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From our Secret Lives of Cepheids program, the prototype Classical Cepheid, $\delta$ 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 $\sim$10--20 times with highest fluxes at $\sim0.90-0.95\phi$, just prior to maximum brightness. Previously, $\delta$ Cep was found as potentially X-ray variable, using XMM-Newton observations (Engle et al. 2014). 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 $\delta$ Cep and not transient or due to a possible coronally-active, cool companion. The Chandra data were combined with prior XMM-Newton observations, and very closely match the previously observed X-ray behavior. From the combined dataset, a $\sim$4$\times$ increase in X-ray flux is measured, reaching a peak $L_X$ = 1.7 $\times$ 10$^{29}$ erg s$^{-1}$ near 0.45$\phi$. The precise X-ray flux phasing with the star's pulsation indicates that the emissions arise from the Cepheid and not a companion. However, it is puzzling that maximum X-ray flux occurs $\sim$0.5$\phi$ ($\sim$3 days) later than the FUV--UV maximum. There are several other potential Cepheid X-ray detections with properties similar to $\delta$ Cep, and comparable X-ray variability is indicated for two other Cepheids: $\beta$ Dor and V473 Lyr. X-ray generating mechanisms in $\delta$ Cep and other Cepheids are discussed. If additional Cepheids are confirmed to show phased X-ray variations, then $\delta$ Cep will be the prototype of new class of pulsation-induced X-ray variables., Comment: 16 pages, 3 figures, 2 tables, accepted for publication in the Astrophysical Journal

We observed Betelgeuse using ALMA's extended configuration in band 7 (f~340 GHz, {\lambda}~0.88 mm), resulting in a very high angular resolution of 18 mas. Using a solid body rotation model of the 28SiO(v=2,J=8-7) line emission, we show that the supergiant is rotating with a projected equatorial velocity of v_eq sin i = 5.47 +/- 0.25 km/s at the equivalent continuum angular radius R_star = 29.50 +/- 0.14 mas. This corresponds to an angular rotation velocity of {\omega} sin i = (5.6 +/- 1.3) x 10^(-9) rad/s. The position angle of its north pole is PA = 48.0 +/- 3.5{\deg}. The rotation period of Betelgeuse is estimated to P/sin i = 36 +/- 8 years. The combination of our velocity measurement with previous observations in the ultraviolet shows that the chromosphere is co-rotating with the star up to a radius of ~10 au (45 mas or 1.5x the ALMA continuum radius). The coincidence of the position angle of the polar axis of Betelgeuse with that of the major ALMA continuum hot spot, a molecular plume, and a partial dust shell (from previous observations) suggests that focused mass loss is currently taking place in the polar region of the star. We propose that this hot spot corresponds to the location of a particularly strong "rogue" convection cell, which emits a focused molecular plume that subsequently condenses into dust at a few stellar radii. Rogue convection cells therefore appear to be an important factor shaping the anisotropic mass loss of red supergiants., Comment: 18 pages, 19 figures, accepted for publication in A&A, update of bibliography and acknowledgements

Multi-wavelength studies of M supergiants have revealed atmospheric structures with a large range of spatial and temporal scales. Focusing on Betelgeuse, these scales and their perplexing connections from the photosphere to the interstellar medium are reviewed. Of particular current interest is the dynamic origin of the ubiquitous and relatively dust-free mass loss. Is it multiple plumes of convection driven ejecta, episodic ejection of molecular reservoirs, or a more steady and uniform flow? With powerful new facilities such as the VLT and ALMA we may begin to understand the connections and answer such puzzles, but ultimately detailed studies of a sample of M supergiants will be needed to disentangle the physics from the stars’ personalities.

We examine the thermal free-free millimetre fluxes expected from non-dusty and non-pulsating K through mid-M giant stars based on our limited understanding of their inhomogeneous chromospheres. We present a semi-analytic model that provides estimates of the radio fluxes for the mm wavelengths (e.g., CARMA, ALMA, JVLA Q-band) based on knowledge of the effective temperatures, angular diameters and chromospheric Mg II h & k emission fluxes. At 250 GHz, the chromospheric optical depths are expected to be significantly less than unity, which means that fluxes across the mm and sub-mm range will have a contribution from the chromospheric mate- rial that gives rise to the ultraviolet emission spectrum, as well as the cool molecular material known to exist above the photosphere. We predict a lower bound to the inferred brightness temperature of red giants based on heating at the basal-flux limit if the upper chromospheres have filling factor 1. Multi-frequency mm observations should provide important new information on the structuring of the inhomogeneous chromospheres, including the boundary layer, and allow tests of competing theoretical models for atmospheric heating. We comment on the suitability of these stars as mm flux calibrators.

