Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...470.1157w&link_type=abstract
Astrophysical Journal v.470, p.1157
Astronomy and Astrophysics
Astronomy
46
Ism: Abundances, Stars: Individual Constellation Name: Epsilon Indi, Stars: Individual Constellation Name: Lambda Andromedae, Ultraviolet: Stars, Stars: Mass Loss, Ism: Kinematics And Dynamics
Scientific paper
We present new observations of the Lyα lines of ɛ Indi (KS V) and λ Andromedae (G8 IV-III + ?). These data were obtained by the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope. Analysis of the interstellar H I and D I absorption lines reveals that the velocities and temperatures inferred from the H I lines are inconsistent with the parameters inferred from the D I lines, unless the H I absorption is assumed to be produced by two absorption components.
One absorption component is produced by interstellar material. For both lines of sight observed, the velocity of this component is consistent with the velocity predicted by the local flow vector. For the E Ind data, the large velocity separation between the stellar emission and the interstellar absorption allows us to measure the H I column density independent of the shape of the intrinsic stellar Lyα profile. This approach permits us to quote an accurate column density and to assess its uncertainty with far more confidence than in previous analyses, for which the errors were dominated by uncertainties in the assumed stellar profiles. For the short (d = 3.46 pc) line of sight to ɛ Ind, the H I column density is found to be log NHI = 18.0±0.1, which implies an average density for the local interstellar medium (LISM) of nHI = 0.094±0.022 cm-3. For the much longer (d = 23 pc) line of sight to λ And, we estimate the H I column density to be log NHi = 18.45±0.15 which corresponds to an average density of nHI = 0.041±0.014 cm-3. The D/H ratios we measure from the data are (1.6±0.4) x 10-5 and (1.7±0.5) x 1O-5 for ɛ Ind and λ And, respectively. These values are consistent with those measured from observations of Capella, Procyon, and α Cen. We measure LISM temperatures of T = 8500±500 K and T = 11,500±500 K from the ɛ Ind and λ And data, respectively. The λ And temperature is significantly higher than temperatures previously measured from GHRS data, which leads us to speculate that the H I and D I absorption lines may be broadened by multiple ISM components with different velocities. The results of our λ And analysis should be considered as tentative, until GHRS observations of the much narrower Mg II and/or Fe II absorption lines can be obtained.
We believe that hot hydrogen surrounding ɛ Ind and λ And is responsible for the second H I absorption component, although we consider this conclusion to be tentative in the case of λ And. These "hydrogen walls" are produced by the interaction of the winds of these stars with the surrounding interstellar material. An anologous solar hydrogen wall has been predicted by recent models of the heliospheric interface region and confirmed by GHRS observations of α Cen. The column densities we measure for the second components are log NHI = 14.2±0.2 and log NHI = 14.8±0.2 for ɛ Ind and λ And, respectively, and the temperatures are 100,000±20000 K and 62,000±18 000 K. These temperatures are too hot for the solar hydrogen wall, and for ɛ Ind the velocity of the second component is clearly inconsistent with the solar hydrogen wall. Thus, for these components we assume a stellar origin, in which the higher temperatures are a consequence of higher interstellar wind velocities in the stellar rest frames. Because the heliospheric models demonstrate the importance of the solar wind in the formation of the solar hydrogen wall, our detection of anologous structure around ɛ Ind and perhaps λ And may constitute a first detection of solar-like winds around dwarf and subgiant stars.
Alexander Russell W.
Linsky Jeffrey L.
Wood Brian E.
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