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Titolo:
CONSTRAINTS ON THE GEOMETRY OF CIRCUMSTELLAR ENVELOPES - OPTICAL INTERFEROMETRIC AND SPECTROPOLARIMETRIC OBSERVATIONS OF 7 BE STARS
Autore:
QUIRRENBACH A; BJORKMAN KS; BJORKMAN JE; HUMMEL CA; BUSCHER DF; ARMSTRONG JT; MOZURKEWICH D; ELIAS NM; BABLER BL;
Indirizzi:
USN OBSERV,NRL,OPT INTERFEROMETER PROJECT,CODE AD5,3450 MASSACHUSETTSAVE NW WASHINGTON DC 20392 UNIV SPACE RES ASSOC WASHINGTON DC 20024 MAX PLANCK INST EXTRATERR PHYS D-85748 GARCHING GERMANY UNIV WISCONSIN,SPACE ASTRON LAB MADISON WI 53706 UNIV WISCONSIN,DEPT ASTRON MADISON WI 53706 UNIV TOLEDO,DEPT PHYS & ASTRON,RITTER OBSERV TOLEDO OH 43606 UNIV DURHAM,DEPT PHYS DURHAM DH1 3LE ENGLAND USN,RES LAB,REMOTE SENSING DIV WASHINGTON DC 20375
Titolo Testata:
The Astrophysical journal
fascicolo: 1, volume: 479, anno: 1997,
parte:, 1
pagine: 477 - 496
SICI:
0004-637X(1997)479:1<477:COTGOC>2.0.ZU;2-F
Fonte:
ISI
Lingua:
ENG
Soggetto:
STELLAR ANGULAR DIAMETERS; VISUAL SURFACE BRIGHTNESS; BASE-LINE INTERFEROMETRY; ROTATING B-STARS; GAMMA-CASSIOPEIAE; SPECTRAL TYPES; CONTINUUM POLARIZATION; WAVELENGTH DEPENDENCE; THOMSON SCATTERING; IRAS OBSERVATIONS;
Keywords:
CIRCUMSTELLAR MATTER; STARS, EMISSION-LINE, BE; TECHNIQUES, INTERFEROMETRIC; TECHNIQUES, POLARIMETRIC;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Science Citation Index Expanded
Citazioni:
74
Recensione:
Indirizzi per estratti:
Citazione:
A. Quirrenbach et al., "CONSTRAINTS ON THE GEOMETRY OF CIRCUMSTELLAR ENVELOPES - OPTICAL INTERFEROMETRIC AND SPECTROPOLARIMETRIC OBSERVATIONS OF 7 BE STARS", The Astrophysical journal, 479(1), 1997, pp. 477-496

Abstract

We have obtained contemporaneous optical interferometry and spectropolarimetry of seven Be stars. The interferometry was done using both continuum and narrow H alpha line filters to observe the circumstellar envelope emission. The envelopes of all seven stars were resolved interferometrically in H alpha. Of these seven stars, the envelopes of fourwere not circularly symmetric and showed clear evidence of elongation. The position angles of the major axis of the elongation were in goodagreement with the disk orientation inferred from the intrinsic polarization data, which samples material within a few stellar radii, and also agreed with previous results from the radio, which samples material out as far as 100 stellar radii but at lower resolution. This finding indicates that the envelope alignment persists over 2 orders of magnitude in radius. Minimum inclination angle estimates from the interferometry are presented for six of the seven stars. Under the assumption that the envelopes are fairly thin circularly symmetric disks, there is a straightforward explanation of the interferometric results. The stars with the greatest elongations, zeta Tau, phi Per, and psi Per, areseen nearly equator-on; gamma Cas is seen at a more intermediate latitude, consistent with previous results; and 48 Per and eta Tau, which are only slightly asymmetric, are nearly pole-on. beta CMi had insufficient coverage to determine whether its envelope is asymmetric. These results are consistent with independent indications of the inclinations based on the polarimetry. Spectral diagnostics have been used in thepast to classify Be stars as pole-on or as shell stars (usually interpreted to indicate an equator-on orientation). Our inclination angle for 48 Per is consistent with its pole-on classification by Slettebak, and the three equator-on stars have all been previously classified as shell stars. However, both gamma Cas and eta Tau have also been previously classified as shell stars, but we find that they have intermediate and pole-on orientations, respectively. We interpret this finding asevidence that, while equator-on stars may preferentially be shell stars, not all shell stars are actually equator-on. Our results show thatthe H alpha emission region extends up to about 12 stellar radii, possibly depending on spectral type. The size of this region correlates with the equivalent width of the H alpha emission. Our results also provide strong constraints for allowed models of Be star envelopes. In particular, for zeta Tau, we derive an upper limit to the disk opening angle of 20 degrees, which limits the vertical extent of the envelope. We also find that the polarization position angle is perpendicular to the interferometric major axis in all cases. This rules out envelope models that are both optically and geometrically thick, since these models produce polarization parallel to the plane of the disk. We conclude that results from the combined interferometry and polarimetry strongly favor the disk paradigm for Be stars over mildly ellipsoidal models.

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Documento generato il 25/11/20 alle ore 04:49:19