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Post Info TOPIC: HR7355
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HR7355
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Title: Basic parameters and properties of the rapidly rotating magnetic helium strong B star HR7355
Authors: Th. Rivinius, R.H.D. Townsend, O. Kochukhov, S. Stefl, D. Baade, L. Barrera, Th. Szeifert

The spectral and magnetic properties and variability of the B2Vnp emission-line magnetosphere star HR7355 were analysed. The object rotates at almost 90% of the critical value, meaning it is a magnetic star for which oblateness and gravity darkening effects cannot be ignored any longer. A detailed modelling of the photospheric parameters indicate that the star is significantly cooler than suggested by the B2 spectral type, with T_eff=17500K atypically cool for a star with a helium enriched surface. The spectroscopic variability of helium and metal lines due to the photospheric abundance pattern is far more complex than a largely dipolar, oblique magnetic field of about 11 to 12kG may suggest. Doppler imaging shows that globally the most He enriched areas coincide with the magnetic poles and metal enriched areas with the magnetic equator. While most of the stellar surface is helium enriched with respect to the solar value, some isolated patches are depleted. The stellar wind in the circumstellar environment is governed by the magnetic field, i.e. the stellar magnetosphere is rigidly corotating with the star. The magnetosphere of HR7355 is similar to the well known \sigma Ori E: the gas trapped in the magnetospheric clouds is fairly dense, and at the limit to being optically thick in the hydrogen emission. Apart from a different magnetic obliquity, HR7355 and the more recently identified HR5907 have virtually identical stellar and magnetic parameters.

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Blobrana


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Title: HR 7355 - another rapidly braking He-strong CP star?
Authors: Z. Mikulasek, J. Krticka, G. W. Henry, S. N. de Villiers, E. Paunzen, M. Zejda

Strong meridional mixing induced by rapid rotation is one reason why all hot main-sequence stars are not chemically peculiar. However, the finding that the He-strong CP star HR 7355 is a rapid rotator complicates this concept. Our goal is to explain the observed behaviour of HR 7355 based on period analysis of all available photometry. Over two years, we acquired 114 new BV observations of HR 7355 at observatories in Arizona, U.S.A and Cape Town, South Africa. We performed period analyses of the new observations along with new analyses of 732 archival measurements from the Hipparcos and ASAS projects. We find that the light curves of HR 7355 in various filters are quite similar, with amplitudes 0.035(4), 0.036(4), and 0.038(3) mag in B, Hp and V, respectively. The light curves are double-peaked, with unevenly deep minima. We substantially refine the rotational period to be P=0.5214410(4) d, indicating that HR 7355 is the most rapidly rotating CP star known. Our period analyses reveal a possible lengthening of the rotational period with dP/dt/P = 2.4(8)x 10^-6/yr. We conclude that the shape and amplitude of HR 7355 light curves are typical of magnetic He-strong CP stars, for which light variations are the result of photospheric spots on the surface of a rotating star. We hypothesise that the light variations are caused mainly by an uneven distribution of overabundant helium on the star's surface. We briefly describe and discuss the cause of the rapid rotational braking of the star.

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Blobrana


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Posts: 131433
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HD 182180
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HR 7355 is a He-rich B2Vn  (early type magnetic) star  with rapid rotation in the constellation Sagittarius.
HR7355 (HD 182180) has  v sin i = 320 km/s and a  period of 0.52 days, estimated from HIPPARCOS data, this makes it the most rapidly rotating early-type magnetic star discovered so far.
The star is rotating so fast it is   almost at the critical limit.

Spectral Type: B2Vn
Magnitude:      5.992
Distance:      403.226 pc

Google Sky file (1kb, kmz)

Position(2000): RA: 19h24m30.1s, DEC: -27°51m56.0s


Title: The most rapidly rotating He-strong emission-line star: HR7355
Authors:  T. Rivinius, S.Stefl, R. Townsend, D. Baade

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Blobrana


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Posts: 131433
Date:
HR7355
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Title: The most rapidly rotating He-strong emission line star: HR7355
Authors: Th. Rivinius (1), S. Stefl (1), R.H.D. Townsend (2), D. Baade (3) ((1) ESO - Chile, (2) Bartol Research Institute, University of Delaware, (3) ESO - Garching)

Using archival spectroscopic and photometric data, we searched for massive stars with Balmer-emission consistent with magnetically confined circumstellar material. HR 7355 is a formerly unknown He-strong star showing Balmer emission. At V=6.02 mag, it is one of the brightest objects simultaneously showing anomalous helium absorption and hydrogen emission. Among similar objects, only sigma Ori E has so far been subjected to any systematic analysis of the circumstellar material responsible for the emission. We argue that the double-wave photometric period of 0.52d corresponds to the rotation period. In tandem with the high projected equatorial velocity, v sin i=320 km/s, this short period suggests that HR 7355 is the most rapidly rotating He-strong star known to date; a class that was hitherto expected to host stars with slow to moderate rotation only.

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