arXiv subjects

T. J. Harries

Publications and source records attributed to T. J. Harries.

At least 19 recordsLinked to original sources

arXiv2022

Hydrogen Emission from Accretion and Outflow in T Tauri Stars

Radiative transfer modelling offers a powerful tool for understanding the enigmatic hydrogen emission lines from T Tauri stars. This work compares optical and near-IR spectroscopy of 29 T Tauri stars with our grid of synthetic line profiles. The archival spectra, obtained with VLT's X-Shooter, provide simultaneous coverage of many optical and infrared hydrogen lines. The observations exhibit similar morphologies of line profiles seen in other studies. We used the radiative transfer code TORUS to create synthetic H$α$, Pa$β$, Pa$γ$, and Br$γ$ emission lines for a fiducial T Tauri model that included axisymmetric magnetospheric accretion and a polar stellar wind. The distribution of Reipurth types and line widths for the synthetic H$α$ lines is similar to the observed results. However, the modelled infrared lines are narrower than the observations by $\approx 80{~\rm kms}^{-1}$, and our models predict a significantly higher proportion ($\approx 90$ per cent) of inverse P-Cygni profiles. Furthermore, our radiative transfer models suggest that the frequency of P-Cygni profiles depends on the ratio of the mass loss to mass accretion rates and blue-shifted sub-continuum absorption was predicted for mass loss rates as low as $10^{-12}~M_{\odot}{\rm~ yr}^{-1}$. We explore the effect of rotation, turbulence, and the contributions from red-shifted absorption in an attempt to explain the discrepancy in widths. Our findings show that, singularly, none of these effects is sufficient to explain the observed disparity. However, a combination of rotation, turbulence, and non-axisymmetric accretion may improve the fit of the models to the observed data.

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arXiv2022

Discovery of a 500 au Protobinary in the Massive Prestellar Core G11.92-0.61 MM2

We present high-resolution (< ~160 au) Atacama Large Millimeter/submillimeter Array (ALMA) 1.3 mm observations of the high-mass prestellar core candidate G11.92-0.61 MM2, which reveal that this source is in fact a protobinary system with a projected separation of 505 au. The binary components, MM2E and MM2W, are compact (radii < 140 au) sources within the partially optically thick dust emission with alpha$_{0.9 cm-1.3 mm}$=2.47-2.94. The 1.3 mm brightness temperatures, T$_b$=68.4/64.6 K for MM2E/MM2W, imply internal heating and minimum luminosities L$_*$ > 24.7 Lsun for MM2E and L$_*$ > 12.6 Lsun for MM2W. The compact sources are connected by a "bridge" of lower-surface-brightness dust emission and lie within more extended emission that may correspond to a circumbinary disk. The circumprotostellar gas mass, estimated from ~0.2" resolution VLA 0.9 cm observations assuming optically thin emission, is 6.8+/-0.9 Msun. No line emission is detected towards MM2E and MM2W in our high-resolution 1.3 mm ALMA observations. The only line detected is 13CO J=2-1, in absorption against the 1.3 mm continuum, which likely traces a layer of cooler molecular material surrounding the protostars. We also report the discovery of a highly asymmetric bipolar molecular outflow that appears to be driven by MM2E and/or MM2W in new deep, ~0.5" resolution (1680 au) ALMA 0.82 mm observations. This outflow, traced by low-excitation CH3OH emission, indicates ongoing accretion onto the protobinary system. Overall, the super-Alfvenic models of Mignon-Risse et al. (2021) agree well with the observed properties of the MM2E/MM2W protobinary, suggesting that this system may be forming in an environment with a weak magnetic field.

