function of envelope mass infall rate and inclination; hence both parameters must be constrained if colors are to be used to infer a source's true evolutionary state. Sources with more opaque envelopes have redder diffuse colors, because the scattered light suffers reddening as it propagates through the envelope. Somewhat counterintuitively, colors are reddest at intermediate inclinations (i ~ 45°-70°) and then become bluer edge-on, where light is ~100% scattered. Thus a source with relatively blue colors could be an evolved source or a younger source oriented edge-on. Importantly, we find that at inclinations where scattered light dominates, it is erroneous to derive extinction A_V from observed colors; fully half of all objects will underestimate A_V by at least an order of magnitude. We use our models to interpret six protostellar sources in the Taurus-Auriga molecular cloud observed with HST NICMOS. Of the six young stellar objects modeled in this paper, five require an infalling envelope to match the colors and should thus be classified as young embedded sources. The remaining source, Haro 6-5B, is a disk source, having already dispersed its envelope.

Rotational periods of T Tauri stars in Taurus-Auriga, south of Taurus-Auriga, and in MBM12

Context. The ROSAT All-Sky Survey detected many young objects outside any known star forming region. Their formation is yet unclear. Aims. In order to improve the knowledge about these X-ray bright objects we aimed at measuring their rotational properties, which are fundamental stellar parameters, and at comparing them to young objects inside molecular clouds. Methods. We monitored photometric variations of 5 T Tauri stars in MBM12 and of 26 young objects in the Taurus-Auriga molecular cloud and south of it. Among the 26 young objects there are 17 weak-line T Tauri stars, 7 zero age main-sequence stars and 2 of unknown type. In addition, 2 main-sequence K-type stars were observed, and one comparison star turned out to be an eclipsing binary. Results. We found periodic variations for most of the targets. The measured periods of the T Tauri stars range from 0.57 to 7.4 days. The photometric variation can be ascribed to rotational modulation caused by spots. For a few of the periodic variables, changes of the light curve profile within several weeks are reported. For one star such changes have been observed in data taken two years apart. The exceptions are two eclipsing systems. One so far unknown system – GSC2.2 N3022313162 – shows a light curve with full phase coverage having both primary and secondary minima well resolved. It has an orbital period of 0.59075 days. From our spectroscopic observations we conclude that it is a main sequence star of spectral type F2 ± 4. We further compared the off-cloud weak-line T Tauri stars to the weak-line T Tauri stars inside the molecular cloud in terms of rotational period distribution. Statistical analysis of the two samples shows that both groups are likely to have the same period distribution.

Mid?Infrared Observations of T Tauri Stars: Probing the Star?Disk Connection in Rotational Evolution

We present mid-IR N-band (λeff = 10.2 μm) photometry of a carefully selected sample of T Tauri stars thought to be single from the Taurus-Auriga molecular cloud. Infrared excesses in these stars are generally attributed to circumstellar dust disks. Combining observations at 2.16 (Ks band) and 10.2 μm (N band) we probe a region in the circumstellar dust disk from a few stellar radii through the terrestrial planet zone (0.02-1.0 AU). By analyzing the distribution of the (Ks - N) color index with respect to previously measured photometric rotation periods we investigate what role circumstellar disks play in the rotational evolution of the central star. The resulting positive correlation between these two variables is consistent with the notion that a star-disk interaction facilitates the regulation of angular momentum during the T Tauri stage. We also demonstrate how including nonsingle stars in such an analysis weakens any correlation in the relation between (Ks - N) color and period. To further understand disk properties we also present spectral energy distributions for a few objects with new ground-based M-band (λeff = 4.8 μm) and Q-band (λeff = 20 μm) data and compare them to a geometrically thin, optically thick disk model.

A Distributed Population of Low?Mass Pre?Main?Sequence Stars near the Taurus Molecular Clouds

We present a drift scan survey covering a ~5 deg by 50 deg region toward the southern portion of the Taurus-Auriga molecular cloud. Data taken in the B,R,I filters with the Quest-2 camera on the Palomar 48-inch telescope were combined with 2MASS near-infrared photometry to select candidate young stars. Follow-up optical spectroscopy of 190 candidates led to identification of 42 new low mass pre-main sequence stars with spectral types M4-M8, of which approximately half exhibit surface gravity signatures similar to known Taurus stars while the other half exhibit surface gravity signatures similar to members of the somewhat older Upper Sco, TW Hya and Beta Pic associations. The pre-main sequence stars are spread over ~35 deg, and many are located well outside of previously explored regions. From assessment of the spatial and proper motion distributions, we argue that the new pre-main sequence stars identified far from the clouds cannot have originated from the vicinity of the 1-2 Myr-old subclusters which contain the bulk of the identified Taurus members, but instead represent a newly-identified area of recent star-formation near the clouds.

