arXiv subjects

J. P. Aufdenberg

Publications and source records attributed to J. P. Aufdenberg.

At least 19 recordsLinked to original sources

arXiv2026

Panchromatic JWST Observations and Models of the Dim Type Iax Supernova 2024vjm at 200 days

We report JWST spectra and photometry of the underluminous SN Iax 2024vjm obtained 202.8 restframe days post-explosion. The spectrum exhibits a rich set of forbidden lines from low-ionization, intermediate-mass, and iron-group elements, notably the [Ni II] 6.64 micron resonance line, which is a direct indicator of stable nickel. Strong CO and SiO emission is detected alongside a warm dust continuum; the spectral properties are consistent with pre-existing rather than newly formed dust. Synthetic spectra were computed with the generalized stellar atmospheres code PHOENIX/1D using simplified ejecta models. The models reproduce the overall spectral energy distribution and the molecular emission features reasonably well, but substantially underestimate the strength of the mid-infrared atomic forbidden lines, leaving the synthetic spectrum dominated by molecular emission. Experiments in which the molecular opacity is suppressed do not recover the forbidden lines; instead, the emission peak migrates to Co and Fe transitions near 2 microns. We attribute this discrepancy to poorly constrained collisional rates and possibly to an excess of iron-group material in the current ejecta models. A prominent feature at 12.8 microns is not well accounted for by the [Ne II] 12.81 micron line, indicating that the 12.8 micron feature may be largely due to [Fe III]. The presence of CO, SiO, and stable nickel together with the non-detection of neon places tight constraints on the total ejecta mass and the nucleosynthetic yields of SNe Iax progenitor systems.

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arXiv2025

Measurement of the photosphere oblateness of $γ$ Cassiopeiae via Stellar Intensity Interferometry with the VERITAS Observatory

We use the stellar intensity interferometry system implemented with the Very Energetic Radiation Imaging Telescope Array System (VERITAS) at Fred Lawrence Whipple Observatory (FLWO) as a light collector to obtain measurements of the rapid rotator star $γ$ Cassiopeiae, at a wavelength of 416 nm. Using data from baselines sampling different position angles, we extract the size, oblateness, and projected orientation of the photosphere. Fitting the data with a uniform ellipse model yields a minor-axis angular diameter of $0.43\pm0.02$ mas, a major-to-minor-radius ratio of $1.28\pm0.04$, and a position angle of $116^\circ\pm5^\circ$ for the axis of rotation. A rapidly-rotating stellar atmosphere model that includes limb and gravity darkening describes the data well with a fitted angular diameter of $0.604^{+0.041}_{-0.034}$ mas corresponding to an equatorial radius of 10.9$^{+0.8}_{-0.6}~R_\odot$, a rotational velocity with a $1~σ$ lower limit at $97.7\%$ that of breakup velocity, and a position angle of $114.7^{+6.4}_{-5.7}$ degrees. These parameters are consistent with H$α$ line spectroscopy and infrared-wavelength Michelson interferometric measurements of the star's decretion disk. This is the first measurement of an oblate photosphere using intensity interferometry.

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arXiv2025

The NewEra model grid

Analyses of stellar spectra, stellar populations, and transit light curves rely on grids of synthetic spectra and center-to-limb variations (limb darkening) from model stellar atmospheres. Extensive model grids from PHOENIX, a generalized non-LTE 1D and 3D stellar atmosphere code, have found widespread use in the astronomical community, however current PHOENIX/1D models have been substantially improved over the last decade. To make these improvements available to the community, we have constructed the NewEra LTE model grid consisting of 37438 models with $2300K \leq T_{eff} \leq 12000K$, $0.0\le log{(g)} \le 6.0$ metallicities [M/H] from $-4.0$ to $+0.5$, and for metallicities $-2.0 \le [M/H] \le 0.0$ additional $α$ element variations from $-0.2 \le [α/{\rm Fe}] \le +1.2$ are included. The models use databases of 851 million atomic lines and 834 billion molecular lines and employ the Astrophysical Chemical Equilibrium Solver for the equation of state. All models in the NewEra grid have been calculated in spherical symmetry because center-to-limb variation differences from plane-parallel models are quite large for giants and not insignificant for dwarfs. All model data are provided in the Hierarchical Data Format 5 (HDF5) format, including low and high sampling rate spectra. These files also include a variety of details about the models, such as the exact abundances and isotopic patterns used and results of the atomic and molecular line selection. Although the model structures have small differences with the previous grid generation, the spectra show significant differences, mostly due to the updates of the molecular line lists.

