arXiv subjects

A. F. Boden

Publications and source records attributed to A. F. Boden.

18 recordsLinked to original sources

arXiv2011

VLBA determination of the distance to nearby star-forming regions V. Dynamical mass, distance and radio structure of V773 Tau A

(ABRIDGED) We present multi-epoch Very Long Baseline Array (VLBA) observations of V773 Tau A, the 51-day binary subsystem in the multiple young stellar system V773 Tau. Combined with previous interferometric and radial velocity measurements, these new data enable us to improve the characterization of the physical orbit of the A subsystem. In particular, we infer updated dynamical masses for the primary and the secondary components of 1.55 pm 0.11 Msun, and 1.293 pm 0.068 Msun, respectively, and an updated orbital parallax distance to the system of 135.7 pm 3.2 pc, all consistent with previous estimates. Using the improved orbit, we can calculate the absolute coordinates of the barycenter of V773 Tau A at each epoch of our VLBA observations, and fit for its trigonometric parallax and proper motion. This provides a direct measurement of the distance to the system almost entirely independent of the orbit modeling. The best fit yields a distance of 129.9 pm 3.2 pc, in good agreement (i.e. within 1 sigma) with the distance estimate based on the orbital fit. Taking the mean value of the orbital and trigonometric parallaxes, we conclude that V773 Tau is located at d =132.8 pm 2.3 pc. The accuracy of this determination is nearly one order of magnitude better than that of previous estimates. In projection, V773 Tau and two other young stars (Hubble 4 and HDE 283572) recently observed with the VLBA are located toward the dark cloud Lynds 1495, in the central region of Taurus. These three stars appear to have similar trigonometric parallaxes, radial velocities, and proper motions, and we argue that the weighted mean and dispersion of their distances (d = 131.4 pc and sigma_d = 2.4 pc) provide a good estimate of the distance to and depth of Lynds 1495 and its associated stellar population.

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arXiv2011

Multi-Epoch Observations of HD69830: High Resolution Spectroscopy and Limits to Variability

The main-sequence solar-type star HD69830 has an unusually large amount of dusty debris orbiting close to three planets found via the radial velocity technique. In order to explore the dynamical interaction between the dust and planets, we have performed multi-epoch photometry and spectroscopy of the system over several orbits of the outer dust. We find no evidence for changes in either the dust amount or its composition, with upper limits of 5-7% (1 $σ$ per spectral element) on the variability of the {\it dust spectrum} over 1 year, 3.3% (1 $σ$) on the broad-band disk emission over 4 years, and 33% (1 $σ$) on the broad-band disk emission over 24 years. Detailed modeling of the spectrum of the emitting dust indicates that the dust is located outside of the orbits of the three planets and has a composition similar to main-belt, C-type asteroids asteroids in our solar system. Additionally, we find no evidence for a wide variety of gas species associated with the dust. Our new higher SNR spectra do not confirm our previously claimed detection of H$_2$O ice leading to a firm conclusion that the debris can be associated with the break-up of one or more C-type asteroids formed in the dry, inner regions of the protoplanetary disk of the HD69830 system. The modeling of the spectral energy distribution and high spatial resolution observations in the mid-infrared are consistent with a $\sim$ 1 AU location for the emitting material.

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arXiv2011

The Gould's Belt distance survey

Very Long Baseline Interferometry (VLBI) observations can provide the position of compact radio sources with an accuracy of order 50 micro-arcseconds. This is sufficient to measure the trigonometric parallax and proper motions of any object within 500 pc of the Sun to better than a few percent. Because they are magnetically active, young stars are often associated with compact radio emission detectable using VLBI techniques. Here we will show how VLBI observations have already constrained the distance to the most often studied nearby regions of star-formation (Taurus, Ophiuchus, Orion, etc.) and have started to provide information on their internal structure and kinematics. We will then briefly describe a large project (called The Gould's Belt Distance Survey) designed to provide a detailed view of star-formation in the Solar neighborhood using VLBI observations.

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arXiv2010

The Massive Progenitor of the Type II-Linear SN 2009kr

We present early-time photometric and spectroscopic observations of supernova (SN) 2009kr in NGC 1832. We find that its properties to date support its classification as Type II-linear (SN II-L), a relatively rare subclass of core-collapse supernovae (SNe). We have also identified a candidate for the SN progenitor star through comparison of pre-explosion, archival images taken with WFPC2 onboard the Hubble Space Telescope (HST) with SN images obtained using adaptive optics (AO) plus NIRC2 on the 10-m Keck-II telescope. Although the host galaxy's substantial distance (~26 Mpc) results in large uncertainties in the relative astrometry, we find that if this candidate is indeed the progenitor, it is a highly luminous (M_V = -7.8 mag) yellow supergiant with initial mass ~18-24 M_sun. This would be the first time that a SN II-L progenitor has been directly identified. Its mass may be a bridge between the upper initial mass limit for the more common Type II-plateau SNe (SNe II-P) and the inferred initial mass estimate for one Type II-narrow SN (SN IIn).