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arXiv2018

G11.92-0.61 MM 1: A fragmented Keplerian disk surrounding a proto-O star

We present high resolution ($\sim$300 au) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the massive young stellar object G11.92-0.61 MM 1. We resolve the immediate circumstellar environment of MM 1 in 1.3 mm continuum emission and CH$_{3}$CN emission for the first time. The object divides into two main sources - MM 1a, which is the source of a bipolar molecular outflow, and MM 1b, located 0.57'' (1920 au) to the South-East. The main component of MM 1a is an elongated continuum structure, perpendicular to the bipolar outflow, with a size of $0.141'' \times 0.050''$ ($480\times170$ au). The gas kinematics toward MM 1a probed via CH$_{3}$CN trace a variety of scales. The lower energy $J=12-11$ $K=3$ line traces extended, rotating gas within the outflow cavity, while the $v$8=1 line shows a clearly-resolved Keplerian rotation signature. Analysis of the gas kinematics and dust emission shows that the total enclosed mass in MM 1a is $40\pm5$ M$_{\odot}$ (where between 2.2-5.8 M$_{\odot}$ is attributed to the disk), while MM 1b is $<0.6$ M$_{\odot}$. The extreme mass ratio and orbital properties of MM 1a and MM 1b suggest that MM 1b is one of the first observed examples of the formation of a binary star via disk fragmentation around a massive young (proto)star.

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arXiv2017

Polarized disk emission from Herbig Ae/Be stars observed using Gemini Planet Imager: HD 144432, HD 150193, HD 163296, and HD 169142

In order to look for signs of on-going planet formation in young disks, we carried out the first J-band polarized emission imaging of the Herbig Ae/Be stars HD 150193, HD 163296, and HD 169142 using the Gemini Planet Imager (GPI), along with new H band observations of HD 144432. We confirm the complex "double ring" structure for the nearly face-on system HD 169142 first seen in H-band, finding the outer ring to be substantially redder than the inner one in polarized intensity. Using radiative transfer modeling, we developed a physical model that explains the full spectral energy distribution (SED) and J- and H-band surface brightness profiles, suggesting that the differential color of the two rings could come from reddened starlight traversing the inner wall and may not require differences in grain properties. In addition, we clearly detect an elongated, off-center ring in HD 163296 (MWC 275), locating the scattering surface to be 18 AU above the midplane at a radial distance of 77 AU, co-spatial with a ring seen at 1.3mm by ALMA linked to the CO snow line. Lastly, we report a weak tentative detection of scattered light for HD 150193 (MWC 863) and a non-detection for HD 144432; the stellar companion known for each of these targets has likely disrupted the material in the outer disk of the primary star. For HD 163296 and HD 169142, the prominent outer rings we detect could be evidence for giant planet formation in the outer disk or a manifestation of large-scale dust growth processes possibly related to snow-line chemistry.

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arXiv2015

TORUS-3DPDR: A self-consistent code treating three-dimensional photoionization and photodissociation regions

The interaction of ionizing and far-ultraviolet radiation with the interstellar medium is of great importance. It results in the formation of regions in which the gas is ionized, beyond which are photodissociation regions (PDRs) in which the gas transitions to its atomic and molecular form. Several numerical codes have been implemented to study these two main phases of the interstellar medium either dynamically or chemically. In this paper we present TORUS-3DPDR, a new self-consistent code for treating the chemistry of three-dimensional photoionization and photodissociation regions. It is an integrated code coupling the two codes TORUS, a hydrodynamics and Monte Carlo radiation transport code, and 3D-PDR, a photodissociation regions code. The new code uses a Monte Carlo radiative transfer scheme to account for the propagation of the ionizing radiation including the diffusive component as well as a ray-tracing scheme based on the HEALPix package in order to account for the escape probability and column density calculations. Here, we present the numerical techniques we followed and we show the capabilities of the new code in modelling three-dimensional objects including single or multiple sources. We discuss the effects introduced by the diffusive component of the UV field in determining the thermal balance of PDRs as well as the effects introduced by a multiple sources treatment of the radiation field. We find that diffuse radiation can positively contribute to the formation of CO. With this new code, three-dimensional synthetic observations for the major cooling lines are possible, for making feasible a detailed comparison between hydrodynamical simulations and observations.