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arXiv2024

An Angular Diameter Measurement of $β$ UMa via Stellar Intensity Interferometry with the VERITAS Observatory

We use the VERITAS imaging air Cherenkov Telescope (IACT) array to obtain the first measured angular diameter of $β$ UMa at visual wavelengths using stellar intensity interferometry (SII) and independently constrain the limb-darkened angular diameter. The age of the Ursa Major moving group has been assessed from the ages of its members, including nuclear member Merak ($β$ UMa), an A1-type subgiant, by comparing effective temperature and luminosity constraints to model stellar evolution tracks. Previous interferometric limb-darkened angular-diameter measurements of $β$ UMa in the near-infrared (CHARA Array, $1.149 \pm 0.014$ mas) and mid-infrared (Keck Nuller, $1.08 \pm 0.07$ mas), together with the measured parallax and bolometric flux, have constrained the effective temperature. This paper presents current VERITAS-SII observation and analysis procedures to derive squared visibilities from correlation functions. We fit the resulting squared visibilities to find a limb-darkened angular diameter of $1.07 \pm 0.04 {\rm (stat)} \pm 0.05$ (sys) mas, using synthetic visibilities from a stellar atmosphere model that provides a good match to the spectrum of $β$ UMa in the optical wave band. The VERITAS-SII limb-darkened angular diameter yields an effective temperature of $9700\pm200\pm 200$ K, consistent with ultraviolet spectrophotometry, and an age of $390\pm 29 \pm 32 $ Myr, using MESA Isochrones and Stellar Tracks (MIST). This age is consistent with $408 \pm 6$ Myr from the CHARA Array angular diameter.

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arXiv2009

Fundamental Stellar Astrophysics Revealed at Very High Angular Resolution

A short review of recent results in long-baseline optical interferometry pertaining to fundamental stellar parameters and the future possibilities this area over the next decade. Included are discussions of accurate stellar masses, links between asteroseismology and interferometry, precise radii and effective temperatures, limb darkening and convection, stellar rotation, and high angular resolution imaging.

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arXiv2008

A near-infrared interferometric survey of debris disc stars. II. CHARA/FLUOR observations of six early-type dwarfs

High-precision interferometric observations of six early-type main sequence stars known to harbour cold debris discs have been obtained in the near-infrared K band with the FLUOR instrument at the CHARA Array. The measured squared visibilities are compared to the expected visibility of the stellar photospheres based on theoretical photospheric models taking into account rotational distortion, searching for potential visibility reduction at short baselines due to circumstellar emission. Our observations bring to light the presence of resolved circumstellar emission around one of the six target stars (zeta Aql) at the 5 sigma level. The morphology of the emission source cannot be directly constrained because of the sparse spatial frequency sampling of our interferometric data. Using complementary adaptive optics observations and radial velocity measurements, we find that the presence of a low-mass companion is a likely origin for the excess emission. The potential companion has a K-band contrast of four magnitudes, a most probable mass of about 0.6 Msun, and is expected to orbit between about 5.5 AU and 8 AU from its host star assuming a purely circular orbit. Nevertheless, by adjusting a physical debris disc model to the observed Spectral Energy Distribution of the zeta Aql system, we also show that the presence of hot dust within 10 AU from zeta Aql, producing a total thermal emission equal to 1.69 +- 0.31% of the photospheric flux in the K band, is another viable explanation for the observed near-infrared excess. Our re-interpretation of archival near- to far-infrared photometric measurements shows however that cold dust is not present around zeta Aql at the sensitivity limit of the IRS and MIPS instruments onboard Spitzer, and urges us to remove zeta Aql from the category of bona fide debris disc stars.

