A. Tannirkulam

Concordia University–Ann Arbor, Ann Arbor, Michigan, United States

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Publications (15)66.94 Total impact

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    ABSTRACT: Wolf-Rayet (WR) stars represent one of the final stages of massive stellar evolution. Relatively little is known about this short-lived phase and we currently lack reliable mass, distance, and binarity determinations for a representative sample. Here we report the first visual orbit for WR 140 (= HD193793), a WC7+O5 binary system known for its periodic dust production episodes triggered by intense colliding winds near periastron passage. The Infrared-Optical Telescope Array and Center for High Angular Resolution Astronomy interferometers resolved the pair of stars in each year from 2003 to 2009, covering most of the highly eccentric, 7.9 year orbit. Combining our results with the recently improved double-line spectroscopic orbit of Fahed et al., we find the WR 140 system is located at a distance of 1.67 ± 0.03 kpc, composed of a WR star with M WR = 14.9 ± 0.5 M ☉ and an O star with M O = 35.9 ± 1.3 M ☉. Our precision orbit yields key parameters with uncertainties ~6× smaller than previous work and paves the way for detailed modeling of the system. Our newly measured flux ratios at the near-infrared H and Ks bands allow a spectral energy distribution decomposition and analysis of the component evolutionary states.
    The Astrophysical Journal Letters 10/2011; 742(1):L1. · 6.35 Impact Factor
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    ABSTRACT: The mid-infrared properties of pre-planetary disks are sensitive to the temperature and flaring profiles of disks for the regions where planet formation is expected to occur. In order to constrain theories of planet formation, we have carried out a mid-infrared (wavelength 10.7 microns) size survey of young stellar objects using the segmented Keck telescope in a novel configuration. We introduced a customized pattern of tilts to individual mirror segments to allow efficient sparse-aperture interferometry, allowing full aperture synthesis imaging with higher calibration precision than traditional imaging. In contrast to previous surveys on smaller telescopes and with poorer calibration precision, we find most objects in our sample are partially resolved. Here we present the main observational results of our survey of 5 embedded massive protostars, 25 Herbig Ae/Be stars, 3 T Tauri stars, 1 FU Ori system, and 5 emission-line objects of uncertain classification. The observed mid-infrared sizes do not obey the size-luminosity relation found at near-infrared wavelengths and a companion paper will provide further modelling analysis of this sample. In addition, we report imaging results for a few of the most resolved objects, including complex emission around embedded massive protostars, the photoevaporating circumbinary disk around MWC 361A, and the subarcsecond binaries T Tau, FU Ori and MWC 1080. Comment: Accepted by Astrophysical Journal. 38 pages. 9 figures
    The Astrophysical Journal 05/2009; · 6.73 Impact Factor
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    ABSTRACT: We present comprehensive models for the Herbig Ae stars MWC 275 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-milliarcsecond 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 MWC 275 and AB Aur. In the absence of shielding of starlight by gas, we show that the gas-dust transition region in these YSOs will have to contain highly refractory dust, sublimating at ~1850 K. Despite having nearly identical structure in the thermal NIR, the outer disks of MWC 275 and AB Aur differ substantially. In contrast to the AB Aur disk, MWC 275 lacks small grains in the disk atmosphere capable of producing significant 10-20 μm emission beyond ~7 AU, forcing the outer regions into the "shadow" of the inner disk.
    The Astrophysical Journal 12/2008; 689(1):513. · 6.73 Impact Factor
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    ABSTRACT: We report the discovery of a circumbinary disk around the Herbig Ae/Be system V892 Tau. Our detailed mid-infrared images were made using segment-tilting interferometry on the Keck I telescope and reveal an asymmetric disk inclined at ~60° with an inner hole diameter of 250 mas (35 AU), approximately 5 times larger than the apparent separation of the binary components. In addition, we report a new measurement along the binary orbit using near-infrared Keck aperture masking, allowing a crude estimate of orbital parameters and the system mass for the first time. The size of the inner hole appears to be consistent with the minimum size prediction from tidal truncation theory, bearing a resemblance to the recently unmasked binary CoKu Tau/4. Our results have motivated a reanalysis of the system spectral energy distribution, concluding the luminosity of this system has been severely underestimated. With further study and monitoring, V892 Tau should prove a powerful testing ground for both predictions of dynamical models for disk-star interactions in young systems with gas-rich disks and for calibrations of pre-main-sequence tracks for intermediate-mass stars.
    The Astrophysical Journal 12/2008; 681(2):L97. · 6.73 Impact Factor
