Chromospheric models for ALTAIR (A7 IV-V)

Ferrero, R. Freire; Gouttebroze, P.; Catalano, S.; Marilli, E.; Bruhweiler, F. C.; Kondo, Y.; Hucht, K. van der; Talavera, A.

doi:10.1086/175238

Cited by 19 publications

(10 citation statements)

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“…We investigate two test models corresponding to the theoretical limits for the gravity-darkening parameter β, namely, 0.08 (convective atmospheres) and 0.25 (radiative atmospheres). To obtain a mean effective temperature compatible with previous works (between 7500 K and 8000 K; see for example Esparmer & North 2003and Ferrero et al 1995) the adopted polar temperatures T p for the test models are 8000 K and 8500 K, corresponding to β = 0.08 and 0.25, respectively. The chosen stellar mass M = 1.8 M is given by Malagnini & Morossi (1990).…”

Section: Results From the χ 2 Analysismentioning

confidence: 85%

“…Such low T eq requires a gravity-darkening exponent β 0.08 (Lucy 1967;Claret 2000c), so that a latitudinal dependent β parameter should be more convenient for Altair (e.g., a continuous variation from the radiative limit β = 0.25 to the convective limit β = 0.08 between the poles and the equator). The hypothesis of a convective equatorial region is supported by several works showing that Altair has a chromosphere and a corona, possibly linked to subphotospheric convective zones (e.g., Ferrero et al 1995).…”

Section: Rotation and Gravity Darkening Lawsmentioning

confidence: 91%

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Gravitational-darkening of Altair from interferometry

Souza¹,

Kervella

Jankov

et al. 2005

A&A

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Interferometric observations have revealed that the rapid rotator Altair is a flattened star with a non-centrally symmetric intensity distribution. In this work we perform for the first time a physically consistent analysis of all interferometric data available so far, corresponding to three different interferometers operating in several spectral bands. These observations include new data (squared visibilities in the H and K bands from VLTI-VINCI) as well as previously published data (squared visibilities in the K band from PTI and squared visibilities, triple amplitudes, and closure phases in the visible between 520 nm and 850 nm from NPOI). To analyze these data we perform a χ 2 minimization using an interferometry-oriented model for fast rotators, which includes Roche approximation, limb-darkening, and von Zeipel-like gravity-darkening. Thanks to the rich interferometric data set available and to this physical model, the main uniqueness problems were avoided. As a result, we show that the observations can only be explained if Altair has a gravity-darkening compatible with the expected value for hot stars, i.e., the von Zeipel effect (T eff ∝ g 0.25 ).

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Section: Results From the χ 2 Analysismentioning

confidence: 85%

Section: Rotation and Gravity Darkening Lawsmentioning

confidence: 91%

Gravitational-darkening of Altair from interferometry

Souza¹,

Kervella

Jankov

et al. 2005

A&A

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“…It has been argued that Altair might undergo substantial surface differential rotation since it was found to be an X-ray source (Ferrero et al 1995). The parameter q 2 /q 1 is also a sensitive indicator for solar-like surface differential rotation with the equator rotating at a higher velocity than the polar regions do.…”

Section: Differential Rotation?mentioning

confidence: 99%

Altair's inclination from line profile analysis

Reiners

Royer

2004

A&A

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Abstract. We present a detailed spectroscopic study of line broadening in the A7IV-V star Altair. In a wavelength region covering 690 Å we reconstruct the overall broadening profile taking into account more than 650 spectral lines. From the broadening profile we determine the projected rotational velocity v sin i, derive an upper limit for the equatorial velocity v from the shape of the profile and search for signatures of differential rotation. Our redetermined value of v sin i is (227 ± 11) km s −1 . Measuring the first two zeros of the Fourier transformed broadening profile yields no signatures of differential rotation. We derive that Altair is seen under an inclination angle higher than i = 68• and it rotates at v < 245 km s −1 or slower than 53% of breakup velocity on a 1σ level.

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“…Their broad, double-horned profile is well fitted by optically thin emission originating on a star with V sin i = 210 km s −1 , that exhibits either strong limb brightening or emitting regions preferentially located towards the equator . A chromosphere covering mainly equatorial regions was also suggested as a possible scenario by Ferrero et al (1995), deduced from the modelling of the H i Ly α line profiles.…”

Section: Discussionmentioning

confidence: 97%

“…Besides X-rays, Ly α emission that is broadened by V sini =220 km/s has also been detected from Altair by IUE (International Ultraviolet Explorer) and was identified as a chromospheric component (Ferrero et al 1995). The analysis of HST/GHRS spectra including Si iii and N v showed the presence of a transition region (Simon & Landsman 1997); also supported by FUSE (Far Ultraviolet Spectroscopic Explorer) spectra that cover prominent C iii and O vi emission lines.…”

Section: Introductionmentioning

confidence: 83%

Altair – the “hottest” magnetically active star in X-rays

Robrade

Schmitt

2009

A&A

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Context. The onset of stellar magnetic activity is related to the operation of dynamo processes that require the development of an outer convective layer. This transition of stellar interior structure is expected to occur in late A-type stars. Aims. The A7 star Altair is one of the hottest magnetically active stars. Its proximity to the Sun allows a detailed investigation of a corona in X-rays for a star with a shallow convection zone. Methods. We used a deep XMM-Newton observation of Altair and analyzed X-ray light curves, spectra, and emission lines. We investigated the temporal behavior and properties of the X-ray emitting plasma and studied the global coronal structure of Altair. Results. Altair's corona with an X-ray luminosity of L X = 1.4 × 10 27 erg/s and an activity level of log L X /L bol = −7.4 is located predominantly at low latitude regions and exhibits X-ray properties that are overall very similar to those of other weakly active stars. The X-ray emission is dominated by cool plasma (1-4 MK) at low density, and elemental abundances exhibit a solar-like FIP effect and Ne/O ratio. The X-ray brightness varies by 30% over the observation, most likely due to rotational modulation and minor activity; in contrast, no strong flares or significant amounts of hot plasma were detected. The X-ray activity level of Altair is apparently close to the saturation level, which is reduced by roughly four orders of magnitude when compared to late-type stars. Conclusions. With its fast rotation, Altair provides an inefficient, but very stable dynamo that mainly operates in convective layers below its "cooler" surface areas around the equator. This dynamo mechanism results in magnetic activity and leads to X-ray properties that are similar to those of the quiescent Sun, despite very different underlying stars.

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Chromospheric models for ALTAIR (A7 IV-V)

Cited by 19 publications

References 0 publications

Gravitational-darkening of Altair from interferometry

Gravitational-darkening of Altair from interferometry

Altair's inclination from line profile analysis

Altair – the “hottest” magnetically active star in X-rays

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