A Long-Lived Accretion Disk Around a Lithium-Depleted Binary T Tauri Star

We present a high dispersion optical spectrum of St 34 and identify the system as a spectroscopic binary with components of similar luminosity and temperature (both M3+/-0.5). Based on kinematics, signatures of accretion, and location on an H-R diagram, we conclude that St 34 is a classical T Tauri star belonging to the Taurus-Auriga T Association. Surprisingly, however, neither component of the binary shows LiI 6708 A, absorption, the most universally accepted criterion for establishing stellar youth. In this uniquely known instance, the accretion disk appears to have survived longer than the lithium depletion timescale. We speculate that the long-lived accretion disk is a consequence of the sub-AU separation companion tidally inhibiting, though not preventing, circumstellar accretion. Comparisons with pre-main sequence evolutionary models imply, for each component of St 34, a mass of 0.37+/-0.08 Msun and an isochronal age of 8+/-3 Myr, which is much younger than the predicted lithium depletion timescale of ~ 25 Myr. Although a distance 38% closer than that of Taurus-Auriga or a hotter temperature scale could reconcile this discrepancy at 21-25 Myr, similar discrepancies in other systems and the implications of an extremely old accreting Taurus-Auriga member suggest instead a possible problem with evolutionary models. Regardless, the older age implied by St 34's depleted lithium abundance is the first compelling evidence for a substantial age spread in this region. Additionally, since St 34's coeval co-members with early M spectral types would likewise fail the lithium test for youth, current membership lists may be incomplete.Comment: 4 pages, including 2 figures. Accepted for publication in ApJ Let

NICMOS Images of the GG Tau Circumbinary Disk

We present deep, near-infrared images of the circumbinary disk surrounding the pre-main-sequence binary star, GG Tau A, obtained with NICMOS aboard the Hubble Space Telescope. The spatially resolved proto-planetary disk scatters roughly 1.5% of the stellar flux, with a near-to-far side flux ratio of ~1.4, independent of wavelength, and colors that are comparable to the central source; all of these properties are significantly different from the earlier ground-based observations. New Monte Carlo scattering simulations of the disk emphasize that the general properties of the disk, such as disk flux, near side to far side flux ratio and integrated colors, can be approximately reproduced using ISM-like dust grains, without the presence of either circumstellar disks or large dust grains, as had previously been suggested. A single parameter phase function is fitted to the observed azimuthal variation in disk flux, providing a lower limit on the median grain size of 0.23 micron. Our analysis, in comparison to previous simulations, shows that the major limitation to the study of grain growth in T Tauri disk systems through scattered light lies in the uncertain ISM dust grain properties. Finally, we use the 9 year baseline of astrometric measurements of the binary to solve the complete orbit, assuming that the binary is coplanar with the circumbinary ring. We find that the estimated 1 sigma range on disk inner edge to semi-major axis ratio, 3.2 < Rin/a < 6.7, is larger than that estimated by previous SPH simulations of binary-disk interactions.Comment: 40 pages, 8 postscript figures, accepted for publication in Ap

Accretion-powered chromospheres in classical T Tauri stars

(Abridged) Optical spectra of classical T Tauri stars (cTTS) are rich in emission lines of low-excitation species that are composed of narrow and broad components, related to two regions with different kinematics, densities, and temperatures. The photospheric spectrum is often veiled by an excess continuous emission. This veiling is usually attributed to radiation from a heated region beneath the accretion shock. The aim of this research is to clarify the nature of the veiling, and whether the narrow chromospheric lines of Fe I and other metals represent a standard chromosphere of a late-type star, or are induced by mass accretion. From high-resolution spectroscopy of DR Tauri we found that the amount of veiling in this star varies from practically nothing to factors more than 10 times the stellar continuum intensity, and that the veiling is caused by both a non-photospheric continuum and chromospheric line emission filling in the photospheric absorption lines. This effect can be shown to exist in several other T Tauri stars. We conclude that enhanced chromospheric emission in cTTS is linked not only to solar-like magnetic activity, but is powered to a greater extent by the accreting gas. We suggest that the area of enhanced chromospheric emission is induced by mass accretion, which modifies the local structure of stellar atmosphere in an area that is more extended than the hot accretion spot. The narrow emission lines from this extended area are responsible for the extra component in the veiling through line-filling of photospheric absorption lines.Comment: 11 pages, 13 figure