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arXiv2006

Tests of stellar model atmospheres by optical interferometry III: NPOI and VINCI interferometry of the M0 giant gamma Sge covering 0.5 - 2.2 microns

Aims: We present a comparison of the visual and NIR intensity profile of the M0 giant gamma Sagittae to plane-parallel ATLAS 9 as well as to plane-parallel & spherical PHOENIX model atmospheres. Methods: We use previously described visual interferometric data obtained with the NPOI in July 2000. We apply the recently developed technique of coherent integration, and thereby obtain visibility data of more spectral channels and with higher precision than before. In addition, we employ new measurements of the K-band diameter of gamma Sagittae obtained with the instrument VINCI at the VLTI in 2002. Results: The spherical PHOENIX model leads to a precise definition of the Rosseland angular diameter and a consistent high-precision diameter value for our NPOI and VLTI/VINCI data sets of Theta_Ross=6.06 pm 0.02 mas, with the Hipparcos parallax corresponding to R_Ross=55 pm 4 R_sun, and with the bolometric flux corresponding to an effective temperature T_eff=3805 pm 55 K. Our visual visibility data close to the first minimum and in the second lobe constrain the limb-darkening effect and are generally consistent with the model atmosphere predictions. The visual closure phases exhibit a smooth transition between 0 and pi. Conclusions: The agreement between the NPOI and VINCI diameter values increases the confidence in the model atmosphere predictions from optical to NIR wavelengths as well as in the calibration and accuracy of both interferometric facilities. The consistent night-by-night diameter values of VINCI give additional confidence in the given uncertainties. The closure phases suggest a slight deviation from circular symmetry, which may be due to surface features, an asymmetric extended layer, or a faint unknown companion.

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arXiv2006

Tests of stellar model atmospheres by optical interferometry IV: VINCI interferometry and UVES spectroscopy of Menkar

We present K-band interferometric and optical spectroscopic observations of Menkar obtained with the instruments VINCI and UVES at Paranal Observatory. Spherically symmetric PHOENIX stellar model atmospheres are constrained by comparison to our interferometric and spectroscopic data, and high-precision fundamental parameters of Menkar are obtained. Our high-precision VLTI/VINCI observations in the first and second lobes of the visibility function directly probe the model-predicted strength of the limb darkening effect in the K-band and the stellar angular diameter. The high spectral resolution of UVES allows us to confront observed and model-predicted profiles of atomic lines and molecular bands. We show that our derived PHOENIX model atmosphere for Menkar is consistent with both the measured strength of the limb-darkening in the near-infrared K-band and the profiles of spectral bands around selected atomic lines and TiO bandheads. At the detailed level of our high spectral resolution, however, noticeable discrepancies between observed and synthetic spectra exist. We obtain a Rosseland angular diameter of Theta_Ross=12.20 mas pm 0.04 mas. Together with the Hipparcos parallax, it corresponds to R_Ross=89 pm 5 R_sun, and together with the bolometric flux to T_eff=3795 K pm 70 K.Our approach illustrates the power of combining interferometry and high-resolution spectroscopy to constrain and calibrate stellar model atmospheres. The simultaneous agreement of the model atmosphere with our interferometric and spectroscopic data increases confidence in the reliability of the modelling of this star, while discrepancies at the detailed level of the high resolution spectra can be used to further improve the underlying model.

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arXiv2006

CHARA Array K'-band Measurements of the Angular Dimensions of Be Star Disks

We present the first K'-band, long-baseline interferometric observations of the northern Be stars gamma Cas, phi Per, zeta Tau, and kappa Dra. The measurements were made with multiple telescope pairs of the CHARA Array interferometer, and in every case the observations indicate that the circumstellar disks of the targets are resolved. We fit the interferometric visibilities with predictions from a simple disk model that assumes an isothermal gas in Keplerian rotation. We derive fits of the four model parameters (disk base density, radial density exponent, disk normal inclination, and position angle) for each of the targets. The resulting densities are in broad agreement with prior studies of the IR excess flux and the resulting orientations generally agree with those from interferometric H-alpha and continuum polarimetric observations. We find that the angular size of the K' disk emission is smaller than that determined for the H-alpha emission, and we argue that the difference is the result of a larger H-alpha opacity and the relatively larger neutral hydrogen fraction with increasing disk radius. All the targets are known binaries with faint companions, and we find that companions appear to influence the interferometric visibilities in the cases of phi Per and kappa Dra. We also present contemporaneous observations of the H-alpha, H-gamma, and Br-gamma emission lines. Synthetic model profiles of these lines that are based on the same disk inclination and radial density exponent as derived from the CHARA Array observations match the observed emission line strength if the disk base density is reduced by approximately 1.7 dex.

