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Post Info TOPIC: T Tauri stars


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Posts: 131433
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RE: T Tauri stars
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Title: Hot Gas Lines in T Tauri Stars
Authors: David R. Ardila (1), Gregory J. Herczeg (2), Scott G. Gregory (3,4), Laura Ingleby (5), Kevin France (6), Alexander Brown (6), Suzan Edwards (7), Christopher Johns-Krull (8), Jeffrey L. Linsky (9), Hao Yang (10), Jeff A. Valenti (11), Hervé Abgrall (12), Richard D. Alexander (13), Edwin Bergin (5), Thomas Bethell (5), Joanna M. Brown (14), Nuria Calvet (5), Catherine Espaillat (14), Lynne A. Hillenbrand (3), Gaitee Hussain (15), Evelyne Roueff (12), Eric R Schindhelm (16), Frederick M. Walter (17) ((1) NASA Herschel Science Center, USA, (2) Peking University, China, (3) California Institute of Technology, USA, (4) University of St Andrews, UK, (5) University of Michigan, USA, (6) University of Colorado, Boulder, USA, (7) Smith College, USA, (8) Rice University, USA, (9) JILA, USA, (10) Central China Normal University, China, (11) Space Telescope Science Institute, USA, (12) Observatoire de Paris, France, (13) University of Leicester, UK, (14) Harvard-Smithsonian Center for Astrophysics, USA, (15) ESO, Germany, (16) Southwest Research Institute, USA, (17) Stony Brook University, USA)

For Classical T Tauri Stars (CTTSs), the resonance lines of N V, Si IV, and C IV, as well as the He II 1640 A line, act as diagnostics of the accretion process. Here we assemble a large high-resolution dataset of these lines in CTTSs and Weak T Tauri Stars (WTTSs). We present data for 35 stars: one Herbig Ae star, 28 CTTSs, and 6 WTTSs. We decompose the C IV and He II lines into broad and narrow Gaussian components (BC & NC). The most common (50 %) C IV line morphology in CTTSs is that of a low-velocity NC together with a redshifted BC. The velocity centroids of the BCs and NCs are such that V_BC > 4 * V_NC, consistent with the predictions of the accretion shock model, in at most 12 out of 22 CTTSs. We do not find evidence of the post-shock becoming buried in the stellar photosphere due to the pressure of the accretion flow. The He II CTTSs lines are generally symmetric and narrow, less redshifted than the CTTSs C IV lines, by ~10 km/sec. The flux in the BC of the He II line is small compared to that of the C IV line, consistent with models of the pre-shock column emission. The observations are consistent with the presence of multiple accretion columns with different densities or with accretion models that predict a slow-moving, low-density region in the periphery of the accretion column. For HN Tau A and RW Aur A, most of the C IV line is blueshifted suggesting that the C IV emission is produced by shocks within outflow jets. In our sample, the Herbig Ae star DX Cha is the only object for which we find a P-Cygni profile in the C IV line, which argues for the presence of a hot (10^5 K) wind. For the overall sample, the Si IV and N V line luminosities are correlated with the C IV line luminosities, although the relationship between Si IV and C IV shows large scatter about a linear relationship and suggests that TW Hya, V4046 Sgr, AA Tau, DF Tau, GM Aur, and V1190 Sco are silicon-poor.

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Title: Bayesian inference of T Tauri star properties using multi-wavelength survey photometry
Authors: Geert Barentsen, J.S. Vink, J. E. Drew, S. E. Sale

There are many pertinent open issues in the area of star and planet formation. Large statistical samples of young stars across star-forming regions are needed to trigger a breakthrough in our understanding, but most optical studies are based on a wide variety of spectrographs and analysis methods, which introduces large biases.
Here we show how graphical Bayesian networks can be employed to construct a hierarchical probabilistic model which allows pre-main sequence ages, masses, accretion rates, and extinctions to be estimated using two widely available photometric survey databases (IPHAS r'/H/i' and 2MASS J-band magnitudes). Because our approach does not rely on spectroscopy, it can easily be applied to homogeneously study the large number of clusters for which Gaia will yield membership lists. We explain how the analysis is carried out using the Markov Chain Monte Carlo (MCMC) method and provide Python source code. We then demonstrate its use on 587 known low-mass members of the star-forming region NGC2264 (Cone Nebula), arriving at a median age of 3.0 Myr, an accretion fraction of 20 ± 2% and a median accretion rate of 10-8.4 solar masses/yr.
The Bayesian analysis formulated in this work delivers results which are in agreement with spectroscopic studies already in the literature, but achieves this with great efficiency by depending only on photometry. It is a significant step forward from previous photometric studies, because the probabilistic approach ensures that nuisance parameters, such as extinction and distance, are fully included in the analysis with a clear picture on any degeneracies.

