skip to main content

SciTech ConnectSciTech Connect

Title: An HST COS 'SNAPSHOT' spectrum of the K supergiant λ Vel (K4Ib-II)

We present a far-ultraviolet spectrum of the K4 Ib-II supergiant λ Vel obtained with the Hubble Space Telescope's Cosmic Origins Spectrograph (COS) as a part of the SNAPshot program 'SNAPing coronal iron' (GO 11687). The observation covers a wavelength region (1326-1467 Å) not previously recorded for λ Vel at a spectral resolving power of R ∼ 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 H I Lyα and the O I (UV 2) triplet emission near λ1304. The molecular CO and H{sub 2} fluorescences are weaker than in the early-K giant α Boo while the Fe II and Cr II lines, also pumped by H I Lyα, are stronger in λ Vel. This pattern of relative line strengths between the two stars is explained by the lower iron-group element abundance in α Boo, which weakens that star's Fe II and Cr II emission without reducing the molecular fluorescences. The λ Vel spectrum shows fluorescent Fe II, Cr II, and H{sub 2} emission similar to that observed in the M supergiant α Ori, but more numerous well-defined narrow emissions from CO.more » The additional CO emissions are visible in the spectrum of λ Vel since that star does not have the cool, opaque circumstellar shells that surround α Ori and produce broad circumstellar CO (A-X) band absorptions that hide those emissions in the cooler star. The presence of Si IV emission in λ Vel indicates a ∼8 × 10{sup 4} K plasma that is mixed into the cooler chromosphere. Evidence of the stellar wind is seen in the C II λλ1334,1335 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 (∼45 versus ∼30 km s{sup –1}) and turbulence (∼27 versus <21 km s{sup –1}) with a more quickly accelerating wind (β = 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{sup –1} relative to the chromospheric emission, suggesting formation in lower levels of the accelerating wind and their widths indicate a higher turbulence in the λ Vel wind compared to α Ori.« less
Authors:
 [1] ;  [2] ;  [3] ; ; ;  [4]
  1. NASA/GSFC Code 667, Greenbelt, MD 20771 (United States)
  2. University of Colorado, CASA, 389-UCB, Boulder, CO 80309 (United States)
  3. School of Physics, Trinity College, Dublin 2 (Ireland)
  4. Deptartment of Physics, Catholic University of America, Washington, DC 20064 (United States)
Publication Date:
OSTI Identifier:
22370478
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 794; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; CARBON MONOXIDE; CHROMOSPHERE; ELEMENT ABUNDANCE; EXCITATION; FAR ULTRAVIOLET RADIATION; FLUORESCENCE; HEAT EXCHANGERS; HYDROGEN; IRON; LYMAN LINES; MASS TRANSFER; RESOLUTION; SIMULATION; SPACE; SPECTRA; STARS; STELLAR WINDS; TELESCOPES; TURBULENCE