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Register a new userVisible and near-infrared spectro-interferometric analysis of the edge-on Be star $omicron$ Aquarii
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Elisson Saldanha da Gama de Almeida
Publication date
2020
fields
Physics
and research's language is
English
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We present a detailed visible and near-IR spectro-interferometric analysis of the Be-shell star $omicron$ Aquarii from quasi-contemporaneous CHARA/VEGA and VLTI/AMBER observations. We measured the stellar radius of $omicron$ Aquarii as 4.0 $pm$ 0.3 $mathrm{R_{odot}}$. We constrained the disk geometry and kinematics using a kinematic model and a MCMC fitting procedure. The disk sizes in H$alpha$ and Br$gamma$ were found to be similar, at $sim$10-12 $mathrm{D_{star}}$, which is uncommon since most results for Be stars show a larger extension in H$alpha$ than in Br$gamma$. We found that the inclination angle $i$ derived from H$alpha$ is significantly lower ($sim$15 deg) than the one derived from Br$gamma$. The disk kinematics were found to be near to the Keplerian rotation in Br$gamma$, but not in H$alpha$. After analyzing all our data using a grid of HDUST models (BeAtlas), we found a common physical description for the disk in both lines: $Sigma_{0}$ = 0.12 g cmtextsuperscript{-2} and $m$ = 3.0. The stellar rotational rate was found to be very close ($sim$96%) to the critical value. Our analysis of multi-epoch H$alpha$ profiles and imaging polarimetry indicates that the disk has been stable for at least 20 years. Compared to Br$gamma$, the data in H$alpha$ shows a substantially different picture that cannot fully be understood using the current physical models of Be star disks. $omicron$ Aquarii presents a stable disk, but the measured $m$ is lower than the standard value in the VDD model for steady-state. Such long-term stability can be understood in terms of the high rotational rate for this star, the rate being a main source for the mass injection in the disk. Our results on the stellar rotation and disk stability are consistent with results in the literature showing that late-type Be stars are more likely to be fast rotators and have stable disks.
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We present near-infrared H and K-band spectro-interferometric observations of the gaseous disk around the primary Be star in the delta Sco binary system, obtained in 2007 (between periastron passages in 2000 and 2011). Observations using the CHARA/MIRC instrument at H-band resolve an elongated disk with a Gaussian FWHM 1.18 x 0.91 mas. Using the Keck Interferometer, the source of the K-band continuum emission is only marginally spatially resolved, and consequently we estimate a relatively uncertain K-band continuum disk FWHM of 0.7 +/- 0.3 mas. Line emission on the other hand, He1 (2.0583 micron) and Br gamma (2.1657 micron), is clearly detected, with about 10% lower visibilities than those of the continuum. When taking into account the continuum/line flux ratio this translates into much larger sizes for the line emission regions: 2.2 +/- 0.4 mas and 1.9 +/- 0.3 mas for He1 and Br gamma respectively. Our KI data also reveal a relatively flat spectral differential phase response, ruling out significant off-center emission. We expect these new measurements will help constrain dynamical models being actively developed in order to explain the disk formation process in the delta Sco system and Be stars in general.
Context. Classical Be stars are hot non-supergiant stars surrounded by a gaseous circumstellar disk that is responsible for the observed infrared-excess and emission lines. The phenomena involved in the disk formation still remain highly debated. Aims. To progress in the understanding of the physical process or processes responsible for the mass ejections and test the hypothesis that they depend on the stellar parameters, we initiated a survey on the circumstellar environment of the brightest Be stars. Methods. To achieve this goal, we used spectro-interferometry, the only technique that combines high spectral (R=12000) and high spatial ($theta_{rm min}$=4,mas) resolutions. Observations were carried out at the Paranal observatory with the VLTI/AMBER instrument. We concentrated our observations on the Br$gamma$ emission line to be able to study the kinematics within the circumstellar disk. Our sample is composed of eight bright classical Be stars : $alpha$ Col, $kappa$ CMa, $omega$ Car, p Car, $delta$ Cen, $mu$ Cen, $alpha$ Ara, and textit{o} Aqr. Results. We managed to determine the disk extension in the line and the nearby continuum for most targets. We also constrained the disk kinematics, showing that it is dominated by rotation with a rotation law close to the Keplerian one. Our survey also suggests that these stars are rotating at a mean velocity of V/V$_{rm c}$,=,0.82,$pm$,0.08. This corresponds to a rotational rate of $Omega/Omega_{rm c}$,=,0.95,$pm$,0.02 Conclusions. We did not detect any correlation between the stellar parameters and the structure of the circumstellar environment. Moreover, it seems that a simple model of a geometrically thin Keplerian disk can explain most of our spectrally resolved K-band data. Nevertheless, some small departures from this model have been detected for at least two objects (i.e, $kappa$ CMa and $alpha$ Col). Finally, our Be stars sample suggests that rotation is the main physical process driving the mass-ejection. Nevertheless, smaller effects from other mechanisms have to be taken into account to fully explain how the residual gravity is compensated.