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arXiv2006

Circumstellar material in the Vega inner system revealed by CHARA/FLUOR

Only a handful of debris disks have been imaged up to now. Due to the need for high dynamic range and high angular resolution, very little is known about the inner planetary region, where small amounts of warm dust are expected to be found. We investigate the close neighbourhood of Vega with the help of infrared stellar interferometry and estimate the integrated K-band flux originating from the central 8 AU of the debris disk. We performed precise visibility measurements at both short (~30 m) and long (~150 m) baselines with the FLUOR beam-combiner installed at the CHARA Array (Mt Wilson, California) in order to separately resolve the emissions from the extended debris disk (short baselines) and from the stellar photosphere (long baselines). After revising Vega's K-band angular diameter (3.202+/-0.005 mas), we show that a significant deficit in squared visibility (1.88+/-0.34%) is detected at short baselines with respect to the best-fit uniform disk stellar model. This deficit can be either attributed to the presence of a low-mass stellar companion around Vega, or as the signature of the thermal and scattered emissions from the debris disk. We show that the presence of a close companion is highly unlikely, as well as other possible perturbations (stellar morphology, calibration), and deduce that we have most probably detected the presence of dust in the close neighbourhood of Vega. The resulting flux ratio between the stellar photosphere and the debris disk amounts to 1.29+/-0.19% within the FLUOR field-of-view (~7.8 AU). Finally, we complement our K-band study with archival photometric and interferometric data in order to evaluate the main physical properties of the inner dust disk. The inferred properties suggest that the Vega system could be currently undergoing major dynamical perturbations.

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arXiv2006

First Results from the CHARA Array VII: Long-Baseline Interferometric Measurements of Vega Consistent with a Pole-On, Rapidly Rotating Star

We have obtained high-precision interferometric measurements of Vega with the CHARA Array and FLUOR beam combiner in the K' band at projected baselines between 103m and 273m. The measured visibility amplitudes beyond the first lobe are significantly weaker than expected for a slowly rotating star characterized by a single effective temperature and surface gravity. Our measurements, when compared to synthetic visibilities and synthetic spectrophotometry from a Roche-von Zeipel gravity-darkened model atmosphere, provide strong evidence for the model of Vega as a rapidly rotating star viewed very nearly pole-on. Our best fitting model indicates that Vega is rotating at ~91% of its angular break-up rate with an equatorial velocity of 275 km/s. Together with the measured vsin(i), this velocity yields an inclination for the rotation axis of 5 degrees. For this model the pole-to-equator effective temperature difference is 2250 K, a value much larger than previously derived from spectral line analyses. The derived equatorial T_eff of 7900 K indicates Vega's equatorial atmosphere may be convective and provides a possible explanation for the discrepancy. The model has a luminosity of ~37 Lsun, a value 35% lower than Vega's apparent luminosity based on its bolometric flux and parallax, assuming a slowly rotating star. The model luminosity is consistent with the mean absolute magnitude of A0V stars. Our model predicts the spectral energy distribution of Vega as viewed from its equatorial plane; a model which may be employed in radiative models for the surrounding debris disk.

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arXiv2006

First Results from the CHARA Array. IV. The Interferometric Radii of Low-Mass Stars

We have measured the angular diameters of six M dwarfs with the CHARA Array, a long-baseline optical interferometer located at Mount Wilson Observatory. Spectral types range from M1.0 V to M3.0 V and linear radii from 0.38 to 0.69 Rsun. These results are consistent with the seven other M-dwarf radii measurements from optical interferometry and with those for sixteen stars in eclipsing binary systems. We compare all directly measured M dwarf radii to model predictions and find that current models underestimate the true stellar radii by up to 15-20%. The differences are small among the metal-poor stars but become significantly larger with increasing metallicity. This suggests that theoretical models for low mass stars may be missing some opacity source that alters the computed stellar radii.