Searching for sub-stellar companion into the LkCa15 proto-planetary disk

Recent sub-millimetric observations at the Plateau de Bure interferometer evidenced a cavity at ~ 46 AU in radius into the proto-planetary disk around the T Tauri star LkCa15 (V1079 Tau), located in the Taurus molecular cloud. Additional Spitzer observations have corroborated this result possibly explained by the presence of a massive (>= 5 MJup) planetary mass, a brown dwarf or a low mass star companion at about 30 AU from the star. We used the most recent developments of high angular resolution and high contrast imaging to search directly for the existence of this putative companion, and to bring new constraints on its physical and orbital properties. The NACO adaptive optics instrument at VLT was used to observe LkCa15 using a four quadrant phase mask coronagraph to access small angular separations at relatively high contrast. A reference star at the same parallactic angle was carefully observed to optimize the quasi-static speckles subtraction (limiting our sensitivity at less than 1.0). Although we do not report any positive detection of a faint companion that would be responsible for the observed gap in LkCa15's disk (25-30 AU), our detection limits start constraining its probable mass, semi-major axis and eccentricity. Using evolutionary model predictions, Monte Carlo simulations exclude the presence of low eccentric companions with masses M >= 6 M Jup and orbiting at a >= 100 AU with significant level of confidence. For closer orbits, brown dwarf companions can be rejected with a detection probability of 90% down to 80 AU (at 80% down to 60 AU). Our detection limits do not access the star environment close enough to fully exclude the presence of a brown dwarf or a massive planet within the disk inner activity (i.e at less than 30 AU). Only, further and higher contrast observations should unveil the existence of this putative companion inside the LkCa15 disk.Comment: 6 pages, 4 figures, accepted for publication in A&

The First Detection of Spatially Resolved Mid-Infrared Scattered Light from a Protoplanetary Disk

We report spatially resolved 11.8 micron images, obtained at the W. M. Keck 10 m telescope, of the protoplanetary disk around the pre--main-sequence star HK Tau B. The mid-infrared morphology and astrometry of HK Tau B with respect to HK Tau A indicate that the flux observed in the mid-infrared from HK Tau B has been scattered off the upper surface of its nearly edge-on disk. This is the first example of a protoplanetary disk observed in scattered light at mid-infrared wavelengths. Monte Carlo simulations of this disk show that the extent (FWHM =0."5, or 70 AU) of the scattered light nebula in the mid-infrared is very sensitive to the dust size distribution. The 11.8 micron measurement can be best modelled by a dust grain population that contains grains on the order of 1.5-3 micron in size; grain populations with exclusively sub-micron grain sizes or power law size distributions that extend beyond 5 micron cannot reproduce the observed morphology. These grains are significantly larger than those expected in the ISM implying that grain growth has occurred; whether this growth is a result of dust evolution within the disk itself or had originally occurred within the dark cloud remains an open question.Comment: 11 pages, 1 postscript figure, accepted for publication in ApJ

Multi-Generational Star Formation in L1551

The L1551 molecular cloud contains two small clusters of Class 0 and I protostars, as well as a halo of more evolved Class II and III YSOs, indicating a current and at least one past burst of star formation. We present here new, sensitive maps of 850 and 450 um dust emission covering most of the L1551 cloud, new CO J=2-1 data of the molecular cloud, and a new, deep, optical image of [SII] emission. No new Class 0/I YSOs were detected. Compact sub-millimetre emitters are concentrated in two sub-clusters: IRS5 and L1551NE, and the HL~Tauri group. Both stellar groups show significant extended emission and outflow/jet activity. A jet, terminating at HH 265 and with a very weak associated molecular outflow, may originate from LkHa 358, or from a binary companion to another member of the HL Tauri group. Several Herbig Haro objects associated with IRS5/NE were clearly detected in the sub-mm, as were faint ridges of emission tracing outflow cavity walls. We confirm a large-scale molecular outflow originating from NE parallel to that from IRS5, and suggest that the "hollow shell" morphology is more likely due to two interacting outflows. We confirm the presence of a prestellar core (L1551-MC) of mass 2-3 Mo north-west of IRS5. The next generation cluster may be forming in this core. The L1551 cloud appears cometary in morphology, and appears to be illuminated and eroded from the direction of Orion, perhaps explaining the multiple episodes of star formation in this cloud. The full paper (including figures) can be downloaded at http://www.jach.hawaii.edu/~gms/l1551/l1551-apj641.pdf, or viewed at http://www.jach.hawaii.edu/~gms/l1551/.Comment: Accepted for publication in The Astrophysical Journal, April 2006 (vol. 641). 27 pages, 17 figure

