arXiv subjects

T. P. Ray

Publications and source records attributed to T. P. Ray.

81 records · Page 5Linked to original sources

arXiv2005

A Galactic Plane Relative Extinction Map from 2MASS

We present three 14400 square degree relative extinction maps of the Galactic Plane (|b|<20degrees) obtained from 2MASS using accumulative star counts (Wolf diagrams). This method is independent of the colour of the stars and the variation of extinction with wavelength. Stars were counted in 3.5'x3.5' boxes, every 20". 1x1degree surrounding fields were chosen for reference, hence the maps represent local extinction enhancements and ignore any contribution from the ISM or very large clouds. Data reduction was performed on a Beowulf-type cluster (in approximately 120 hours). Such a cluster is ideal for this type of work as areas of the sky can be independently processed in parallel. We studied how extinction depends on wavelength in all of the high extinction regions detected and within selected dark clouds. On average a power law opacity index (β) of 1.0 to 1.8 in the NIR was deduced. The index however differed significantly from region to region and even within individual dark clouds. That said, generally it was found to be constant, or to increase, with wavelength within a particular region.

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arXiv2004

Classical T Tauri stars as sources of parsec-scale optical outflows

Previous studies of young stellar objects (YSOs) have uncovered a number of associated parsec-scale optical outflows, the majority of which are driven by low-mass, embedded Class I sources. Here we examine more evolved Classical T Tauri stars (CTTSs), i.e. Class II sources, to determine whether these are also capable of driving parsec-scale outflows. Five such sources are presented here - CW Tau, DG Tau, DO Tau, HV Tau C and RW Aur, all of which show optical evidence for outflows of the order of 1pc (24' at the distance of Taurus-Auriga). These sources were previously known only to drive "micro-jets" or small-scale outflows < 1' in length. A parsec-scale outflow from a less evolved source (DG Tau B) which was noted in the course of this work is also included here. Examination of the five newly discovered large-scale outflows from CTTSs shows that they have comparable morphologies, apparent dynamical timescales and degrees of collimation to those from less evolved sources. There is also strong evidence that these outflows have blown out of their parent molecular clouds. Finally we note that the "fossil record" provided by these outflows suggests their sources could have undergone FU Orionis-type outbursts in the past.

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arXiv2003

Paschen beta emission as a tracer of outflow activity from T-Tauri stars, as compared to Optical Forbidden emission

The Paschen beta (1.2822 micron) emission line found in the near-infrared spectra of T-Tauri stars (TTSs) is believed to trace the accretion of material onto the central star. We present spectroscopic data which suggests that this may not always be the case. The technique of spectro-astrometry is used by us to measure positional displacements in the Pab emission from four T-Tauri stars, namely DG Tau, V536 Aql, LkHalpha 321 and RW Aur. We also observed the optical forbidden emission from these sources. Forbidden emission lines are formed in the outflows that accompany the evolution of protostars and so are ideal to use as a comparison to confirm that the measured offsets in the Pab emission are indeed due to outflowing material. Models based on the magnetospheric accretion theory have been the most successful to date in explaining the origin of atomic hydrogen emission lines. Yet we see that the line profiles of the sources showing displacement in their Pab emission all have features that the magnetospheric accretion model has so far failed to explain, such as broad full width half maxima, large wings and an absence of red shifted absorption features. The failure of the models to explain the presence of large extended wings in the line profiles is particularly interesting in the context of this study as in all cases it is in the extended wings that we measure offsets in position with respect to the source.

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arXiv2003

Detection of a warm molecular wind in DG Tauri

We detect near-infrared H2 emission in DG Tau using the Infrared Camera and Spectrograph (IRCS) on the 8.2-m SUBARU telescope. The spectra obtained along the jet axis show that the centroidal position of the 1-0 S(1) emission is offset by 0.2" from the star towards the jet, while those obtained perpendicular to the jet axis show a marginal extension, indicating that the emission line region has a typical width of ~0.6". Their line profiles show a peak velocity of ~15 km s^-1 blueshifted from the systemic velocity. These results indicate that the emission originates from a warm molecular wind with a flow length and width of ~40 and ~80 AU, respectively. The line flux ratios I_{1-0 S(0)}/I_{1-0 S(1)} and an upper limit for I_{2-1 S(1)}/I_{1-0 S(1)}) suggest that the flow is thermalized at a temperature of ~2000 K, and is likely heated by shocks or ambipolar diffusion. The observed velocity and spatial extension suggest that the H2 and forbidden line emission originate from different components of the same flow, i.e., a fast and partially ionised component near the axis and a slow molecular component surrounding it. Such a flow geometry agrees with model predictions of magneto-centrifugal driven winds.

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arXiv2002

The kinematic relationship between disk and jet in the DG Tauri system

We present high angular resolution millimeter wavelength continuum and 13CO(2-1) observations of the circumstellar disk surrounding the TTauri star DG Tauri. We show that the velocity pattern in the inner regions of the disk is consistent with Keplerian rotation about a central 0.67 Msun star. The disk rotation is also consistent with the toroidal velocity pattern in the initial channel of the optical jet, as inferred from HST spectra of the first de-projected 100 AU from the source. Our observations support the tight relationship between disk and jet kinematics postulated by the popular magneto-centrifugal models for jet formation and collimation.