Recently, it has been proposed that the winds of non-pulsating and non-dusty K and M giants and supergiants may be driven by some form of magnetic pressure acting on highly clumped wind material. While many researchers believe that magnetic processes are responsible for cool evolved stellar winds, existing MHD and Alfven wave-driven wind models have magnetic fields that are essentially radial and tied to the photosphere. The clumped magnetic wind scenario is quite different in that the magnetic flux is also being carried away from the star with the wind. We test this clumped wind hypothesis by computing continuum radio fluxes from the ζ Aur semiempirical model of Baade et al., which is based on wind-scattered line profiles. The radio continuum opacity is proportional to the electron density squared, while the line scattering opacity is proportional to the gas density. This difference in proportionality provides a test for the presence of large clumping factors. We derive the radial distribution of clump factors (CFs) for ζ Aur by comparing the nonthermal pressures required to produce the semiempirical velocity distribution with the expected thermal pressures. The CFs are ~5 throughout the sub-sonic inner wind region and then decline outward. These implied clumping factors lead to excess radio emission at 2.0 cm, while at 6.2 cm it improves agreement with the smooth unclumped model. Smaller clumping factors of ~2 lead to better overall agreement but also increase the discrepancy at 2 cm. These results do not support the magnetic clumped wind hypothesis and instead suggest that inherent uncertainties in the underlying semiempirical model probably dominate uncertainties in predicted radio fluxes. However, new ultraviolet line and radio continuum observations are needed to test the new generations of inhomogeneous magnetohydrodynamic wind models.

The solar Mg I emission lines at 12 micrometer have already been observed and analyzed well. Previous modeling attempts for other stars have, however, been made only for Procyon and two cool evolved stars, with unsatisfactory results for the latter. We present high-resolution observational spectra for the K giants Pollux, Arcturus, and Aldebaran, which show strong Mg I emission lines at 12 micrometer as compared to the Sun. We also present the first observed stellar emission lines from Mg I at 18 micrometer and from Al I, Si I, and presumably Ca I at 12 micrometer. To produce synthetic line spectra, we employ standard non-LTE modeling for trace elements in cool stellar photospheres. We compute model atmospheres with the MARCS code, apply a comprehensive magnesium model atom, and use the radiative transfer code MULTI to solve for the magnesium occupation numbers in statistical equilibrium. We successfully reproduce the observed Mg I emission lines simultaneously in the giants and in the Sun, but show how the computed line profiles depend critically on atomic input data and how the inclusion of energy levels with n > 9 and collisions with neutral hydrogen are necessary to obtain reasonable fits.

We report the detection of thermal continuum radio emission from the K0 III coronal giant Pollux ($\beta$ Gem) with the Karl G. Jansky Very Large Array (VLA). The star was detected at 21 and 9 GHz with flux density values of $150\pm21$ and $43\pm8\,\mu$Jy, respectively. We also place a $3\sigma_{\mathrm{rms}}$ upper limit of $23\,\mu$Jy for the flux density at 3 GHz. We find the stellar disk-averaged brightness temperatures to be approximately 9500, 15000, and $

The "hybrid chromosphere" star α Trianguli Australis (K2 Ib-IIa) displays atypical coronal properties for its class: elevated soft X-ray emission, flare outbursts, and a superhot (T ~ 10 MK) spectrum. The Wide Field Planetary Camera 2 (WFPC2) on board the Hubble Space Telescope (HST) now has found a faint object close to the red supergiant, 0.4'' nearly due south and about 1% its intensity in the F160BW ultraviolet filter, consistent with a G0 dwarf. Potentially, the "young sun" companion to the massive primary could completely dominate the coronal luminosity. The X-ray centroid in a contemporaneous Chandra High-Resolution Camera (HRC) pointing is offset in the direction of the faint UV secondary, but measurements of the radio counterpart of a serendipitous X-ray source 12'' southeast (SE) of α TrA, obtained with the Australia Telescope Compact Array (ATCA), suggest a conflicting offset.

We examine the little-known phenomenon of orbitally modulated Si i emission at λ 3905.523 Å and λ 4102.936 Å in composite-spectrum binaries, with specific reference to ζ Aurigae (K4 Ib + B5 V). The emission is detected in the isolated spectrum of the B-type dwarf secondary, and while λ 4102 Å is heavily blended with Hδ, λ 3905 Å falls in the B-star's featureless continuum. The narrowness of the emission (vturb ≃ 6 km s−1) demonstrates that it originates in the upper photosphere or deep chromosphere of the K star primary. We propose that photoexcitation by the hot star's UV continuum, followed by recombination and cascades, leads to resonant scattering and subsequent pumping of lower opacity transitions in the singlet and triplet systems of Si i. This process channels the UV continuum into select narrow emission lines. We have also identified weaker photoexcited emission of Fe ii at λ 3938.289 Å. The strengths, positions, and widths of the λ 3905 Å emission line vary with orbital phase owing to changes in the dilution of the irradiating flux and in the geometrical aspect of the irradiated hemisphere. Utilizing the inherent spatial resolution provided by the illuminated patch, and assuming that the K star is spherical with isotropic emission, yields vsin i ∼ 5.7 km s−1. Evidence of tidal distortion was deduced from the timing of the rapidly rising phase of the emission just after periastron. Increasing the diagnostic potential requires radiative transfer modelling of the formation and centre-to-limb variation of the emission.