The Accretion Disk of the Lithium-depleted Young Binary St 34

We presented the infrared spectrum of the young binary system St 34 obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The IRS spectrum clearly shows excess dust emission, consistent with the suggestion of White & Hillenbrand that St 34 is accreting from a circumbinary disk. The disk emission of St 34 is low in comparison with the levels observed in typical T Tauri stars; silicate features at 10 and 20 microns are much weaker than typically seen in T Tauri stars; and excess emission is nearly absent at the shortest wavelengths observed. These features of the infrared spectrum suggest substantial grain growth (to eliminate silicate features) and possible settling of dust to the disk midplane (to reduce the continuum excess emission levels), along with a relatively evacuated inner disk, as expected due to gravitational perturbations by the binary system. Although the position of St 34 in the H-R diagram suggests an age of 8f Myr, assuming that it lies at the distance of the Taurus-Auriga molecular clouds, White & Hillenbrand could not detect any Li I absorption, which would indicate a Li depletion age of roughly 25 Myr or more. We suggest that St 34 is closer than the Taurus clouds by about 30-40 pc and has an age roughly consistent with Li depletion models. Such an advanced age would make St 34 the oldest known low-mass pre-main-sequence object with a dusty accretion disk. The persistence of optically thick dust emission well outside the binary orbit may indicate a failure to make giant planets that could effectively remove dust particles.

On the alignment of Classical T Tauri stars with the magnetic field in the Taurus-Auriga molecular cloud

In this paper we readdress the issue of the alignment of Classical T Tauri stars (CTTS) with the magnetic field in the Taurus-Auriga molecular cloud. Previous studies have claimed that the jet axis of active young stellar objects (YSO), projected in the plane of the sky, is aligned preferentially along the projected direction of the local magnetic field. We re-examine this issue in view of the numerous high angular resolution images of circumstellar disks and micro-jets now available. The images show that T Tauri stars as a group are oriented randomly with respect to the local magnetic field, contrary to previous claims. This indicates that the magnetic field may play a lesser role in the final stages of collapse of an individual prestellar core than previously envisioned. The current database also suggests that a subsample of CTTS with resolved disks but without observations of bright and extended outflows have a tendency to align perpendicularly to the magnetic field. We discuss the possibility that this may trace a less favorable topology, e.g., quadrupolar, for the magnetic field in the inner disk, resulting in a weaker collimated outflow.

The Cradle of the Solar System

The recent discovery of decay products of 60Fe in meteorites challenges conventional wisdom about the environment in which the Sun and planets formed. Rather than a region like the well-studied Taurus-Auriga molecular cloud, the solar system must have formed instead in a region more like the Eagle nebula--a region that contained one or more massive stars that went supernova, injecting newly synthesized radionuclides into the nascent solar system. In their Perspective, Hester et al. discuss a scenario by which the solar system--and other low-mass stars like the Sun--could have formed. Radiant energy from massive, luminous stars first compresses surrounding interstellar gas, triggering the formation of Sun-like stars, then quickly disperses this material, exposing newborn stars and their protoplanetary disks to harsh radiation from the massive stars. When the massive stars go supernova, they pelt surrounding protoplanetary disks with ejecta laden with the products of stellar nucleosynthesis that are required to explain the isotopic composition we see today.

Spectroscopic Survey of the Galactic Open Cluster NGC 6871. I. New Emission-Line Stars

We analyze spectra of 44 emission-line stars detected in a low-resolution optical spectroscopic survey of the galactic open cluster NGC 6871. The survey of 1217 stars is complete to V = 14.9 and includes stars with V < 16.5 between the zero-age main sequence and the 107 yr pre?main-sequence (PMS) isochrone. Of the 44 emission-line stars in this survey, 28 show obvious emission in H? and 16 have weak H? absorption (compared with H?). We use the reddening to separate foreground and background stars from the cluster members; the position in the HR diagram or the presence of forbidden emission lines ([N II] and [S II]) then yields the evolutionary status of the emission-line stars. A comparison of the H? spectral index distribution in NGC 6871 with the distribution of young stars in the Taurus-Auriga molecular cloud indicates that the late-type PMS stars in our sample may be weak-line T Tauri stars. Many of these stars show [S II] (36%) and [N II] (45%) emission.

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.

Barnard’s Merope Nebula Revisited: New Observational Results

IC 349 is a small, fan-shaped reflection nebula located only 30'' from 23 Tau; its nucleus is, by a factor 15, the brightest area of the Pleiades nebulosity. We propose that IC 349 is a fragment of the Taurus-Auriga molecular cloud that has been encountered by the Pleiades in that cluster's southward motion and is being illuminated and shaped by the radiation field of 23 Tau. New Hubble Space Telescope multicolor imagery and the structure, colors, and surface brightness of IC 349 are discussed in terms of that hypothesis. What is known of the proper motion of the nebula, what can be inferred of the properties of the nebula from its color, and what is expected from radiation pressure theory appear to be compatible with this cloudlet-encounter hypothesis.