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arXiv2014

Line and continuum radiative transfer modelling of AA Tau

We present photometric and spectroscopic models of the Classical T Tauri star AA Tau. Photometric and spectroscopic variability present in observations of AA Tau is attributed to a magnetically induced warp in the accretion disc, periodically occulting the photosphere on an 8.2--day timescale. Emission line profiles show signatures of both infall, attributed to magnetospherically accreting material, and outflow. Using the radiative transfer code TORUS, we have investigated the geometry and kinematics of AA Tau's circumstellar disc and outflow, which is modelled here as a disc wind. Photometric models have been used to constrain the aspect ratio of the disc, the offset angle of the magnetosphere dipole with respect to the stellar rotation axis, and the inner radius of the circumstellar disc. Spectroscopic models have been used to constrain the wind and magnetosphere temperatures, wind acceleration parameter, and mass loss rate. We find observations are best fitted by models with a mass accretion rate of $5\times10^{-9}$ M$_\odot$ yr$^{-1}$, a dipole offset of between $10^\circ$ and $20^\circ$, a magnetosphere that truncates the disc from 5.2 to 8.8 R$_\star$, a mass-loss-rate to accretion-rate ratio of ~ 0.1, a magnetosphere temperature of 8500 -- 9000 K, and a disc wind temperature of 8000 K.

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arXiv2012

Rotating Wolf-Rayet stars in a post RSG/LBV phase. An evolutionary channel towards long-duration GRBs?

(shortened) We investigate the properties of Galactic WR stars and their environment to identify evolutionary channels that may lead to the formation of LGRBs. To this purpose we compile available information on the spectropolarimetric properties, the presence of CS ejecta, and the CS velocities in the environment of Galactic WR stars. We use linear line-depolarization as an indicator of rotation, nebular morphology as an indicator of stellar ejecta, and velocity patterns in UV absorption features as an indicator of increased velocities in the CS environment. We find that the ~23% WR stars with "possible ejecta nebulae" dominate the population of WR stars with spectropolarimetric signatures of rotation, while WR stars without such nebulae only rarely show indications of rotation. The corresponding objects are most likely in an early stage after a preceding RSG or LBV phase, and have not yet lost their angular momenta due to the strong mass-loss in the WR phase. From their photometric periods we estimate rotation parameters in the range omega = 0.04...0.25, corresponding to moderate rotation speeds of 36...120 km/s. These values are very uncertain, but comply with the specific surface angular momentum requirement for LGRB progenitors. Our results indicate that, in the Galaxy, mainly "young" WR stars shortly after a RSG/LBV phase show spectropolarimetric signatures of rotation. Their rotation rates are thus likely enhanced with respect to the majority of Galactic WR stars. According to their estimated specific surface angular momenta, a subgroup of stars exploding in this phase may represent an evolutionary channel towards LGRBs at high metallicities, comparable to the Galaxy.

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arXiv2011

In pursuit of gamma-ray burst progenitors: the identification of a sub-population of rotating Wolf-Rayet stars

Long gamma-ray bursts involve the most powerful cosmic explosions since the Big Bang. Whilst it has been established that GRBs are related to the death throes of massive stars, the identification of their progenitors has proved challenging. Theory suggests that rotating Wolf-Rayet stars are the best candidates, but their strong stellar winds shroud their surfaces, preventing a direct measurement of their rotation. Fortunately, linear spectropolarimetry may be used to probe the flattening of their winds due to stellar spin. Spectropolarimetry surveys show that an 80% majority of WR stars have spherically symmetric winds and are thus rotating slowly, yet a small 20% minority display a spectropolarimetric signature indicative of rotation. Here we find a highly significant correlation between WR objects that carry the signature of stellar rotation and the subset of WR stars with ejecta nebulae that have only recently transitioned from a red sugergiant or luminous blue variable phase. As these youthful WR stars have yet to spin-down due to mass loss, they are the best candidate GRB progenitors identified to date. When we take recently published WR ejecta nebula numbers we find that five out of the six line-effect WR stars are surrounded by ejecta nebulae. The statistics imply that the null hypothesis of no correlation between line-effect WR stars and ejecta nebulae can be rejected at the 0.0004% level. Given that four line-effect and WR ejecta nebula have spectroscopically been confirmed to contain nucleosynthetic products, we argue that the correlation is both statistically significant and physically convincing. The implication is that we have identified a WR sub-population that fulfills the necessary criteria for making GRBs. Finally, we discuss the potential of identifying GRB progenitors via spectropolarimetry with extremely large telescopes.

