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We describe recent spectroscopic observations of red giant stars made by the Space Telescope Imaging Spectrograph (STIS) instrument on board the Hubble Space Telescope, which has provided spatially resolved observations of the warm chromospheric winds that predominate for early K to mid-M giants. The H i Ly α lines of a set of 11 red giants observed with the STIS/E140M echelle grating are first analyzed to ascertain wind H i column densities and total wind mass-loss rates. The M giants have estimated mass-loss rates of $\dot{M}=(14\mbox{--}86)\times {10}^{-11}$ M _⊙ yr ^−1 , while the K giants with detected wind absorption have weaker winds with $\dot{M}=(1.5\mbox{--}2.8)\times {10}^{-11}$ M _⊙ yr ^−1 . We use long-slit spectra of H i Ly α for two particular red giants, α Tau (K5 III) and γ Cru (M3.5 III), to study the spatial extent of the Ly α emission. From these data we estimate limits for the extent of detectable emission, which are r = 193 R _* for γ Cru and r = 44 R _* for α Tau. The cross-dispersion emission profiles in the STIS echelle spectra of the larger sample of red giants also show evidence for spatial resolution, not only for H i Ly α but for other lines with visible wind absorption, such as Fe ii , Mg ii , Mg i , O i , and C ii . We characterize the nature of these spatial signatures. The spatial extent is far more apparent for the M giants than for the K giants, consistent with the stronger winds found for the M giants from the Ly α analysis.

From our Secret Lives of Cepheids program, the prototype Classical Cepheid, δ Cep, is found to be an X-ray source with periodic pulsation-modulated X-ray variations. This finding complements our earlier reported phase-dependent FUV–UV emissions of the star that increase ∼10–20 times with highest fluxes at , just prior to maximum brightness. Previously δ Cep was found as potentially X-ray variable, using XMM-Newton observations. Additional phase-constrained data were secured with Chandra near X-ray emission peak, to determine if the emission and variability were pulsation-phase-specific to δ Cep and not transient or due to a possible coronally active, cool companion. The Chandra data were combined with prior XMM-Newton observations, and were found to very closely match the previously observed X-ray behavior. From the combined data set, a ∼4 increase in X-ray flux is measured, reaching a peak = 1.7 × 1029 erg s−1 near 0.45ϕ. The precise X-ray flux phasing with the star’s pulsation indicates that the emissions arise from the Cepheid and not from a companion. However, it is puzzling that the maximum X-ray flux occurs ∼0.5ϕ (∼3 days) later than the FUV–UV maximum. There are several other potential Cepheid X-ray detections with properties similar to δ Cep, and comparable X-ray variability is indicated for two other Cepheids: β Dor and V473 Lyr. X-ray generating mechanisms in δ Cep and other Cepheids are discussed. If additional Cepheids are confirmed to show phased X-ray variations, then δ Cep will be the prototype of a new class of pulsation-induced X-ray variables. [ABSTRACT FROM AUTHOR]

We report on an ultraviolet spectroscopic survey of red giants observed by the Hubble Space Telescope, focusing on spectra of the Mg ii h and k lines near 2800 Å in order to study stellar chromospheric emission, winds, and astrospheric absorption. We focus on spectral types between K2 III and M5 III, a spectral type range with stars that are noncoronal, but possessing strong, chromospheric winds. We find a very tight relation between Mg ii surface flux and photospheric temperature, supporting the notion that all K2-M5 III stars are emitting at a basal flux level. Wind velocities (Vw) are generally found to decrease with spectral type, with Vw decreasing from ∼40 km s−1 at K2 III to ∼20 km s−1 at M5 III. We find two new detections of astrospheric absorption, for σ Pup (K5 III) and γ Eri (M1 III). This absorption signature had previously only been detected for α Tau (K5 III). For the three astrospheric detections, the temperature of the wind after the termination shock (TS) correlates with Vw, but is lower than predicted by the Rankine–Hugoniot shock jump conditions, consistent with the idea that red giant TSs are radiative shocks rather than simple hydrodynamic shocks. A full hydrodynamic simulation of the γ Eri astrosphere is provided to explore this further. [ABSTRACT FROM AUTHOR]