The extended atmosphere of the M supergiant Betelgeuse is complex with cool plasma dominating the structure by mass and small amounts of embedded hotter chromospheric plasma. A major challenge is to understand the interrelationship and juxtaposition of these different components, which in turn may provide clues to the nature of the process of nonradiative heating and the mechanisms that drive mass loss. We examine the chromospheric C II] λ2325 multiplet emission line electron density diagnostic using spatially scanned HST STIS echelle spectra. Escape probability models for the electron density-sensitive line ratios reveal that the mean electron density decreases by 0.6 dex as the sight line goes from disk center to ±75 mas. Radiative transfer simulations using spherical model atmospheres show that this trend can be explained if the electron density declines with radius by nearly 2 dex across ΔR ~ 2R*. The emission profiles indicate that the chromospheric material corotates with the star and then becomes decoupled by ±75 mas from disk center. We find no evidence for radial outflow in the chromospheric plasma. We find that the strongest C II] λ2325 emission lines are opacity broadened and that the gradient of atmospheric turbulence is surprisingly small. Using empirical constraints, we derive a relation between the relative C II column densities in the cool and chromospheric atmospheric components and the excitation temperature. These UV chromospheric results and previous radio analyses suggest that the chromosphere is pervasive but has a small filling factor at ~3R*, suggestive of confinement and heating in magnetic structures.

Studies of the winds from single K and early M evolved stars indicate that these flows typically reach a significant fraction of their terminal velocity within the first couple of stellar radii. The most detailed spatially resolved information of the extended atmospheres of these spectral types comes from the ζ Aur eclipsing binaries. However, the wind acceleration inferred for the evolved primaries in these systems appears significantly slower than for stars of similar spectral type. Since there are no successful theories for mass loss from K and early M evolved stars, it is important to place strong empirical constraints on potential models and determine whether this difference in acceleration is real or an artifact of the analyses. We have undertaken a radio continuum monitoring study of ζ Aurigae (K4 Ib + B5 V) using the Very Large Array to test the wind density model of Baade et al. that is based on Hubble Space Telescope (HST) Goddard High Resolution Spectrograph ultraviolet spectra. ζ Aur was monitored at centimeter wavelengths over a complete orbital cycle, and flux variations during the orbit are found to be of similar magnitude to variations at similar orbital phases in the adjacent orbit. During eclipse, the flux does not decrease, showing that the radio emission originates from a volume substantially larger than R ~ (150 R⊙)3 surrounding the B star. Using the one-dimensional density model of the K4 Ib primary's wind derived from HST spectral line profile modeling and electron temperature estimates from previous optical and new HST studies, we find that the predicted radio fluxes are consistent with those observed. Three-dimensional hydrodynamic simulations indicate that the accretion flow perturbations near the B star do not contribute significantly to the total radio flux from the system, consistent with the radio eclipse observations. Our radio observations confirm the slow wind acceleration for the evolved K4 Ib component. ζ Aur's velocity structure does not appear to be typical of single stars with similar spectral types. This highlights the need for more comprehensive multiwavelength studies for both single stars, which have been sadly neglected, and other ζ Aur systems to determine if its wind properties are typical.

Multifrequency spatially-resolved radio continuum observations of the M supergiants, α Ori (M2 Iab) and α Sco (M1.5 Iab + B2.5 V), have been obtained using the VLA A array and VLA+Pie Town configurations, to study changes in the extended (i.e. scale 1 − 10 stellar radii) atmospheres of these stars and to model the conditions in their wind acceleration regions. Strong modelling constraints on the atmospheric thermal properties are derived, because the radio emission is resolved at multiple wavelengths. Changes seen in the α Ori source flux density and radio visibility data occurring on several year timescales are described, based on observations obtained in 2002 February and April and in 1996 December. The need for multicomponent models of the plasma conditions in both the warm and cool gas around α Ori is discussed. The radio properties of the α Sco system, both of the M supergiant itself and in the H II region surrounding the B-type companion, provide important tools for estimating conditions within the M supergiant's wind.