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    ABSTRACT: We have constructed a detailed radiative transfer disk model which reproduces the main features of the spectrum of the outbursting young stellar object FU Orionis from ~4000 Å to ~8 μm. Using an estimated visual extinction AV ~ 1.5, a steady disk model with a central star mass ~0.3 M☉, and a mass accretion rate ~2 × 10-4 M☉ yr-1, we can reproduce the SED of FU Ori quite well. Higher values of extinction used in previous analysis (AV ~ 2.1) result in SEDs which are less well fitted by a steady disk model, but might be explained by extra energy dissipation of the boundary layer in the inner disk. With the mid-infrared spectrum obtained by the IRS on board the Spitzer Space Telescope, we estimate that the outer radius of the hot, rapidly accreting inner disk is ~1 AU, using disk models truncated at this outer radius. Inclusion of radiation from a cooler irradiated outer disk might reduce the outer limit of the hot inner disk to ~0.5 AU. In either case, the radius is inconsistent with a pure thermal instability model for the outburst. Our radiative transfer model implies that the central disk temperature Tc ≥ 1000 K out to ~0.5-1 AU, suggesting that the magnetorotational instability can be supported out to that distance. Assuming that the ~100 yr decay timescale in brightness of FU Ori represents the viscous timescale of the hot inner disk, we estimate the viscosity parameter to be α ~ 0.2-0.02 in the outburst state, consistent with numerical simulations of the magnetorotational instability in disks. The radial extent of the high- region is inconsistent with the model of Bell & Lin, but may be consistent with theories incorporating both gravitational and magnetorotational instabilities.
    The Astrophysical Journal 12/2008; 669(1):483. · 6.73 Impact Factor
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    ABSTRACT: We present observations of the T Tauri stars BP Tau, DG Tau, DI Tau, GM Aur, LkCa 15, RW Aur, and V830 Tau, using long baseline infrared interferometry at K band (2.2 μm) from the Keck Interferometer. The target sources have a range of mass accretion rates and excess near-infrared emission. The interferometer is most sensitive to extended emission on characteristic size scales of 1-5 mas. All sources show evidence for resolved K-band emission on these scales, although a few of the sources are marginally consistent with being unresolved. We calculate the infrared excess based on fitting stellar photosphere models to the optical photometry and estimate the physical size of the emission region using simple geometric models for the sources with a significant infrared excess. Assuming that the K-band-resolved emission traces the inner edge of the dust disk, we compare the measured characteristic sizes to predicted dust sublimation radii and find that the models require a range of dust sublimation temperatures and possibly optical depths within the inner rim to match the measured radii.
    The Astrophysical Journal 12/2008; 635(2):1173. · 6.73 Impact Factor
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    ABSTRACT: We present new K-band long-baseline interferometer observations of three young stellar objects of the FU Orionis class, namely, V1057 Cyg, V1515 Cyg, and Z CMa-SE, obtained at the Keck Interferometer during its commissioning science period. The interferometer clearly resolves the source of near-infrared emission in all three objects. Using simple geometric models, we derive size scales (0.5-4.5 AU) for this emission. All three objects appear significantly more resolved than expected from simple models of accretion disks tuned to fit the broadband optical and infrared spectrophotometry. We explore variations in the key parameters that are able to lower the predicted visibility amplitudes to the measured levels and conclude that accretion disks alone do not reproduce the spectral energy distributions and K-band visibilities simultaneously. We conclude that either disk models are inadequate to describe the near-infrared emission or additional source components are needed. We hypothesize that large-scale emission (tens of AU) in the interferometer field of view is responsible for the surprisingly low visibilities. This emission may arise in scattering by large envelopes believed to surround these objects.
    The Astrophysical Journal 12/2008; 641(1):547. · 6.73 Impact Factor
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    ABSTRACT: We report the first scientific results from the Michigan Infrared Combiner (MIRC), including the first resolved image of a main-sequence star besides the Sun. Using the CHARA Array, MIRC was able to clearly resolve the well-known elongation of Altair's photosphere due to centrifugal distortion, and was also able to unambiguously image the effect of gravity darkening. In this report, we also show preliminary images of the interacting binary β Lyr and give an update of MIRC performance.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
    07/2008;