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arXiv2006

Hubble Space Telescope Observations of SV Cam: II. First Derivative Lightcurve Modelling using PHOENIX and ATLAS Model Atmospheres

The variation of the specific intensity across the stellar disc is essential input parameter in surface brightness reconstruction techniques such as Doppler imaging, where the relative intensity contributions of different surface elements are important in detecting starspots. We use PHOENIX and ATLAS model atmospheres to model lightcurves derived from high precision (S/N ~ 5000) HST data of the eclipsing binary SV Cam (F9V + K4V), where the variation of specific intensity across the stellar disc will determine the contact points of the binary system lightcurve. For the first time we use chi^2 comparison fits to the first derivative profiles to determine the best-fitting model atmosphere. We show the wavelength dependence of the limb darkening and that the first derivative profile is sensitive to the limb-darkening profile very close to the limb of the primary star. It is concluded that there is only a marginal difference (< 1sigma) between the chi^2 comparison fits of the two model atmospheres to the HST lightcurve at all wavelengths. The usefulness of the second derivative of the light-curve for measuring the sharpness of the primary's limb is investigated, but we find that the data are too noisy to permit a quantitative analysis.

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arXiv2005

A "Combination Nova" Outburst in Z Andromedae: Nuclear Shell Burning Triggered by a Disk Instability

We describe observational evidence for a new kind of interacting-binary-star outburst that involves both an accretion instability and an increase in thermonuclear shell burning on the surface of an accreting white dwarf. We refer to this new type of eruption as a combination nova. In late 2000, the prototypical symbiotic star Z Andromedae brightened by roughly two magnitudes in the optical. We observed the outburst in the radio with the VLA and MERLIN, in the optical both photometrically and spectroscopically, in the far ultraviolet with FUSE, and in the X-rays with both Chandra and XMM. The two-year-long event had three distinct stages. During the first stage, the optical rise closely resembled an earlier, small outburst that was caused by an accretion-disk instability. In the second stage, the hot component ejected an optically thick shell of material. In the third stage, the shell cleared to reveal a white dwarf whose luminosity remained on the order of 10^4 Lsun for approximately one year. The eruption was thus too energetic to have been powered by accretion alone. We propose that the initial burst of accretion was large enough to trigger enhanced nuclear burning on the surface of the white dwarf and the ejection of an optically thick shell of material. This outburst therefore combined elements of both a dwarf nova and a classical nova. Our results have implications for the long-standing problem of producing shell flashes with short recurrence times on low-mass white dwarfs in symbiotic stars.

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arXiv2005

On the Limb Darkening, Spectral Energy Distribution, and Temperature Structure of Procyon

We have fit synthetic visibilities from 3-D (CO5BOLD + PHOENIX) and 1-D (PHOENIX, ATLAS 12) model stellar atmospheres of Procyon (F5 IV) to high-precision interferometric data from the VLTI Interferometer (K-band) and from the Mark III interferometer (500 nm and 800 nm). These data sets provide a test of theoretical wavelength dependent limb-darkening predictions. The work of Allende Prieto et al. has shown that the temperature structure from a spatially and temporally averaged 3-D hydrodynamical model produces significantly less limb darkening at 500 nm relative to the temperature structure of a 1-D MARCS model atmosphere with a standard mixing-length approximation for convection. Our direct fits to the interferometric data confirm this prediction. A 1-D ATLAS 12 model with ``approximate overshooting'' provides the required temperature gradient. We show, however, that 1-D models cannot reproduce the ultraviolet spectrophotometry below 160 nm with effective temperatures in the range constrained by the measured bolometric flux and angular diameter. We find that a good match to the full spectral energy distribution can be obtained with a composite model consisting of a weighted average of twelve 1-D model atmospheres based on the surface intensity distribution of a 3-D granulation simulation. We emphasize that 1-D models with overshooting may realistically represent the mean temperature structure of F-type stars like Procyon, but the same models will predict redder colors than observed because they lack the multicomponent temperature distribution expected for the surfaces of these stars.