The mechanism responsible for driving the ubiquitous winds of cool giant and supergiant stars remains to be established. To this end, we are constructing semi-empirical models of the extended outer atmospheres (‘chromospheres’) and winds of selected red supergiants. These models are constrained by analyses of the UV line spectra of single stars, and of red supergiants in binaries that eclipse their main-sequence companions: the ζ Aur and VV Cep stars. These detached binaries are well-separated, with no evidence of mass transfer. The C II] 2325 Å line profiles of the binaries are similar to those of comparable single stars, suggesting that the chromospheres remain relatively unperturbed by binarity. However, it is unclear how much binarity disturbs the wind: binary observations suggest a gradual acceleration (β ∼ 3), but line profile analyses of single red supergiants imply a rapid acceleration (β < 1). To date, we have obtained extensive series of HST/GHRS and STIS observations of three eclipsing red supergiant binaries: ζ Aur, HR 2554 and VV Cep. In this paper, we focus on ζ Aur, and present observations and modelling results for this eclipsing binary.

Spatially resolved multi-wavelength centimeter continuum observations of cool evolved stars can not only constrain the morphology of the radio emitting regions, but can also directly probe the mean gas temperature at various depths of the star's extended atmosphere. Here, we use the Very Large Array (VLA) in the A configuration with the Pie Town (PT) Very Long Baseline Array (VLBA) antenna to spatially resolve the extended atmosphere of Betelgeuse over multiple epochs at 0.7, 1.3, 2.0, 3.5, and 6.1 cm. The extended atmosphere deviates from circular symmetry at all wavelengths while at some epochs we find possible evidence for small pockets of gas significantly cooler than the mean global temperature. We find no evidence for the recently reported e-MERLIN radio hotspots in any of our multi-epoch VLA/PT data, despite having sufficient spatial resolution and sensitivity at short wavelengths, and conclude that these radio hotspots are most likely interferometric artefacts. The mean gas temperature of the extended atmosphere has a typical value of 3000 K at 2 R∗ and decreases to 1800 K at 6 R∗, in broad agreement with the findings of the single epoch study from Lim et al. (1998, Nature, 392, 575). The overall temperature profile of the extended atmosphere between 2 R∗ ≲ r ≲ 6 R∗ can be described by a power law of the form Tgas(r) ∝ r-0.6, with temporal variability of a few 100 K evident at some epochs. Finally, we present over 12 yr of V band photometry, part of which overlaps our multi-epoch radio data. We find a correlation between the fractional flux density variability at V band with most radio wavelengths. This correlation is likely due to shock waves induced by stellar pulsations, which heat the inner atmosphere and ionize the more extended atmosphere through radiative means. Stellar pulsations may play an important role in exciting Betelgeuse's extended atmosphere.

We report on a Far Ultraviolet Spectroscopic Explorer (FUSE) spectroscopic observation of the pre–main-sequence star T Tauri, the defining object of pre–main-sequence stars. The FUSE satellite is the only UV instrument currently available that can acquire high-quality spectra between 912 and 1185 A. The spectrum of T Tau was acquired between 2000 January 15 and 16. Serendipitous observing conditions resulted in a very low noise spectrum that allowed the detection of several weak features, including the first detection of H2 Werner-band emission in an extrasolar object.

We have constructed a detailed mean density and temperature model for the extended outer atmosphere of the O-rich supergiant Betelgeuse (??Ori [M2 Iab]), which extends from 1.0 to 10.0 stellar radii. A one-dimensional model is based on fitting NRAO VLA centimeter visibility data, and two-dimensional models are constructed using the intensity contours of the 0.7 cm observations of Lim et al. As one moves in toward the star from about 10 R* the mean electron temperature increases to a value of 3800 K, then declines down below Teff, and then rises to photospheric values. The peak mean model temperature is less than the typical chromospheric temperatures found in previous models. Observations of H? and chromospheric ultraviolet (UV) emission show that higher temperature components must also exist, but they do not dominate the weighted mean temperature structure. We tentatively identify the radius where the temperature distribution peaks (R 1.45 R*) with the dominant chromospheric UV emission region and find an areal filling factor of ?. In the extended semiempirical model the dominant source of electrons is from photoionized metals and is dominated by carbon. The low ionization of hydrogen leads to a dominance by H- (free-free) opacity at centimeter wavelengths. We derive simple estimates of the radio spectral indices for other similar M supergiants. We have constructed two-dimensional models to examine whether the intensity asymmetry observed at 0.7 cm is most likely to result from density or temperature variations. Adopting an elliptical two-dimensional model, a density asymmetry along the axes of symmetry would need to be 20?:?1. If we assume the radial wind velocity is independent of angle the integrated mass-loss rate is only a factor of ~2 greater than that derived from the one-dimensional model. However, previous H? speckle observations that sample the same spatial regions suggest the asymmetry observed at 0.7 cm is not due to such a large-scale density asymmetry. A modest change in temperature can more easily provide the asymmetry, increasing both the opacity and the thermal source term. If the radial density structure is assumed to be the same as in 1992 September, when HST/GHRS spectra were obtained, then the Fe II wind absorption features provide an estimate of the mass-loss rate of 3.1(?1.3) ? 10-6 M? yr-1. This further implies that the cool material dominates the mass of the extended atmosphere and that the radio-emitting region is within the base of the outflow observed in the circumstellar layers. A simple silicate dust model is constructed and the semiempirical model suggests an onset of dust formation at R 33 R* where Tdust ~ 360 K. This region lies outside the semiempirical model but simple extrapolations suggest that at this radius Te ~ 220 K, and the mean hydrogen density nH ~ 3 ? 106 cm-3. We address the difficult question of whether the mean thermal model based on the radio data can be consistent with the observed off-limb H? scattering emission if inhomogeneities are present.