Be stars are rapid rotators surrounded by a gaseous disk envelope whose origin is still under debate. This envelope is responsible for observed emission lines and large infrared excess. To progress in the understanding of the physical processes involved in the disk formation, we estimate the disk parameters for a sample of Be stars and search for correlations between these parameters and stellar properties. We performed spectro-interferometric observations of 26 Be stars in the region of the Br$gamma$ line to study the kinematical properties of their disks through the Doppler effect. Observations were performed at the Paranal observatory with the VLTI/AMBER interferometer. This instrument provides high spectral and high spatial resolutions. We modeled 18 Be stars with emission in the Br$gamma$ line. The disk kinematic is described by a quasi-Keplerian rotation law, with the exception of HD28497 that presents a one-arm density-wave structure. Using a combined sample, we derived a mean value for the velocity ratio V/Vc=0.75, and found that rotation axes are probably randomly distributed in the sky. Disk sizes in the line component model are in the range of 2-13 stellar radii and do not correlate with the effective temperature or spectral type. However, we found that the maximum size of a stable disk correlates with the rotation velocity at the inner part of the disk and the stellar mass. We found that, on average, the Be stars of our combined sample do not rotate at their critical velocity. However, the centrifugal force and mass of the star defines an upper limit size for a stable disk configuration. For a given rotation, high-mass Be stars tend to have more compact disks than their low-mass counterparts. It would be interesting to follow up the evolution of the disk size in variable stars to better understand the formation and dissipation processes of their circumstellar disks.
We present an analysis of the visible through near infrared spectrum of Eta Carinae and its ejecta obtained during the Eta Carinae Campaign with the UVES at the ESO VLT. This is a part of larger effort to present a complete Eta Carinae spectrum, and extends the previously presented analyses with the HST/STIS in the UV (1240-3159 A) to 10,430 A. The spectrum in the mid and near UV is characterized by the ejecta absorption. At longer wavelengths, stellar wind features from the central source and narrow emission lines from the Weigelt condensations dominate the spectrum. However, narrow absorption lines from the circumstellar shells are present. This paper provides a description of the spectrum between 3060 and 10,430 A, including line identifications of the ejecta absorption spectrum, the emission spectrum from the Weigelt condensations and the P-Cygni stellar wind features. The high spectral resolving power of VLT/UVES enables equivalent width measurements of atomic and molecular absorption lines for elements with no transitions at the shorter wavelengths. However, the ground based seeing and contributions of nebular scattered radiation prevent direct comparison of measured equivalent widths in the VLT/UVES and HST/STIS spectra. Fortunately, HST/STIS and VLT/UVES have a small overlap in wavelength coverage which allows us to compare and adjust for the difference in scattered radiation entering the instruments apertures. This paper provides a complete online VLT/UVES spectrum with line identifications and a spectral comparison between HST/STIS and VLT/UVES between 3060 and 3160 A.
The Visible and Near Infrared (VNIR) is one of the modules of EChO, the Exoplanets Characterization Observatory proposed to ESA for an M-class mission. EChO is aimed to observe planets while transiting by their suns. Then the instrument had to be designed to assure a high efficiency over the whole spectral range. In fact, it has to be able to observe stars with an apparent magnitude Mv= 9-12 and to see contrasts of the order of 10-4 - 10-5 necessary to reveal the characteristics of the atmospheres of the exoplanets under investigation. VNIR is a spectrometer in a cross-dispersed configuration, covering the 0.4-2.5 micron spectral range with a resolving power of about 330 and a field of view of 2 arcsec. It is functionally split into two channels respectively working in the 0.4-1 and 1.0-2.5 micron spectral ranges. Such a solution is imposed by the fact the light at short wavelengths has to be shared with the EChO Fine Guiding System (FGS) devoted to the pointing of the stars under observation. The spectrometer makes use of a HgCdTe detector of 512 by 512 pixels, 18 micron pitch and working at a temperature of 45K as the entire VNIR optical bench. The instrument has been interfaced to the telescope optics by two optical fibers, one per channel, to assure an easier coupling and an easier colocation of the instrument inside the EChO optical bench.
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