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arXiv2007

The Palomar Testbed Interferometer Calibrator Catalog

The Palomar Testbed Interferometer (PTI) archive of observations between 1998 and 2005 is examined for objects appropriate for calibration of optical long-baseline interferometer observations - stars that are predictably point-like and single. Approximately 1,400 nights of data on 1,800 objects were examined for this investigation. We compare those observations to an intensively studied object that is a suitable calibrator, HD217014, and statistically compare each candidate calibrator to that object by computing both a Mahalanobis distance and a Principal Component Analysis. Our hypothesis is that the frequency distribution of visibility data associated with calibrator stars differs from non-calibrator stars such as binary stars. Spectroscopic binaries resolved by PTI, objects known to be unsuitable for calibrator use, are similarly tested to establish detection limits of this approach. From this investigation, we find more than 350 observed stars suitable for use as calibrators (with an additional $\approx 140$ being rejected), corresponding to $\gtrsim 95%$ sky coverage for PTI. This approach is noteworthy in that it rigorously establishes calibration sources through a traceable, empirical methodology, leveraging the predictions of spectral energy distribution modeling but also verifying it with the rich body of PTI's on-sky observations.

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arXiv2007

The circumbinary disk of HD 98800 B: Evidence for disk warping

The quadruple young stellar system HD 98800 consists of two spectroscopic binary pairs with a circumbinary disk around the B component. Recent work by Boden and collaborators using infrared interferometry and radial velocity data resulted in a determination of the physical orbit for HD 98800 B. We use the resulting inclination of the binary and the measured extinction toward the B component stars to constrain the distribution of circumbinary material. Although a standard optically and geometrically thick disk model can reproduce the spectral energy distribution, it can not account for the observed extinction if the binary and the disk are co-planar. We next constructed a dynamical model to investigate the influence of the A component, which is not in the Ba-Bb orbital plane, on the B disk. We find that these interactions have a substantial impact on the inclination of the B circumbinary disk with respect to the Ba-Bb orbital plane. The resulting warp would be sufficient to place material into the line of sight and the non-coplanar disk orientation may also cause the upper layers of the disk to intersect the line of sight if the disk is geometrically thick. These simulations also support that the dynamics of the Ba-Bb orbit clear the inner region to a radius of ~3 AU. We then discuss whether the somewhat unusual properties of the HD 98800 B disk are consistent with material remnant from the star formation process or with more recent creation by collisions from larger bodies.

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arXiv2007

Dynamical Masses for Pre-Main Sequence Stars: A Preliminary Physical Orbit for V773 Tau A

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 $\pm$ 2.4 deg, and allows us to infer the component dynamical masses and system distance. In particular we find component masses of 1.54 $\pm$ 0.14 and 1.332 $\pm$ 0.097 M$_{\sun}$ for the Aa (primary) and Ab (secondary) components respectively. Our modeling 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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arXiv2007

First Space-Based Microlens Parallax Measurement: Spitzer Observations of OGLE-2005-SMC-001

We combine Spitzer and ground-based observations to measure the microlens parallax of OGLE-2005-SMC-001, the first such space-based determination since S. Refsdal proposed the idea in 1966. The parallax measurement yields a projected velocity \tilde v ~ 230 km/s, the typical value expected for halo lenses, but an order of magnitude smaller than would be expected for lenses lying in the Small Magellanic Cloud (SMC) itself. The lens is a weak (i.e., non-caustic-crossing) binary, which complicates the analysis considerably but ultimately contributes additional constraints. Using a test proposed by Assef et al. (2006), which makes use only of kinematic information about different populations but does not make any assumptions about their respective mass functions, we find that the likelihood ratio is L_halo/L_SMC = 20. Hence, halo lenses are strongly favored but SMC lenses are not definitively ruled out. Similar Spitzer observations of additional lenses toward the Magellanic Clouds would clarify the nature of the lens population. The Space Interferometry Mission could make even more constraining measurements.