The structures of the outer atmospheres of red giants are very complex. The notion of large optically thick molecular spheres around the stars (MOLspheres) has been invoked in order to explain e.g. spectro-interferometric observations. However, high-resolution spectra in the mid-IR do not easily fit into this picture. They rule out any large sphere of water vapour in LTE surrounding red giants. Our aim here is to investigate high-resolution, mid-infrared spectra for a range of red giants, from early-K to mid M. We have recorded 12 microns spectra of 10 well-studied bright red giants, with TEXES on the IRTF. We find that all giants in our study cooler than 4300 K, spanning a range of effective temperatures, show water absorption lines stronger than expected. The strengths of the lines vary smoothly with spectral type. We identify several spectral features in the wavelength region that undoubtedly are formed in the photosphere. From a study of water-line ratios of the stars, we find that the excitation temperatures, in the line-forming regions, are several hundred Kelvin lower than expected from a classical photospheric model. This could either be due to an actually lower temperature structure in the outer regions of the photospheres caused by, for example, extra cooling, or due to non-LTE level populations, affecting the source function and line opacities. We have demonstrated that these diagnostically interesting water lines are a general feature of red giants across spectral types, and we argue for a general explanation of their formation rather than explanations requiring specific properties. Since the water lines are neither weak (filled in by emission) nor appear in emission, as predicted by LTE MOLsphere models in their simplest forms, the evidence for the existence of such large optically-thick, molecular spheres enshrouding the stars is weakened. (abbreviated), Comment: Accepted for publication in A&A

Cool, evolved stars have copious, enriched winds. The structure of these winds and the way they are accelerated is not well known. We need to improve our understanding by studying the dynamics from the pulsating stellar surface to about 10 stellar radii, where radiation pressure on dust is fully effective. Some red supergiants have highly asymmetric nebulae, implicating additional forces. We retrieved ALMA Science Verification data providing images of sub-mm line and continuum emission from VY CMa. This enables us to locate water masers with milli-arcsec precision and resolve the dusty continuum. The 658-, 321- and 325-GHz masers lie in irregular, thick shells at increasing distances from the centre of expansion. For the first time this is confirmed as the stellar position, coinciding with a compact peak offset to the NW of the brightest continuum emission. The maser shells (and dust formation zone) overlap but avoid each other on tens-au scales. Their distribution is broadly consistent with excitation models but the conditions and kinematics appear to be complicated by wind collisions, clumping and asymmetries., Comment: Letter 4 pages, 5 figures plus appendix with 3 figures. Accepted by Astronomy and Astrophysics Letters

We have obtained ultraviolet spectra of the Herbig Ae star HD104237 using the Goddard High Resolution Spectrograph (GHRS) on HST. The high temperature emission and absorption lines show remarkable outflow absorption features, which have very similar profiles that are essentially independent of formation temperature. The profiles are not those expected from a spherically-symmetric stellar wind but have optically-thick absorption to −280 km s−1 and a high velocity plateau extending to −375 km s−1. The profile shape is a manifestation of the nonspherical geometry of the flow. The observed UV emission is too strong to be associated with the hot X–ray emitting plasma seen by ASCA and probably is generated by the interaction of the innermost part of an accretion disk with the corotating outermost magnetospheric field. The outflow is almost certainly the inner part of a biconical disk wind.