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Title: A Search for Giant Planet Companions to T Tauri Stars
Authors: Christopher J. Crockett (1), Naved I. Mahmud (2), L. Prato (3), Christopher M. Johns-Krull (2), Daniel T. Jaffe (4), Patrick M. Hartigan (2), Charles A. Beichman (5,6) ((1) USNO, (2) Lowell Observatory, (3) Rice University, (4) University of Texas at Austin, (5) JPL, (6) NExScI)

We present results from an ongoing multiwavelength radial velocity (RV) survey of the Taurus-Auriga star forming region as part of our effort to identify pre--main sequence giant planet hosts. These 1-3 Myr old T Tauri stars present significant challenges to traditional RV surveys. The presence of strong magnetic fields gives rise to large, cool star spots. These spots introduce significant RV jitter which can mimic the velocity modulation from a planet-mass companion. To distinguish between spot-induced and planet-induced RV modulation, we conduct observations at ~6700 Angstroms and ~2.3 microns and measure the wavelength dependence (if any) in the RV amplitude. CSHELL observations of the known exoplanet host Gl 86 demonstrate our ability to detect not only hot Jupiters in the near infrared but also secular trends from more distant companions. Observations of nine very young stars reveal a typical reduction in RV amplitude at the longer wavelengths by a factor of ~2-3. While we can not confirm the presence of planets in this sample, three targets show different periodicities in the two wavelength regions. This suggests different physical mechanisms underlying the optical and K band variability.

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Title: A Young Planet Search in Visible and IR Light: DN Tau, V836 Tau, and V827 Tau
Authors: L. Prato, M. Huerta, C. M. Johns-Krull, N. Mahmud, D. T. Jaffe, P. Hartigan

In searches for low-mass companions to late-type stars, correlation between radial velocity variations and line bisector slope changes indicates contamination by large starspots. Two young stars demonstrate that this test is not sufficient to rule out starspots as a cause of radial velocity variations. As part of our survey for substellar companions to T Tauri stars, we identified the ~2 Myr old planet host candidates DN Tau and V836 Tau. In both cases, visible light radial velocity modulation appears periodic and is uncorrelated with line bisector span variations, suggesting close companions of several M_Jup in these systems. However, high-resolution, infrared spectroscopy shows that starspots cause the radial velocity variations. We also report unambiguous results for V827 Tau, identified as a spotted star on the basis of both visible light and infrared spectroscopy. Our results suggest that infrared follow up observations are critical for determining the source of radial velocity modulation in young, spotted stars.

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Title: Accretion properties of T Tauri stars in sigma Ori
Authors: T. Gatti, A. Natta, S. Randich, L. Testi, G. Sacco

Accretion disks around young stars evolve in time with time scales of few million years. We present here a study of the accretion properties of a sample of 35 stars in the ~3 million year old star-forming region sigma Ori. Of these, 31 are objects with evidence of disks, based on their IR excess emission. We use near-IR hydrogen recombination lines (Pa_gamma) to measure their mass accretion rate. We find that the accretion rates are significantly lower in sigma Ori than in younger regions, such as rho-Oph, consistently with viscous disk evolution. The He I 1.083 micron line is detected (either in absorption or in emission) in 72% of the stars with disks, providing evidence of accretion-powered activity also in very low accretors, where other accretion indicators disappear.

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Title: Interacting coronae of two T Tauri stars: first observational evidence for solar-like helmet streamers
Authors: M. Massi, E. Ros, K. M. Menten, M. Kaufman Bernado', G. Torricelli-Ciamponi, J. Neidhofer, A. Boden, D. Boboltz, A. Sargent, G.Torres

Context:  The young binary system V773 Tau A exhibits a persistent radio flaring activity that gradually increases from a level of a few mJy at apoastron to more than 100 mJy at periastron. Interbinary collisions between very large (> 15 R) magnetic structures anchored on the two rotating stars of the system have been proposed to be the origin of these periodic radio flares. Magnetic structures extended over tens of stellar radii, that can also account for the observed fast decay of the radio flares, seem to correspond to the typical solar semi-open quite extended magnetic configurations called helmet streamers.
Aims:  We aim to find direct observational evidence for the postulated, solar-like, coronal topologies.
Methods: We performed seven-consecutive-day VLBI observations at 8.4 GHz using an array consisting of the VLBA and the 100-m Effelsberg telescope.
Results: Two distinctive structures appear in the radio images here presented. They happen to be associated with the primary and secondary stars of the V773 Tau A system. In one image (Fig.2-B) the two features are extended up to 18 R each and are nearly parallel revealing the presence of two interacting helmet streamers. One image (Fig.2-E) taken a few hours after a flare monitored by the 100-m Effelsberg telescope shows one elongated fading structure substantially rotated with respect to those seen in the B run. The same decay scenario is seen in Fig.2-G for the helmet streamer associated with the other star.
Conclusions: This is the very first direct evidence revealing that even if the flare origin is magnetic reconnection due to interbinary collision, both stars independently emit in the radio range with structures of their own. These structures are helmet streamers, observed for the first time in stars other than the Sun.

