Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres

Abstract

We search the Hubble Space Telescope (HST) archive for previously unanalyzed observations of stellar H I Lyα emission lines, our primary purpose being to look for new detections of Lyα absorption from the outer heliosphere, and to also search for analogous absorption from the astrospheres surrounding the observed stars. The astrospheric absorption is of particular interest because it can be used to study solar-like stellar winds that are otherwise undetectable. We find and analyze 33 HST Lyα spectra in the archive. All the spectra were taken with the E140M grating of the Space Telescope Imaging Spectrograph (STIS) instrument on board HST. The HST/STIS spectra yield 4 new detections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD 165185) and 7 new detections of astrospheric absorption (EV Lac, 70 Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling the previous number of heliospheric and astrospheric detections. When combined with previous results, 10 of 17 lines of sight within 10 pc yield detections of astrospheric absorption. This high detection fraction implies that most of the ISM within 10 pc must be at least partially neutral, since the presence of H I within the ISM surrounding the observed star is necessary for an astrospheric detection. In contrast, the detection percentage is only 9.7% (3 out of 31) for stars beyond 10 pc. Our Lyα analyses provide measurements of ISM H I and D I column densities for all 33 lines of sight, and we discuss some implications of these results. Finally, we measure chromospheric Lyα fluxes from the observed stars. We use these fluxes to determine how Lyα flux correlates with coronal X-ray and chromospheric Mg II emission, and we also study how Lyα emission depends on stellar rotation.

Atmospheric escape from HD 189733b observed in H I Lyman-α: detailed analysis of HST/STIS September 2011 observations

“…4). The resulting profile shows a double peak emission, which is usually seen in the Lyman-α line of cool stars, and can be interpreted as self-absorption in the chromosphere of the star (Wood et al 2005).…”

“…As a result, the spectrally resolved line, at the resolution of the G140M spectrograph, is composed of two peaks separated by this deep absorption. In order to estimate the Lyman-α line flux and profile, we fitted the line profile as observed in September 2011, following the method of Wood et al (2005) and Ehrenreich et al (2011). The best result was obtained by modeling the stellar emission line using two Voigt profiles with the same width and damping constant.…”

Search for indications of stellar mass ejections using FUV spectra

“…Wood et al (2005) investigated ISM and astrospheric absorption in Lyα profiles of more than 60 late-type stars. These authors showed that the ISM absorption in Lyα is much stronger than the astrospheric absorption.…”

The Galactic Environment of the Sun: Interstellar Material Inside and Outside of the Heliosphere

“…2.1.2). This heliospheric hydrogen produces a Lyα absorption signature that has been detected in some HST spectra (Linsky and Wood 1996;Wood et al 2005). The heliospheric contribution to the hydrogen wall can be completely obscured by the interstellar absorption if the ISMa column density is too high and the ISMa absorption therefore too broad.…”

“…Most detections of heliospheric absorption sample this population, and originate near the upwind direction of the CHISM flow, because this is where the heating, compression, and deceleration of the ISMa neutrals is strongest. Deceleration of the H • flow in the radial direction is required to separate the heliospheric absorption from that of the ISMa (Wood et al 2005). Several studies have demonstrated that hydrodynamic models of the heliosphere can reproduce the observed kinematic characteristics of the hydrogen wall absorption, depending on exactly what is assumed about the properties of the CHISM (Gayley et al 1997;Wood et al 2000aWood et al , 2007.…”