AN HST COS %26quot;SNAPSHOT%26quot; SPECTRUM OF THE K SUPERGIANT lambda Vel (K4Ib-II)*

作者：Carpenter Kenneth G^*; Ayres Thomas R; Harper Graham M; Kober Gladys; Nielsen Krister E; Wahlgren Glenn M

来源：Astrophysical Journal, 2014, 794(1): 41.

DOI：10.1088/0004-637X/794/1/41

• stars: atmospheres
• stars: chromospheres
• stars individual (lambda Vel, alpha Boo, alpha On)
• stars: late-type
• stars: mass-loss
• supergiants

We present a far-ultraviolet spectrum of the K4 lb-II supergiant Vel obtained with the Hubble Space Telescope%26apos;s Cosmic Origins Spectrograph (COS) as a part of the SN, APshot program %26quot;SNAPing coronal iron%26quot; (GO 11687). The observation covers a wavelength region (1326-1467 angstrom) not previously recorded for Vel at a spectral resolving power of R similar to 20,000 and displays strong emission and absorption features, superposed on a bright chromospheric continuum. Fluorescent excitation is responsible for much of the observed emission, mainly powered by strong Hi Ly alpha and the 0 (UV 2) triplet emission near),1304. The molecular CO and H-2 fluorescences are weaker than in the early-K giant alpha Boo while the Fell and Cr II lines, also pumped by HI Ly alpha, are stronger in lambda Vel. This pattern of relative line strengths between the two stars is explained by the lower iron-group element abundance in a Boo, which weakens that star%26apos;s Fell and Cr II emission without reducing the molecular fluorescences. The lambda Vel spectrum shows fluorescent Fe ii, Cr ii, and H-2 emission similar to that observed in the M supergiant alpha Ori, but more numerous well-defined narrow emissions from CO. The additional CO emissions are visible in the spectrum of Vel since that star does not have the cool, opaque circumstellar shells that surround a On and produce broad circumstellar CO (A-X) band absorptions that hide those emissions in the cooler star. The presence of Si w emission in Vel indicates a similar to 8 x 10(4) K plasma that is mixed into the cooler chromosphere. Evidence of the stellar wind is seen in the C)1334,1335 lines and in the blueshifted Fe II and Ni ii wind absorption lines. Line modeling using Sobolev with Exact Integration for the C ii lines indicates a larger terminal velocity (similar to 45 versus similar to 30 km s(-1)) and turbulence (similar to 27 versus %26lt;21 km s(-1)) with a more quickly accelerating wind (t = 0.35 versus 0.7) at the time of this COS observation in 2010 than derived from Goddard High Resolution Spectrograph data obtained in 1994. The Fe ii and Ni ii absorptions are blueshifted by 7.6 km s(-1) relative to the chromospheric emission, suggesting formation in lower levels of the accelerating wind and their widths indicate a higher turbulence in the Vel wind compared to a On.

• 出版日期2014-10-10