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    ABSTRACT: The mid- to far-infrared emission of the outbursting FU Orionis objects has been attributed either to a flared outer disk or to an infalling envelope. We revisit this issue using detailed radiative transfer calculations to model the recent, high signal-to-noise data from the IRS instrument on the {Spitzer Space Telescope}. In the case of FU Ori, we find that a physically-plausible flared disk irradiated by the central accretion disk matches the observations. Building on our previous work, our accretion disk model with outer disk irradiation by the inner disk reproduces the spectral energy distribution between ~4000 angstroms to ~40 microns. Our model is consistent with near-infrared interferometry but there are some inconsistencies with mid-infared interferometric results. Including the outer disk allows us to refine our estimate of the outer radius of the outbursting, high mass accretion rate disk in FU Ori as ~ 0.5 AU, which is a crucial parameter in assessing theories of the FU Orionis phenomenon. We are able to place an upper limit on the mass infall rate of any remnant envelope infall rate to ~ 7e-7 Msun/yr assuming a centrifugal radius of 200 AU. The FUor BBW 76 is also well modelled by a 0.6 AU inner disk and a flared outer disk. However, V1515 Cyg requires an envelope with an outflow cavity to adequately reproduce the IRS spectrum. In contrast with the suggestion by Green et al., we do not require a flattened envelope to match the observations; the inferred cavity shape is qualitatively consistent with typical protostellar envelopes. This variety of dusty structures suggests that the FU Orionis phase can be present at either early or late stages of protostellar evolution. Comment: 30 pages, 9 figures, Accepted by ApJ
    The Astrophysical Journal 06/2008; · 6.73 Impact Factor
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    ABSTRACT: 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. Comment: 12 pages, 4 figures, Accepted for publication in ApJL
    The Astrophysical Journal 03/2008; 677(1):L51. · 6.73 Impact Factor
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    ABSTRACT: Using the sub-milli-arcsecond resolution of the CHARA interferometer array and combining light with the 2-telescope combiner CHARA Classic, we have detected strong near-infrared (NIR) emission interior to the dust-sublimation radius of Herbig Ae stars MWC275 and AB Aur. The large contribution of this emission component, which we argue to be hot gas, to the total NIR spectral energy distribution (SED) is not predicted by current models of the dust evaporation front, indicating that the NIR disk is more complicated than expected. Furthermore, we demonstrate that the structure of the evaporation front in MWC275 is time variable, making single epoch, large uv coverage observations critical to decoding front geometry. With the commissioning of CHARA Michigan Phase Tracker in the summer of 2008, the Michigan Infrared Combiner (a 6 telescope combiner at CHARA) will become an ideal instrument for studying the evaporation front, achieving the required sensitivities to begin the first "true" interferometric imaging of the gas-dust transition region in young stellar objects (YSOs). Here, we summarize results on the evaporation front structure obtained with CHARA Classic and describe future prospects with CHARA MIRC in elucidating morphology of the gas-dust transition region.
    Proc SPIE 01/2008;
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    A. Tannirkulam, T. J. Harries, J. D. Monnier
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    ABSTRACT: 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.
    The Astrophysical Journal 02/2007; 661. · 6.73 Impact Factor
  • A. Tannirkulam, J. D. Monnier, T. J. Harries
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    ABSTRACT: Circumstellar disk models for Herbig Ae/Be stars have evolved in sophistication over the last few years. Up until now these models have concentrated on fitting either SEDs or, more recently, interferometry data. Self-consistent models that explain both data jointly will require an understanding of how coagulation, settling and spatial segregation of dust affect the density and temperature structure of the disk. In this poster, we explore disk models that take these effects into account, illustrating how simultaneous fitting of SEDs, near-IR interferometry and mid-IR interferometry can solve some long-standing problems with model parameter degeneracies.
    12/2005;
  • 01/2005;
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    ABSTRACT: Using the Keck-1 telescope, we have measured the sizes of Herbig Ae/Be disks in the thermal infrared, a wavelength range which probes the region of the circumstellar disk where gas giant planets are expected to form. Previous attempts to obtain precisely calibrated imaging data with Keck/LWS have been frustrated by unstable atmospheric seeing which affects the time-averaged PSF. To overcome this, we have carried-out a novel sparse-aperture interferometry experiment to enable robust mid-IR diffraction-limited imaging. Specifically, selected subsets of Keck-1 primary segments have been independently repointed using custom programming, yielding the well-established calibration advantages of sparse-aperture Fizeau interferometry. We present inital results and compare to recent results from nulling interferometry.
    11/2004; 36:1367.

Publication Stats

231 Citations
66.94 Total Impact Points

Institutions

  • 2008–2011
    • Concordia University–Ann Arbor
      Ann Arbor, Michigan, United States
    • Centro de Investigaciones de Astronomia (CIDA)
      Mérida, Estado Mérida, Venezuela
  • 2004–2009
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States