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Posts: 131433
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V773 Tau A
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Title: Dynamical Masses for Pre-Main Sequence Stars: A Preliminary Physical Orbit for V773 Tau A
Authors: A.F. Boden, G. Torres, A.I. Sargent, R.L. Akeson, J.M. Carpenter, D.A. Boboltz, M. Massi, A. M. Ghez, D.W. Latham, K.J Johnston, K.M. Menten, E. Ros

We report on interferometric and radial-velocity observations of the double-lined 51-d period binary (A) component of the quadruple pre-main sequence (PMS) system V773 Tau. With these observations we have estimated preliminary visual and physical orbits of the V773 Tau A subsystem. Among other parameters, our orbit model includes an inclination of 66.0 ± 2.4 deg, and allows us to infer the component dynamical masses and system distance. In particular we find component masses of 1.54 ± 0.14 and 1.332 ± 0.097 M_{\sun} for the Aa (primary) and Ab (secondary) components respectively.
Our modelling of the subsystem component spectral energy distributions finds temperatures and luminosities consistent with previous studies, and coupled with the component mass estimates allows for comparison with PMS stellar models in the intermediate-mass range. We compare V773 Tau A component properties with several popular solar-composition models for intermediate-mass PMS stars. All models predict masses consistent to within 2-sigma of the dynamically determined values, though some models predict values that are more consistent than others.

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Posts: 131433
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2MASS J16281370-2431391
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Title: Deep Spitzer spectroscopy of the `Flying Saucer' edge-on disk: Large grains beyond 50 AU
Authors: Klaus M. Pontoppidan, Karl R. Stapelfeldt, Geoffrey A. Blake, Ewine F. van Dishoeck, Cornelis P. Dullemond

We present deep Spitzer-IRS low-resolution (lambda/Delta lambda ~ 100) 5-35 micron spectroscopy of the edge-on disk  ''the Flying Saucer'' (2MASS J16281370-2431391) in the Ophiuchus molecular cloud. The spectral energy distribution exhibits the characteristic two-peak shape predicted for a circumstellar disk viewed very close to edge-on. The short-wavelength peak is entirely due to photons scattered off the surface of the disk, while the long-wavelength peak is due to thermal emission from the disk itself. The Spitzer spectrum represents the first spectroscopic detection of scattered light out to 15 micron from a bona-fide, isolated edge-on disk around a T Tauri star. The depth and the wavelength of the mid-infrared "valley" of the SED give direct constraints on the size distribution of large grains in the disk. Using a 2D continuum radiative transfer model, we find that a significant amount of 5-10 micron-sized grains is required in the surface layers of the disk at radii of 50-300 AU. The detection of relatively large grains in the upper layers implies that vertical mixing is effective, since grain growth models predict the grains would otherwise settle deep in the disk on short time scales. Additionally, we tentatively detect the 9.66 micron S(3) line of H2 and the 11.2 micron emission feature due to PAHs.

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Posts: 131433
Date:
T Tauri stars
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Title: Spitzer-IRS spectra of disks around T Tauri stars II. PAH emission features
Authors: V.C. Geers, J.-C. Augereau, K.M. Pontoppidan, C.P. Dullemond, R. Visser, J.E. Kessler-Silacci, N.J. Evans II, E.F. van Dishoeck, G.A. Blake, A.C.A. Boogert, J.M. Brown, F. Lahuis, B. Merin

Aims: We search for PAH features towards T Tauri stars and compare them with surveys of Herbig Ae/Be stars. The presence and strength of the PAH features are interpreted with disk radiative transfer models exploring the PAH feature dependence on the incident UV radiation, PAH abundance and disk parameters.
Methods: Spitzer Space Telescope 5-35 micron spectra of 54 pre-main sequence stars with disks were obtained, consisting of 38 T Tauri, 7 Herbig Ae/Be and 9 stars with unknown spectral type.
Results: Compact PAH emission is detected towards at least 8 sources of which 5 are Herbig Ae/Be stars. The 11.2 micron PAH feature is detected in all of these sources, as is the 6.2 micron PAH feature where short wavelength data are available. However, the 7.7 and 8.6 micron features appear strongly in only 1 of these 4 sources. PAH emission is observed towards at least 3 T Tauri stars (8% detection rate). The lowest mass source with PAHs in our sample is T Cha (G8). All 4 sources in our sample with evidence for dust holes in their inner disk show PAH emission, increasing the feature/continuum ratio. Typical 11.2 micron line intensities are an order of magnitude lower than those observed for the more massive Herbig Ae/Be stars. Measured line fluxes indicate PAH abundances that are factors of 10-100 lower than standard interstellar values. Conversely, PAH features from disks exposed to stars with Teff < =4200K without enhanced UV are predicted to be below the current detection limit, even for high PAH abundances. Disk modelling shows that the 6.2 and 11.2 micron features are the best PAH tracers for T Tauri stars, whereas the 7.7 and 8.6 micron bands have low feature over continuum ratios due to the strongly rising silicate emission.

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