VLBA determination of the distance to nearby star-forming regions II. Hubble 4 and HDE 283572 in Taurus

The non-thermal 3.6 cm radio continuum emission from the naked T Tauri stars Hubble 4 and HDE 283572 in Taurus has been observed with the Very Long Baseline Array (VLBA) at 6 epochs between September 2004 and December 2005 with a typical separation between successive observations of 3 months. Thanks to the remarkably accurate astrometry delivered by the VLBA, the trajectory described by both stars on the plane of the sky could be traced very precisely, and modeled as the superposition of their trigonometric parallax and uniform proper motion. The best fits yield distances to Hubble 4 and HDE 283572 of 132.8 +/- 0.5 and 128.5 +/- 0.6 pc, respectively. Combining these results with the other two existing VLBI distance determinations in Taurus, we estimate the mean distance to the Taurus association to be 137 pc with a dispersion (most probably reflecting the depth of the complex) of about 20 pc.Comment: 21 pages, 4 figues, accepted in ApJ (Dec 20, 2007 issue

A Parallactic Distance of 389 +24/-21 parsecs to the Orion Nebula Cluster from Very Long Baseline Array Observations

We determine the parallax and proper motion of the flaring, non-thermal radio star GMR A, a member of the Orion Nebula Cluster, using Very Long Baseline Array observations. Based on the parallax, we measure a distance of 389 +24/-21 parsecs to the source. Our measurement places the Orion Nebula Cluster considerably closer than the canonical distance of 480 +/- 80 parsecs determined by Genzel et al. (1981). A change of this magnitude in distance lowers the luminosities of the stars in the cluster by a factor of ~ 1.5. We briefly discuss two effects of this change--an increase in the age spread of the pre-main sequence stars and better agreement between the zero-age main-sequence and the temperatures and luminosities of massive stars.Comment: 10 pages, 4 figures, emulateapj, accepted to Ap

Possible detection of a magnetic field in T Tauri

Medium-resolution $(R\simeq 15000)$ circular spectropolarimetry of T Tauri is presented. The star was observed twice: on November 11, 1996 and January 22, 2002. Weak circular polarization has been found in photospheric absorption lines, indicating a mean surface longitudinal magnetic field $B_{\|}$ of $160\pm 40$ G and $140\pm 50$ G at the epoch of the first and second observations respectively. While these values are near the detection limit of our apparatus, we belive that they are real. In any case one can conclude from our data that $B_{\|}$ of T Tau does not significantly exceed 200 G, which is much less than surface magnetic field strength of the star ($>2.3$ kG) found by Guenther et al. (1999) and Johns-Krull et al. (2000). We discuss possible reasons of this difference.Comment: 5 pages, 3 figure

VLBA determination of the distance to nearby star-forming regions I. The distance to T Tauri with 0.4% accuracy

In this article, we present the results of a series of twelve 3.6-cm radio continuum observations of T Tau Sb, one of the companions of the famous young stellar object T Tauri. The data were collected roughly every two months between September 2003 and July 2005 with the Very Long Baseline Array (VLBA). Thanks to the remarkably accurate astrometry delivered by the VLBA, the absolute position of T Tau Sb could be measured with a precision typically better than about 100 micro-arcseconds at each of the twelve observed epochs. The trajectory of T Tau Sb on the plane of the sky could, therefore, be traced very precisely, and modeled as the superposition of the trigonometric parallax of the source and an accelerated proper motion. The best fit yields a distance to T Tau Sb of 147.6 +/- 0.6 pc. The observed positions of T Tau Sb are in good agreement with recent infrared measurements, but seem to favor a somewhat longer orbital period than that recently reported by Duchene et al. (2006) for the T Tau Sa/T Tau Sb system.Comment: 24 pages, 3 pages, AASTEX format, accepted for publication in Ap