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arXiv2009

On the presence and absence of disks around O-type stars

(abridged) As the favoured progenitors of long-duration gamma-ray bursts, massive stars may represent our best signposts of individual objects in the early Universe, but special conditions seem required to make these bursters, which might originate from the progenitor's rapid rotation and associated asymmetry. To obtain empirical constraints on the interplay between stellar rotation and wind asymmetry, we perform linear Halpha spectropolarimetry on a sample of 18 spectroscopically peculiar massive O stars, including OVz, Of?p, Oe, and Onfp stars, supplemented by an earlier sample of 20 O supergiants. Despite their rapid rotation (with vsin(i) up to 400 km/s) most O-type stars are found to be spherically symmetric, but with notable exceptions amongst specific object classes. We divide the peculiar O stars into four distinct categories: Groups III and IV include the Oe stars and Onfp stars, which are on the high-end tail of the O star rotation distribution and have in the past been claimed to be embedded in disks. Here we report the detection of a classical depolarization ``line effect'' in the Oe star HD 45314, but the overall incidence of line effects amongst Oe stars is significantly lower (1 out of 6) than amongst Be stars. The chance that the Oe and Be datasets are drawn from the same parent population is negligible (with 95% confidence). This implies there is as yet no evidence for a disk hypothesis in Oe stars, providing relevant constraints on the physical mechanism that is responsible for the Be phenomenon. Finally, we find that 3 out of 4 of the group IV Onfp stars show evidence for complex polarization effects, which are likely related to rapid rotation, and we speculate on the evolutionary links to B[e] stars.

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arXiv2009

Benchmark problems for continuum radiative transfer. High optical depths, anisotropic scattering, and polarisation

Solving the continuum radiative transfer equation in high opacity media requires sophisticated numerical tools. In order to test the reliability of such tools, we present a benchmark of radiative transfer codes in a 2D disc configuration. We test the accuracy of seven independently developed radiative transfer codes by comparing the temperature structures, spectral energy distributions, scattered light images, and linear polarisation maps that each model predicts for a variety of disc opacities and viewing angles. The test cases have been chosen to be numerically challenging, with midplane optical depths up 10^6, a sharp density transition at the inner edge and complex scattering matrices. We also review recent progress in the implementation of the Monte Carlo method that allow an efficient solution to these kinds of problems and discuss the advantages and limitations of Monte Carlo codes compared to those of discrete ordinate codes. For each of the test cases, the predicted results from the radiative transfer codes are within good agreement. The results indicate that these codes can be confidently used to interpret present and future observations of protoplanetary discs.

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arXiv2008

A Tale of Two Herbig Ae stars -MWC275 and AB Aurigae: Comprehensive Models for SED and Interferometry

We present comprehensive models for the Herbig Ae stars MWC275 and AB Aur that aim to explain their spectral energy distribution (from UV to millimeter) and long baseline interferometry (from near-infrared to millimeter) simultaneously. Data from the literature, combined with new mid-infrared (MIR) interferometry from the Keck Segment Tilting Experiment, are modeled using an axisymmetric Monte Carlo radiative transfer code. Models in which most of the near-infrared (NIR) emission arises from a dust rim fail to fit the NIR spectral energy distribution (SED) and sub-milli-arcsecond NIR CHARA interferometry. Following recent work, we include an additional gas emission component with similar size scale to the dust rim, inside the sublimation radius, to fit the NIR SED and long-baseline NIR interferometry on MWC275 and AB Aur. In the absence of shielding of star light by gas, we show that the gas-dust transition region in these YSOs will have to contain highly refractory dust, sublimating at ~1850K. Despite having nearly identical structure in the thermal NIR, the outer disks of MWC275 and AB Aur differ substantially. In contrast to the AB Aur disk, MWC275 lacks small grains in the disk atmosphere capable of producing significant 10-20micron emission beyond ~7AU, forcing the outer regions into the "shadow" of the inner disk

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arXiv2008

Strong Near-Infrared Emission Interior to the Dust-Sublimation Radius of Young Stellar Objects MWC275 and AB Aur

Using the longest optical-interferometeric baselines currently available, we have detected strong near-infrared (NIR) emission from inside the dust-destruction radius of Herbig Ae stars MWC275 and AB Aur. Our sub-milli-arcsecond resolution observations unambiguously place the emission between the dust-destruction radius and the magnetospheric co-rotation radius. We argue that this new component corresponds to hot gas inside the dust-sublimation radius, confirming recent claims based on spectrally-resolved interferometry and dust evaporation front modeling.