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arXiv2005

Semi-empiric Radiative Transfer Modeling of FUSE Stellar Spectra

We present an overview of radiative transfer modeling efforts to interpret spectra of a variety of stellar objects observed with FUSE. Detailed radiative transfer modeling of high ion emission line profiles of C III and O VI observed in the far-UV spectrum provides a powerful means to probe the thermal and dynamic properties of high-temperature plasmas in the atmospheres of stars. We model asymmetric emission lines of C III lam977 observed in spectra of luminous cool stars such as Alpha Aqr, to infer the wind- and microturbulence velocity structures of the upper chromosphere. Semi-empiric radiative transfer models that include transition region temperature conditions, are further developed based on detailed fits to O VI resonance emission lines in the supergiant Alpha Aqr, the classical Cepheid variable Beta Dor, and to self-absorbed O VI emission lines in the cataclysmic variable SW UMa. We observe that the C III resonance line profile of Alpha Aqr assumes a remarkable asymmetric shape, reminiscent of P Cygni type profiles observed in hot luminous supergiants. The model calculations indicate outflow velocities above ~140 km/s at kinetic temperatures of 65 kK and higher. Based on detailed model fits to the narrow red-shifted and self-absorbed O VI emission lines of SW UMa we compute that the gas- and electron-density exceed the density conditions of the upper solar transition region by about three orders of magnitude. We discuss how detailed semi-empiric fits to emission lines observed with the high spectral resolution of FUSE can provide reliable constraints on the mass-loss or mass-accretion rates in these objects.

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arXiv2005

Direct Evidence for a Polar Spot on SV Cam

We have used spectrophotometric data from the Hubble Space Telescope to eclipse-map the primary component of the RS CVn binary SV Cam over 9 HST orbits. We find from these observations and the HIPPARCOS parallax that the surface flux in the eclipsed low-latitude region of the primary is about 30% lower than computed from a PHOENIX model atmosphere at the effective temperature that best fits the spectral energy distribution of the eclipsed flux. This can only be accounted for if about a third of the primary's surface is covered with unresolved dark star-spots. Extending this to the full surface of the primary, we find that even taking into account this spot filling factor there is an additional flux deficit on the primary star. This can only be explained if there is a large polar spot on the primary star extending from the pole to latitude 48 degrees.

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arXiv2003

Tests of stellar model atmospheres by optical interferometry: VLTI/VINCI limb-darkening measurements of the M4 giant psi phe

We present K-band interferometric measurements of the limb-darkened intensity profile of the M4 giant star psi Phoenicis obtained with VLTI/VINCI. High-precision squared visibility amplitudes in the second lobe of the visibility function were obtained employing two 8.2 m UTs. This succeeded one month after light from UTs was first combined for interferometric fringes. In addition, we sampled the visibility function at small spatial frequencies using the 40cm test siderostats. Our measurement constrains the diameter of the star as well as its CLV. We construct a spherical hydrostatic PHOENIX model atmosphere based on spectrophotometric data from the literature and confront its CLV prediction with our interferometric measurement. We compare as well CLV predictions by plane-parallel hydrostatic PHOENIX, ATLAS9, and ATLAS12 models. We find that the Rosseland angular diameter as predicted by comparison of the spherical PHOENIX model with spectrophotometry is in good agreement with our interferometric diameter measurement. The shape of our measured visibility function in the second lobe is consistent with all considered PHOENIX and ATLAS model predictions, and significantly different from UD and FDD models. We derive high-precision fundamental parameters for psi Phe, namely a Rosseland angular diameter of 8.13 +/- 0.2 mas, with the Hipparcos parallax corresponding to a Rosseland linear radius R of 86 +/- 3 Rsun and an effective temperature of 3550 +/- 50 K, with R corresponding to a luminosity of log (L/Lsun)=3.02 +/- 0.06. Together with evolutionary models, these values are consistent with a mass of 1.3 +/- 0.2 Msun, and a surface gravity of log g = 0.68 +/- 0.11.

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