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arXiv2006

Physical Orbit for Lambda Virginis and a Test of Stellar Evolution Models

Lambda Virginis (LamVir) is a well-known double-lined spectroscopic Am binary with the interesting property that both stars are very similar in abundance but one is sharp-lined and the other is broad-lined. We present combined interferometric and spectroscopic studies of LamVir. The small scale of the LamVir orbit (~20 mas) is well resolved by the Infrared Optical Telescope Array (IOTA), allowing us to determine its elements as well as the physical properties of the components to high accuracy. The masses of the two stars are determined to be 1.897 Msun and 1.721 Msun, with 0.7% and 1.5% errors respectively, and the two stars are found to have the same temperature of 8280 +/- 200 K. The accurately determined properties of LamVir allow comparisons between observations and current stellar evolution models, and reasonable matches are found. The best-fit stellar model gives LamVir a subsolar metallicity of Z=0.0097, and an age of 935 Myr. The orbital and physical parameters of LamVir also allow us to study its tidal evolution time scales and status. Although currently atomic diffusion is considered to be the most plausible cause of the Am phenomenon, the issue is still being actively debated in the literature. With the present study of the properties and evolutionary status of LamVir, this system is an ideal candidate for further detailed abundance analyses that might shed more light on the source of the chemical anomalies in these A stars.

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arXiv2005

First Results from the CHARA Array. III. Oblateness, Rotational Velocity and Gravity Darkening of Alderamin

We present observations of the A7IV-V star Alderamin ($α$ Cep, HR 8162, HD 203280) from the Georgia State University CHARA Array. These infrared interferometric angular size measurements indicate a non-circular projected disk brightness distribution for this known rapid rotator. The interferometric observations are modeled as arising from an elongated rigid atmosphere, with apparent polar and equatorial radii of $r_p=0.6753^{+0.0119}_{-0.0135}$ milliarcseconds (mas) and $r_e=0.8767^{+0.0293}_{-0.0183}$ mas, respectively, for a difference of $201\pm 32$ microarcseconds ($μ$as), and with an axial ratio of $r_e/r_p =1.298\pm 0.051$. Using the Hipparcos distance of $14.96\pm 0.11$ pc, these angular measures translate to $2.18\pm0.05$ and $2.82\pm0.10$ $R_\odot$. The inclination of Alderamin to the line of sight indicated by this modeling is effectively edge-on ($i=88.2^{+1.8}_{-13.3}$). The star has a true rotational velocity of $283\pm 10$ km/s ($\sim83$% of breakup velocity), and a polar temperature of roughly 8400 K. Significantly, a necessary aspect of this modeling is a determination of the gravity darkening coefficient, which at a value of $β=0.084^{+0.026}_{-0.049}$ is consistent with a convective photosphere, as expected for an A7IV-V star. Our detailed characterization of this object allows us to investigate various scenarios for the angular momentum history of Alderamin and the appropriateness of certain stellar evolution models.

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arXiv2005

Keck Interferometer observations of classical and weak line T Tauri stars

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 microns) 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 to 5 millarcseconds. 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.

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arXiv2005

Dynamical Masses for Low-Mass Pre-Main Sequence Stars: A Preliminary Physical Orbit for HD 98800 B

We report on Keck Interferometer observations of the double-lined binary (B) component of the quadruple pre-main sequence (PMS) system HD 98800. With these interferometric observations combined with astrometric measurements made by the Hubble Space Telescope Fine Guidance Sensors (FGS), and published radial velocity observations we have estimated preliminary visual and physical orbits of the HD 98800 B subsystem. Our orbit model calls for an inclination of 66.8 $\pm$ 3.2 deg, and allows us to infer the masses and luminosities of the individual components. In particular we find component masses of 0.699 $\pm$ 0.064 and 0.582 $\pm$ 0.051 M$_{\sun}$ for the Ba (primary) and Bb (secondary) components respectively. Modeling of the component SEDs finds temperatures and luminosities in agreement with previous studies, and coupled with the component mass estimates allows for comparison with PMS models in the low-mass regime with few empirical constraints. Solar abundance models seem to under-predict the inferred component temperatures and luminosities, while assuming slightly sub-solar abundances bring the models and observations into better agreement. The present preliminary orbit does not yet place significant constraints on existing pre-main sequence stellar models, but prospects for additional observations improving the orbit model and component parameters are very good.

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arXiv2000

A Preliminary Visual Orbit of BY Draconis

We report on the preliminary determination of the visual orbit of the double-lined spectroscopic binary system BY Draconis with data obtained by the Palomar Testbed Interferometer in 1999. BY Dra is a nearly equal-mass double-lined binary system whose spectroscopic orbit is well known. We have estimated the visual orbit of BY Dra from our interferometric visibility data fit both separately and in conjunction with archival radial velocity data. Our BY Dra orbit is in good agreement with the spectroscopic results. Due to the orbit's face-on orientation, the physical parameters implied by a combined fit to our visibility data and radial velocity data do not yet result in precise component masses and a system distance, but with continued interferometric monitoring we hope to improve the mass estimates to better than 10% determinations.