We present a full determination of the fundamental stellar and orbital parameters of the eclipsing binary ζ Aurigae (K4 Ib + B5 V) using recent observations with the Hubble Space Telescope Goddard High Resolution Spectrograph (GHRS) and the Mark III long-baseline optical interferometer. The information obtained from spectroscopic and interferometric measurements is complementary, and the combination permits a complete determination of the stellar masses, the absolute semimajor axis of the orbit, and the distance. A complete solution requires that both components be visible spectroscopically, and this has always been difficult for the ζ Aur systems. The ζ Aur K star primary presents no difficulty, and accurate radial velocities are readily obtainable in the optical. However, the B star secondary is more problematic. Ground-based radial velocity measurements are hampered by the difficulty of working with the composite spectrum in the blue-violet region, the small number of suitable lines in the generally featureless optical spectrum of the B star, and the great width of the few available lines (the Balmer lines of hydrogen and a few weak He I lines) due to rapid rotation. We avoid the worst of these problems by using GHRS observations in the ultraviolet, where the K star Flux is negligible and the intrinsic B star spectrum is more distinctive, and obtain the most accurate determination of the B star radial velocity amplitude to date. We also analyze published photometry of previous eclipses and near-eclipse phases of ζ Aur in order to obtain eclipse durations, which fix the length of the eclipse chord and therefore determine the orbit inclination. The long-baseline interferometry (LBI) yields, in conjunction with the spectroscopic solution, the distance to the system and thus the absolute stellar radius of the resolved K supergiant primary star, ζ Aur A. The secondary is not resolved by LBI, but its angular (and absolute) radius is found by Fitting the model stellar flux plus an interstellar extinction model to the flux-calibrated GHRS data. We Find MK = 5.8 ± 0.2 M☉, MB = 4.8 ± 0.2 M☉, RK = 148 ± 3 R☉ and RB = 4.5 ± 0.3 RB for the masses and radii of the ζ Aur stars. We determine the distance to ζ Aur to be 261 ± 3 pc. Additionally, we refine the stellar parameters of the B star secondary presented in the 1995 spectroscopic study of Bennett, Brown, & Linsky. We also determine the effective temperature of the K star primary using values of the bolometric flux, angular diameter, and interstellar extinction derived in this study. The positions of the ζ Aur stars on the theoretical H-R diagram are compared to current evolutionary model tracks, and the resulting good agreement provides a strong check of the internal self-consistency of this analysis and the accuracy of the theoretical models. The ζ Aurigae stars are confirmed to be coeval with an age of 80 ± 15 Myr.

We have detected [Fe II] 17.94 um and 24.52 um emission from a sample of M supergiants using TEXES on the IRTF. These low opacity emission lines are resolved at R = 50, 000 and provide new diagnostics of the dynamics and thermodynamics of the stellar wind acceleration zone. The [Fe II] lines, from the first excited term, are sensitive to the warm plasma where energy is deposited into the extended atmosphere to form the chromosphere and wind outflow. These diagnostics complement previous KAO and ISO observations which were sensitive to the cooler and more extended circumstellar envelopes. The turbulent velocities, Vturb is about 12 to 13 km/s, observed in the [Fe II] forbidden lines are found to be a common property of our sample, and are less than that derived from the hotter chromospheric C II] 2325 Angstrom lines observed in alpha Ori, where Vturb is about 17 to 19 km/s. For the first time, we have dynamically resolved the motions of the dominant cool atmospheric component discovered in alpha Ori from multi-wavelength radio interferometry by Lim et al. (1998). Surprisingly, the emission centroids are quite Gaussian and at rest with respect to the M supergiants. These constraints combined with model calculations of the infrared emission line fluxes for alpha Ori imply that the warm material has a low outflow velocity and is located close to the star. We have also detected narrow [Fe I] 24.04 um emission that confirms that Fe II is the dominant ionization state in alpha Ori's extended atmosphere., 79 pages including 10 figures and 2 appendices. Accepted by ApJ

We present observations of the X‐ray and ultraviolet emission from a sample of dwarf stars in the Arcturus Moving Group. The Arcturus Moving Group is very likely a remnant of the merger of a dwarf galaxy with the Milky Way Galaxy in the distant past. This kinematically distinct group has members located very close to the Sun, allowing study of stellar activity on very old (7–8 Gyr), low metallicity stars that would typically not be possible. Our sample has metallicities between 0.4 and 0.04 solar, spectral types F9—mid‐M, and distances less than 25 pc from the Sun. We have detected X‐ray emission from five AMG dwarfs with the Chandra ACIS‐S S3 back‐illuminated detector and for four stars have measured or placed upper limits on the C IV UV1 emission flux using GALEX GRISM spectra. The measured X‐ray luminosities are comparable to the minimum solar Lx (range 6–20 1026 erg s−1 for 0.24–2.0 keV) and for the late G/early K stars are also similar to that of the inactive, more metal rich ([Fe/H] = −0.42] G8 dwar...

