Measurements of stellar mass loss rates are used to assess how wind strength varies with coronal activity and age for solar-like stars. Mass loss generally increases with activity, but we find evidence that winds suddenly weaken at a certain activity threshold. Very active stars are often observed to have polar starspots, and we speculate that the magnetic field geometry associated with these spots may be inhibiting the winds. Our inferred mass-loss/age relation represents an empirical estimate of the history of the solar wind. This result is important for planetary studies as well as solar/stellar astronomy, since solar wind erosion may have played an important role in the evolution of planetary atmospheres.

Collisions between the winds of solar-like stars and the local ISM result in a population of hot hydrogen gas surrounding these stars. Absorption from this hot H I can be detected in high resolution Lyα spectra of these stars from the Hubble Space Telescope. The amount of absorption can be used as a diagnostic for the stellar mass loss rate. We present new mass loss rate measurements derived in this fashion for four stars (ǫ Eri, 61 Cyg A, 36 Oph AB, and 40 Eri A). Combining these measurements with others, we study how mass loss varies with stellar activity. We find that for the solar-like GK dwarfs, the mass loss per unit surface area is correlated with X-ray surface flux. Fitting a power law to this relation yields Ṁ ∝ F 1.15±0.20x . The active M dwarf Proxima Cen and the very active RS CVn system λ And appear to be inconsistent with this relation. Since activity is known to decrease with age, the above power law relation for solar-like stars suggests that mass loss decreases with time. We infer a power law relation of Ṁ ∝ t −2.00±0.52 . This suggests that the solar wind may have been as much as 1000 times more massive in the distant past, which may have had important ramifications for the history of planetary atmospheres in our solar system, that of Mars in particular.

Analyses of spectra obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite, together with spectra from the Copernicus and IMAPS instruments, reveal an unexplained very wide range in the observed deuterium/hydrogen (D/H) ratios for interstellar gas in the Galactic disk beyond the Local Bubble. We argue that spatial variations in the depletion of deuterium onto dust grains can explain these local variations in the observed gas-phase D/H ratios. We present a variable deuterium depletion model that naturally explains the constant measured values of D/H inside the Local Bubble, the wide range of

We present an empirical dynamical model of the LISM based on 270 radial velocity measurements for 157 sight lines toward nearby stars. Physical parameter measurements (i.e., temperature, turbulent velocity, depletions) are available for 90 components, or one-third of the sample, enabling initial characterizations of the physical properties of LISM clouds. The model includes 15 warm clouds located within 15 pc of the Sun, each with a different velocity vector. We derive projected morphologies of all clouds and estimate the volume filling factor of warm partially ionized material in the LISM to be between $5.5% and 19%. Relative velocities of potentially interacting clouds are often supersonic, consistent with heating, turbulent, and metal depletion properties. Cloud-cloud collisions may be responsible for the filamentary morphologies found in $ 1 3 of LISM clouds, the distribution of clouds along the boundaries of the two nearest clouds (LIC and G), the detailed shape and heating of the Mic Cloud, the location of nearby radio scintillation screens, and the location of an LISM cold cloud. Contrary to previous claims, the Sun appears to be located in the transition zone between the LIC and G Cloud.

We have determined the correlation between observed stellar X-ray luminosities, bolometric luminosities, and projected rotational velocities v sin / for stars of various spectral types and luminosity classes observed by the Einstein Observatory. There are two clearly defined patterns of behavior observed. Early type stars (03-A5) have X-ray luminosities which are proportional to bolometric luminosity. The proportionality constant is (1.4±0.3)X10~7 and is independent of luminosity class. In contrast, late type stars (G to M) have X-ray luminosities strongly dependent on rotation rate [L x~(v sin /) 1-9±0 ' 5 ] and independent of bolometric luminosity; this relation for late type stars is again found to be independent of luminosity class. This dependence is equivalent to a relation/^ ~Q 2 between the X-ray surface flux and the stellar angular velocity. F stars as a class are intermediate, having X-ray luminosities substantially higher than would be predicted on the basis of the early type star L x-L bol relation, but substantially lower than expected from the late type velocity dependence. The location of RS CVn stars as a class is discussed with respect to the dependence of X-ray luminosity on rotation.

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.

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Ground-and space-based planet searches employing radial velocity techniques and transit photometry have detected thousands of planet-hosting stars in the Milky Way. With so many planets discovered, the next step toward identifying potentially habitable planets is atmospheric characterization. While the Sun-Earth system provides a good framework for understanding the atmospheric chemistry of Earth-like planets around solar-type stars, the observational and theoretical constraints on the atmospheres of rocky planets in the habitable zones (HZs) around low-mass stars (K and M dwarfs) are relatively few. The chemistry of these atmospheres is controlled by the shape and absolute flux of the stellar spectral energy distribution (SED), however, flux distributions of relatively inactive low-mass stars are poorly understood at present. To address this issue, we have executed a panchromatic (X-ray to mid-IR) study of the SEDs of 11 nearby planet-hosting stars, the Measurements of the Ultraviolet Spectral Characteristics of Low-mass Exoplanetary Systems (MUSCLES) Treasury Survey. The MUSCLES program consists visible observations from Hubble and ground-based observatories. Infrared and astrophysically inaccessible wavelengths (EUV and Lyα) are reconstructed using stellar model spectra to fill in gaps in the observational data. In this overview and the companion papers describing the MUSCLES survey, we show that energetic radiation (X-ray and ultraviolet) is present from magnetically active stellar atmospheres at all times for stars as late as M6. The emission line luminosities of C IV and Mg II are strongly correlated with band-integrated luminosities and we present empirical relations that can be used to estimate broadband FUV and XUV (≡X-ray + EUV) fluxes from individual stellar emission line measurements. We find that while the slope of the SED, FUV/NUV, increases by approximately two orders of magnitude form early K to late M dwarfs (≈0.01-1), the absolute FUV and XUV flux levels at their corresponding HZ distances are constant to within factors of a few, spanning the range 10-70 erg cm −2 s −1 in the HZ. Despite the lack of strong stellar activity indicators in their optical spectra, several of the M dwarfs in our sample show spectacular UV flare emission in their light curves. We present an example with flare/quiescent ultraviolet flux ratios of the order of 100:1 where the transition region energy output during the flare is comparable to the total quiescent luminosity of the star E flare (UV) ∼ 0.3 L * Δt (Δt = 1 s). Finally, we interpret enhanced L(line)/L Bol ratios for C IV and N V as tentative observational evidence for the interaction of planets with large planetary mass-to-orbital distance ratios (M plan /a plan ) with the transition regions of their host stars.