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arXiv1999

The Visual Orbit and Evolutionary State of 12 Bootes

We report on the determination of the visual orbit of the double-lined spectroscopic binary system 12 Bootes with data obtained by the Palomar Testbed Interferometer in 1998 and 1999. 12 Boo is a nearly equal-mass double-lined binary system whose spectroscopic orbit is well known. We have estimated the visual orbit of 12 Boo from our interferometric visibility data fit both separately and in conjunction with archival radial velocity data. Our 12 Boo orbit is in good agreement with the spectroscopic results, and the physical parameters implied by a combined fit to our visibility data and radial velocity data result in precise component masses. In particular, the orbital parallax of the system is determined to be 27.09 $\pm$ 0.41 mas, and masses of the two components are determined to be 1.435 $\pm$ 0.023 M$_{\sun}$ and 1.409 $\pm$ 0.020 M$_{\sun}$, respectively. Somewhat remarkably, even though the two components are nearly equal mass, the system exhibits a significant brightness difference between the components in the near infrared and visible. We attribute this brightness difference to evolutionary differences between the two components in their transition between main sequence and giant evolutionary phases, and based on theoretical isochrones we can estimate a system age. Further, because the atmospheres of the two components are becoming more convective, we project the system components are currently at or near synchronous rotation, and the system orbit is in the process of circularizing.

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arXiv1999

The Visual Orbit of 64 Piscum

We report on the determination of the visual orbit of the double-lined spectroscopic binary system 64 Piscum with data obtained by the Palomar Testbed Interferometer in 1997 and 1998. 64 Psc is a nearly equal-mass double-lined binary system whose spectroscopic orbit is well known. We have estimated the visual orbit of 64 Psc from our interferometric visibility data. Our 64 Psc orbit is in good agreement with the spectroscopic results, and the physical parameters implied by a combined fit to our interferometric visibility data and radial velocity data of Duquennoy and Mayor result in precise component masses that agree well with their spectral type identifications. In particular, the orbital parallax of the system is determined to be 43.29 \pm 0.46 mas, and masses of the two components are determined to be 1.223 \pm 0.021 M_{\sun} and 1.170 \pm 0.018 M_{\sun}, respectively. Nadal et al. put forward arguments of temporal variability in some of the orbital elements of 64 Psc, presumably explained by an undetected component in the system. While our visibility data does not favor the Nadal temporal variability inference, neither is it definitive in excluding it. Consequently we have performed both high dynamic-range near-infrared imaging and spectroscopy of potential additional companions to the 64 Psc system. Our imaging and spectroscopic data do not support the conjecture of an additional component to 64 Psc, but we did identify a faint object with unusual red colors and spectra.

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arXiv1998

The Visual Orbit of iota Pegasi

We have determined the visual orbit for the spectroscopic binary iota~Pegasi with interferometric visibility data obtained by the Palomar Testbed Interferometer in 1997. iota~Pegasi is a double-lined binary system whose minimum masses and spectral typing suggests the possibility of eclipses. Our orbital and component diameter determinations do not favor the eclipse hypothesis: the limb-to-limb separation of the two components is 0.151 +/- 0.069 mas at conjunction. Our conclusion that the iota~Peg system does not eclipse is supported by high-precision photometric observations. The physical parameters implied by our visual orbit and the spectroscopic orbit of Fekel and Tomkin (1983) are in good agreement with those inferred by other means. In particular, the orbital parallax of the system is determined to be 86.9 +/- 1.0 mas, and masses of the two components are determined to be 1.326 +/- 0.016 M_sun and 0.819 +/- 0.009 M_sun respectively.

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arXiv1998

An Interferometric Search for Bright Companions to 51 Pegasi

We report on a near-infrared, long-baseline interferometric search for luminous companions to the star 51 Pegasi conducted with the Palomar Testbed Interferometer. Our data is completely consistent with a single-star hypothesis. We find no evidence to suggest a luminous companion to 51 Pegasi, and can exclude a companion brighter than a $Δ$K of 4.27 at the 99% confidence level for the 4.2-day orbital period indicated by spectroscopic measurements. This $Δ$K corresponds to an upper limit in the companion M$_K$ of 7.30, in turn implying a main-sequence companion mass less than 0.22 M$_{\sun}$.

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arXiv1998

Astrometric Observation of MACHO Gravitational Microlensing

Following previous suggestions of other researchers, this paper discusses the prospects for astrometric observation of MACHO gravitational microlensing events. We derive the expected astrometric observables for a simple microlensing event with either a dark or self-luminous lens, and demonstrate that accurate astrometry can determine the lens mass, distance, and proper motion in a very general fashion. In particular we argue that in limited circumstances ground-based, narrow-angle differential astrometric techniques are sufficient to measure the lens mass directly, and other lens properties (distance, transverse motion) by applying an independent model for the source distance and motion. We investigate the sensitivity of differential astrometry in determining lens parameters by Monte Carlo methods, and derive a quasi-empirical relationship between astrometric accuracy and mass uncertainty.

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