Several recent papers have argued for warm, semi-detached, molecular layers surrounding red giant and supergiant stars, a concept known as a MOLsphere. Spectroscopic and interferometric analyses have often corroborated this general picture. Here, we present high-resolution spectroscopic data of pure rotational lines of water vapor at 12 microns for the supergiant mu Cephei. This star has often been used to test the concept of molecular layers around supergiants. Given the prediction of an isothermal, optically thick water-vapor layer in Local Thermodynamic Equilibrium around the star (MOLsphere), we expected the 12 micron lines to be in emission or at least in absorption but filled in by emission from the molecular layer around the star. Our data, however, show the contrary; we find definite absorption. Thus, our data do not easily fit into the suggested isothermal MOLsphere scenario. The 12 micron lines, therefore, put new, strong constraints on the MOLsphere concept and on the nature of water seen in signatures across the spectra of early M supergiants. We also find that the absorption is even stronger than that calculated from a standard, spherically symmetric model photosphere without any surrounding layers. A cool model photosphere, representing cool outer layers is, however, able to reproduce the lines, but this model does not account for water vapor emission at 6 microns. Thus, a unified model for water vapor on mu Cephei appears to be lacking. It does seem necessary to model the underlying photospheres of these supergiants in their whole complexity. The strong water vapor lines clearly reveal inadequacies of classical model atmospheres., Accepted for publication in the Astrophysical Journal

We report on multi-wavelength observations spanning radio to X-ray wavelengths of the M dwarf flare star, EV Lacertae, probing the characteristics of the outer atmospheric plasma from the upper chromosphere to the corona. We detect the star at a wavelength of 2 cm (15 GHz) for the first time. UV and FUV line profiles show evidence of nonthermal broadening, and the velocity width appear to peak at lower temperatures than in the Sun; this trend is confirmed in another active M dwarf flare star. Electron density measurements indicate nearly constant electron pressures between $\log T=$5.2 and 6.4. At higher coronal temperatures, there is a sharp increase of two orders of magnitude in density (n$_{e}\sim10^{13}$ cm$^{-3}$ at $\log T=$6.9). X-ray, EUV, FUV and NUV spectra constrain the DEM from the upper chromosphere through the corona. The coronal pressures are inconsistent with the assumption of hydrostatic equilibrium, either through EM modeling or application of scaling laws, and imply large conductive loss rates and a large energy input at the highest temperatures. The timescales for radiative and conductive losses in EV Lac's upper atmosphere imply that significant continued heating must occur for the corona to maintain its quiescent properties. The high frequency radio detection requires the high temperature X-ray-emitting coronal plasma to be spatially distinct from the radio emission source. Length scales in the low-temperature corona are markedly larger than those in the high-temperature corona, further suggestions of an inhomogeneous mixture of thermal and nonthermal coronal plasma., Comment: accepted, Astrophysical Journal

We present and analyze FUSE observations of six solar analogs. These are single, main-sequence G0-5 strs selected as proxies for the Sun at several stages of its main-sequence lifetime. The emission features in the FUSE 920-1180 A wavelength range allow for a critical probe of the hot plasma over three decades in temperature. Using the flux ratio CIII 1176/977 as diagnostics, we investigate the dependence of the electron pressure of the transition region as a function of the rotation period, age and magnetic activity. The results from these solar proxies indicate that the electron pressure of the stellar ~10^5-K plasma decreases by a factor of about 70 between the young, fast-rotating magnetically active star and the old, slow-rotating inactive star. Also, the observations indicate that the average surface fluxes of emission features strongly decrease with increasing stellar age and longer rotation period. The emission flux evolution with age or rotation period is well fitted by power laws, which become steeper from cooler chromospheric (10^4 K) to hotter coronal (10^7 K) plasma. The relationship for the integrated (920-1180 A) FUSE flux indicates that the solar far-ultraviolet emissions were about twice the present value 2.5 Gyr ago and about 4 times the present value 3.5 Gyr ago. Note also that the FUSE/FUV flux of the Zero-Age Main Sequence Sun could have been higher by as much as 50 times. Our analysis suggests that the strong FUV emissions of the young Sun may have played a crucial role in the developing planetary system, in particular through the photoionization, photochemical evolution and possible erosion of the planetary atmospheres. (abridged), 15 pages, 8 figures, accepted for publication in ApJ