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arXiv2002

HST/STIS Spectroscopy of the Optical Outflow from DG Tau: Indications for Rotation in the Initial Jet Channel

We have carried out a kinematical, high angular resolution (~ 0".1) study of the jet from DG Tau within 0."5 from the source (or 110 AU along this flow). We analysed line profiles extracted from a set of seven spectra taken with STIS on board the Hubble Space Telescope, with the slits parallel to the jet axis but displaced transversely every 0".07. For the flow of moderate velocity (-70 km/s), we have found systematic differences in the radial velocities of lines emitted on alternate sides of the jet axis. The results are corrected for the effects due to uneven illumination of the slit. The relative Doppler shifts range from 5 to 20 km/s. If this is interpreted as rotation, the flow is then rotating clockwise looking from the jet towards the source and the derived toroidal velocities are in the range 6 - 15 km/s. Using recent estimates of the mass loss rate, one obtains for the considered velocity regime, an angular momentum flux of ~ 3.8x10E-5 M_sun/yr AU km/s. Our findings may constitute the first detection of rotation in the initial channel of a jet flow. The derived values appear to be consistent with the predictions of popular magneto-centrifugal jet-launching models, although we cannot exclude transverse outflow asymmetries other than rotation.

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arXiv2002

A Global Jet/Circulation Model for Young Stars

Powerful, highly collimated jets, surrounded by bipolar molecular outflows, are commonly observed near Young Stellar Objects (YSOs). In the usual theoretical picture of star formation, a jet is ejected from a magnetized accretion disk, with a molecular outflow being driven either by the jet or by a wider wind coming from the disk. Here, we propose an alternative global model for the flows surrounding YSOs. In addition to a central accretion-ejection engine driving the jet, the molecular outflow is powered by the infalling matter and follows a circulation pattern around the central object without necessarily being entrained by a jet. It is shown that the model produces a heated pressure-driven outflow with magneto-centrifugal acceleration and collimation. We report solutions for the three different parts of this self-similar model, i.e. the jet, the infalling envelope and the circulating matter that eventually forms the molecular outflow. This new picture of the accretion/outflow phase provides a possible explanation for several observed properties of YSO outflows. The most relevant ones are the presence of high mass molecular outflows around massive protostars, and a realistic fraction (typically 0.1) of the accretion flow that goes into the jet.

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arXiv1999

On the transfer of momentum from stellar jets to molecular outflows

While it is generally thought that molecular outflows from young stellar objects (YSOs) are accelerated by underlying stellar winds or highly collimated jets, the actual mechanism of acceleration remains uncertain. The most favoured model, at least for low and intermediate mass stars, is that the molecules are accelerated at jet-driven bow shocks. Here we investigate, through high resolution numerical simulations, the efficiency of this mechanism in accelerating ambient molecular gas without causing dissociation. The efficiency of the mechanism is found to be surprisingly low suggesting that more momentum may be present in the underlying jet than previously thought. We also compare the momentum transferring efficiencies of pulsed versus steady jets. We find that pulsed jets, and the corresponding steady jet with the same average velocity, transfer virtually the same momentum to the ambient gas. The additional momentum ejected sideways from the jet beam in the case of the pulsed jet only serves to accelerate post-shock jet gas which forms a, largely atomic, sheath around the jet beam. For both the steady and pulsing jets, we find a power law relationship between mass and velocity ($m(v) \propto v^{-γ}$) which is similar to what is observed. We also find that increasing the molecular fraction in the jet decreases $γ$ as one might expect. We reproduce the so-called Hubble law for molecular outflows and show that it is almost certainly a local effect in the presence of a bow shock.

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arXiv1997

Numerical Simulations of the Kelvin-Helmholtz instability in radiatively cooled jets

We present the results of simulations of the development of the Kelvin-Helmholtz (KH) instability in a cooled, slab symmetric system. The parameters were chosen to approximate the physical conditions typically found in jets from young stellar objects (YSOs). The effect of different methods of maintaining the initial equilibrium were examined for varying density. In addition, the effect of adjusting the width of the shear layer between the jet and ambient material was studied and found not to have significant long-term effects on the development of the instability. We find that, in general, cooling acts to: increase the level of mixing between jet and ambient material through the `breaking' of KH induced waves on the surface of the jet; increase the amount of momentum transferred from jet material to ambient material; increase the time taken for shocks to develop in the flow; reduce the strength of these shocks; reduce the rate of decollimation of momentum flux. The first and second of these results appear to contradict the conclusions of Rossi et al. (1997) who carried out a similar study to ours but in cylindrical symmetry. It is found, however, that the differences between slab and cylindrical symmetry, while insignificant in the linear regime, explain the apparent discrepancy between our results and those of Rossi et al.\ (1997) in the non-linear regime.

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