A Large-Scale Objective-Prism and X-Ray Survey in Taurus-Auriga

We present the results of a moderately deep, wide-field optical/X-ray survey extending over ~60° in the Taurus-Auriga molecular cloud complex. Our observations are sensitive enough to allow us to detect lower mass and older pre–main-sequence stars that could have been missed in previous surveys. We identify four new T Tauri stars, two in the area of L1500 and L1503, one in L1538, and one in L1544. X-rays were detected from three near-IR sources, one of them a very reddened Class II T Tauri star. Four X-ray sources have no known optical/IR known counterpart; they could be very reddened pre–main-sequence stars. Our findings are consistent with the idea that little (if any) star formation has taken place in L1537 and L1538. Although we found one slightly older T Tauri star (~3–4 Myr old), our survey yields no evidence for any significant number of T Tauri stars, regardless of mass and age, outside the main groups of young stars in Taurus. We conclude that the M stars comprising the low-mass end of the ROSAT All-Sky Survey (RASS) source population must be older than ~10 Myr and most probably unrelated to the ongoing process of star formation in Taurus. Instead, the bulk of the RASS sources seen toward regions like Taurus seem to trace the history of star formation over a longer period in the solar vicinity.

Newly discovered candidate weak-line T Tauri stars in the surrounding area of the Taurus-Auriga region

We present results of an extensive search for weak-line T Tauri stars (WTTS) in the outskirts of the Taurus-Auriga molecular cloud on the basis of the ROSAT All-Sky Survey Bright Source Catalog. Our surveyed region extends from 2h 40m to 5h 40m in right ascension and from 10° to 40° in declination, with the central part of Taurus-Auriga (4 5h , 15 34°), accomplished by Wichmann et al. (1996), excluded. Within a sky coverage of about 103 square degrees, 219 X-ray sources fulfil the criteria for selecting program sources suggested by Neuhauser et al. (1995a), and 164 of these X-ray sources were found to have at least one optical counterpart with E magnitude brighter than 16. Low-resolution spectroscopic observation has been carried out in order to discard early type stars and galaxies from the sample, additional intermediate-resolution spectra of a sub-sample of 156 late type optical counterparts were obtained for spectral classification and for the calculation of the equivalent width of H emission and LiI line absorption at 6707 A. Excluding 2 previously identified WTTS, a total of 75 new candidate WTTS and one possible classical T Tauri star have been discovered in our study. The majority of the newly found Li-rich optical counterparts are believed to be PMS stars rather than ZAMS stars as those of the Pleiades.

The Bright Accretion Rings on Magnetic T Tauri Stars

We model the properties of bright rings on accreting classical T Tauri stars whose magnetospheres are tilted with respect to the axis of rotation. These rings have interesting geometries and azimuthal intensity profiles depending on β, the inclination angle of the dipole field, ξ, the ratio of the star's radius to the accretion radius, and , the ratio of the star's luminosity to the total ring luminosity. As the star rotates, the rings may be occulted periodically, and the apparent brightness of the star may vary as a function of time. With knowledge of i, the observer's inclination, it is possible to predict Δm, the amplitude of magnitude variations due to the occultation. Conversely, measurements of Δm and either i or can constrain the values of the other variables. To observe any variation i must be greater than zero. Stars viewed nearly pole-on (i ≈ 10°) must have β 50° to allow any significant variation. On the other hand, with i 60°, even a very small β ≈ 5° can produce a significant Δm. We find that stars that are as bright as their accretion rings ( ≈ 1) cannot produce variations greater than ≈ 0.8 mag; to observe 2 mag variations ≈ 0.1 is required, independently of i or β. We predict a distribution of Δm that is consistent with observed populations of T Tauri stars in the Orion Nebula and in the Taurus-Auriga Molecular Cloud.

A survey of optical and near-infrared jets in taurus embedded sources.

view Abstract Citations (64) References (129) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A survey of optical and near-infrared jets in taurus embedded sources. Gomez, Mercedes ; Whitney, Barbara A. ; Kenyon, Scott J. Abstract We describe optical ([S II], 6732 Å) and near-infrared (H2, 2.122 μm) imaging observations of 22 embedded protostars in the Taurus-Auriga molecular cloud. Our survey identifies 4 new [S II] emission sources and 4 new H2 sources. We detect three emission knots in both [S II] and H2 one object with H2 emission has no [S II] emission. These results increase the number of class I sources with optical or near-IR outflows to 13. We also detect [S II] emission in two additional class II sources, ZZ Tau IRS and IC 2087 IRS. When combined with previous results, the frequency of optical outflows among class H sources, ∼10%, is much smaller than the frequency among class I sources, 60%. The large difference in the frequency of optical and molecular outflows indicates that outflow activity decreases with age among pre-main-sequence stars. The jets in the Taurus-Auriga protostars are poorly collimated, with multiple knots indicative of large opening angles, ∼10°-40°. The image morphologies are complex and suggest a time dependence in the direction of ejection. A simple precessing jet can account for the sinusoidal chain of [S II] knots-HH 31-associated with 04248+2612. The precessing jets, together with the good alignment between the axes of the jets, molecular outflows, and reflection nebulae, support current models where jets produce molecular outflows and bipolar cavities in the dense infalling cloud cores surrounding class I sources. Publication: The Astronomical Journal Pub Date: September 1997 DOI: 10.1086/118545 Bibcode: 1997AJ....114.1138G Keywords: SURVEYS; INFRARED: STARS; STARS: PRE-MAIN SEQUENCE; ISM: JETS AND OUTFLOWS full text sources ADS | data products SIMBAD (87)