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arXiv2008

Magnetospheric accretion on the T Tauri star BP Tauri

From observations collected with the ESPaDOnS and NARVAL spectropolarimeters, we report the detection of Zeeman signatures on the classical T Tauri star BP Tau. Circular polarisation signatures in photospheric lines and in narrow emission lines tracing magnetospheric accretion are monitored throughout most of the rotation cycle of BP Tau at two different epochs in 2006. We observe that rotational modulation dominates the temporal variations of both unpolarised and circularly polarised spectral proxies tracing the photosphere and the footpoints of accretion funnels. From the complete data sets at each epoch, we reconstruct the large-scale magnetic topology and the location of accretion spots at the surface of BP Tau using tomographic imaging. We find that the field of BP Tau involves a 1.2 kG dipole and 1.6 kG octupole, both slightly tilted with respect to the rotation axis. Accretion spots coincide with the two main magnetic poles at high latitudes and overlap with dark photospheric spots; they cover about 2% of the stellar surface. The strong mainly-axisymmetric poloidal field of BP Tau is very reminiscent of magnetic topologies of fully-convective dwarfs. It suggests that magnetic fields of fully-convective cTTSs such as BP Tau are likely not fossil remants, but rather result from vigorous dynamo action operating within the bulk of their convective zones. Preliminary modelling suggests that the magnetosphere of BP Tau extends to distances of at least 4 R* to ensure that accretion spots are located at high latitudes, and is not blown open close to the surface by a putative stellar wind. It apparently succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of BP Tau.

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arXiv2007

The Inner Rim of YSO Disks: Effects of dust grain evolution

Dust-grain growth and settling are the first steps towards planet formation. An understanding of dust physics is therefore integral to a complete theory of the planet formation process. In this paper, we explore the possibility of using the dust evaporation front in YSO disks (`the inner rim') as a probe of the dust physics operating in circumstellar disks. The geometry of the rim depends sensitively on the composition and spatial distribution of dust. Using radiative transfer and hydrostatic equilibrium calculations we demonstrate that dust growth and settling can curve the evaporation front dramatically (from a cylindrical radius of about 0.5 AU in the disk mid-plane to 1.2 AU in the disk upper layers for an A0 star). We compute synthetic images and interferometric visibilities for our representative rim models and show that the current generation of near-IR long-baseline interferometers (VLTI, CHARA) can strongly constrain the dust properties of circumstellar disks, shedding light on the relatively poorly understood processes of grain growth, settling and turbulent mixing.

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arXiv2006

Star formation in Perseus: II. SEDs, classification and lifetimes

Working with the submillimetre continuum map of the Perseus molecular cloud (Hatchell et al. 2005), we aimed to determine the evolutionary stage of each submm core in Perseus, and investigate the lifetimes of these phases. We compile spectral energy distributions (SEDs) from 2MASS, Spitzer IRAC, Michelle, IRAS, SCUBA and Bolocam data. Sources are classified starless/protostellar on the basis of infrared and/or outflow detections and Class I/Class 0 on the basis of Tbol, Lbol/Lsmm and F_{3.6}/F_{850}. In order to investigate the dependence of these evolutionary indicators on mass, we construct radiative transfer models of Class 0 sources. Of the submm cores, 56/103 (54%) are confirmed protostars on the basis of infrared emission or molecular outflows. Of these, 22 are classified Class 1 on the basis of three evolutionary indicators, 34 are Class 0, and the remaining 47 are assumed starless. Perseus contains a much greater fraction of Class 0 sources than either Taurus or Rho Oph. Comparing the protostellar with the T Tauri population, the lifetime of the protostellar phase in Perseus is 0.25-0.67 Myr (95% confidence limits). The relative lifetime of the Class 0 and Class 1 phases are similar. We find that for the same source geometry but different masses, evolutionary indicators such as Tbol vary their value. It is therefore not always appropriate to use a fixed threshold to separate Class 0 and Class I sources. More modelling is required to determine the observational characteristics of the Class 0/Class I boundary over a range of masses.

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arXiv2006

The Keck Aperture Masking Experiment: Near-Infrared Sizes of Dusty Wolf-Rayet Stars

We report the results of a high angular resolution near-infrared survey of dusty Wolf-Rayet stars using the Keck-1 Telescope, including new multi-wavelength images of the pinwheel nebulae WR 98a, WR 104, and WR 112. Angular sizes were measured for an additional 8 dusty WR stars using aperture masking interferometry, allowing us to probe characteristics sizes down to ~20 milliarcseconds (~40 AU for typical sources). With angular sizes and specific fluxes, we can directly measure the wavelength-dependent surface brightness and size relations for our sample. We discovered tight correlations of these properties within our sample which could not be explained by simple spherically-symmetric dust shells or even the more realistic ``pinwheel nebula'' (3-D) radiative transfer model, when using optical constants of Zubko. While the tightly-correlated surface brightness relations we uncovered offer compelling indirect evidence of a shared and distinctive dust shell geometry amongst our sample, long-baseline interferometers should target the marginally-resolved objects in our sample in order to conclusively establish the presence or absence of the putative underyling colliding wind binaries thought to produce the dust shells around WC Wolf-Rayets.

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arXiv2006

Magnetospheric Accretion in Classical T Tauri Stars

The inner 0.1 AU around accreting T Tauri stars hold clues to many physical processes that characterize the early evolution of solar-type stars. The accretion-ejection connection takes place at least in part in this compact magnetized region around the central star, with the inner disk edge interacting with the star's magnetosphere thus leading simultaneously to magnetically channeled accretion flows and to high velocity winds and outflows. The magnetic star-disk interaction is thought to have strong implications for the angular momentum evolution of the central system, the inner structure of the disk, and possibly for halting the migration of young planets close to the stellar surface. We review here the current status of magnetic field measurements in T Tauri stars, the recent modeling efforts of the magnetospheric accretion process, including both radiative transfer and multi-D numerical simulations, and summarize current evidence supporting the concept of magnetically-channeled accretion in young stars. We also discuss the limits of the models and highlight observational results which suggest that the star-disk interaction is a highly dynamical and time variable process in young stars.

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arXiv2005

Spectroscopic study of the long-period dust-producing WC7pd+O9 binary HD192641

We present the results of an optical spectroscopic study of the massive Wolf-Rayet binary WR137. These data cover the dust-formation maximum in 1997. Combining all available measurements of radial velocities, we derive, for the first time, a spectroscopic orbit with period 13.05 +/- 0.18 years. The resulting masses, adopting i=67degrees, are M(O)= 20 +/- 2 Mo and M(WR)= 4.4+/- 1.5 Mo. These appear, respectively, around normal and on the low side for the given spectral types. Analysis of the intense multi-site spectroscopic monitoring in 1999 shows that the CIII5696 and CIV5802/12 lines have the highest intrinsic variability levels. The periodogram analysis yields a small-amplitude modulation in the absorption troughs of the CIV5802/12 and HeI5876 lines with a period of 0.83 days, which could be related either to pulsations or large-scale rotating structures as seen in the WN4 star EZ Canis Majoris (WR6).Wavelet analysis of the strong emission lines of CIII5696 and CIV5802/12 enabled us to isolate and follow for several hours small structures associated with density enhancements within the wind of the Wolf-Rayet star. Cross-correlating the variability patterns seen in different lines, we find a weak but significant correlation between the varability in emission lines with different ionization potential, i.e. in lines formed at different distances from the WR stellar core. Adopting a beta wind-velocity law, from the motion of individual subpeaks we find beta around 5, which is significantly larger than the canonical value of 1 usually found in O-star winds.

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