The Kepler satellite provides a unique window into stellar temporal variability by observing a wide variety of stars with multi-year, near-continuous, high precision, optical photometric time series. While most Kepler targets are faint stars with poorly known physical properties, many unexpected discoveries should result from a long photometric survey of such a large area of sky. During our Kepler Guest Observer programs that monitored late-type stars for starspot and flaring variability, we discovered a previously unknown dwarf nova that lies within a few arcseconds of the mid-G dwarf star KIC 5438845. This dwarf nova underwent nine outbursts over a 4 year time span. The two largest outbursts lasted ~17–18 days and show strong modulations with a 110.8 minute period and a declining amplitude during the outburst decay phase. These properties are characteristic of an SU UMa-type cataclysmic variable. By analogy with other dwarf nova light curves, we associate the 110.8 minute (1.847 hr) period with the superhump period, close to but slightly longer than the orbital period of the binary. No precursor outbursts are seen before the super-outbursts and the overall super-outburst morphology corresponds to Osaki & Meyer "Case B" outbursts, which are initiated when the outer edge of the disk reaches the tidal truncation radius. "Case B" outbursts are rare within the Kepler light curves of dwarf novae. The dwarf nova is undergoing relatively slow mass transfer, as evidenced by the long intervals between outbursts, but the mass transfer rate appears to be steady, because the smaller "normal" outbursts show a strong correlation between the integrated outburst energy and the elapsed time since the previous outburst. At super-outburst maximum the system was at V ~ 18, but in quiescence it is fainter than V ~ 22, which will make any detailed quiescent follow-up of this system difficult.

The processes leading to dust formation and the subsequent role it plays in driving mass-loss in cool evolved stars is an area of intense study. Here, we present high resolution ALMA Science Verification data of the continuum emission around the highly evolved oxygen-rich red supergiant VY CMa. These data enable us to study the dust in its inner circumstellar environment at a spatial resolution of 129 mas at 321 GHz and 59 mas at 658 GHz, allowing us to trace dust on spatial scales down to 11 R$_{\star}$ (71 AU). Two prominent dust components are detected and resolved. The brightest dust component, C, is located 334 mas (61 R$_{\star}$) south-east of the star and has a dust mass of at least $2.5\times 10^{-4} $M$_{\odot}$. It has an emissivity spectral index of $\beta =-0.1$ at its peak, implying that it is either optically thick at these frequencies with a cool core of $T_{d}\lesssim 100$ K, and/or contains very large dust grains. Interestingly, not a single molecule in the ALMA data has emission close to the peak of this massive dust clump. The other main dust component, VY, is located at the position of the star and contains a total dust mass of $4.0 \times 10^{-5} $M$_{\odot}$. It also contains a weaker dust feature extending over $60 $R$_{\star}$ to the north with the total component having a typical emissivity spectral index of $\beta =0.7$. We find that $>17%$ of the dust mass around VY CMa is located in clumps ejected within a more quiescent roughly spherical stellar wind, with a quiescent dust mass loss rate of $5 \times 10^{-6}$ M$_{\odot} $yr$^{-1}$. The observations suggest a continuous preferentially directed mass-loss from the star over many decades and do not support current models of convective driven only mass loss in red supergiant stars. We thus suggest other forces, i.e., MHD disturbances, are also needed to explain the observations. ispartof: Astronomy & Astrophysics vol:573 pages:5- status: published

We present a far-ultraviolet spectrum of the K4 Ib-II supergiant (Lambda)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 A) not previously recorded for (Lambda)Vel at a spectral resolving power of R approx. 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 Ly(alpha) and the O I (UV 2) triplet emission near (Lambda)1304. The molecular CO and H2 fluorescences are weaker than in the early-K giant (alpha) Boo while the Fe II and Cr II lines, also pumped by H I Ly(alpha), are stronger in (Lambda)Vel. This pattern of relative lines strengths between the two stars is explained by the lower iron-group element abundance in (alpha) Boo, which weakens that star's Fe II and Cr II emission without reducing the molecular fluorescences. The (Lambda)Vel spectrum shows fluorescent Fe II, Cr II, and H2 emission similar to that observed in the M supergiant (alpha) Ori, but more numerous well-defined narrow emissions from CO. The additional CO emissions are visible in the spectrum of (Lambda)Vel since that star does not have the cool, opaque circumstellar shells that surround 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 (Lambda)Vel indicates a approx.8 10(exp 4) K plasma that is mixed into the cooler chromosphere. Evidence of the stellar wind is seen in the CII (Lambda)(Lambda)1334,1335 lines and in the blueshifted Fe II and Ni II wind absorption lines. Line modeling using Sobolev with Exact Integration for the Cii lines indicates a larger terminal velocity (approx.45 versus approx.30 km/s) and turbulence (approx.27 versus

The origin of 'cosmic' fluorine is uncertain, but there are three proposed production sites/mechanisms: AGB stars, $\nu$ nucleosynthesis in Type II supernovae, and/or the winds of Wolf-Rayet stars. The relative importance of these production sites has not been established even for the solar neighborhood, leading to uncertainties in stellar evolution models of these stars as well as uncertainties in the chemical evolution models of stellar populations. We determine the fluorine and oxygen abundances in seven bright, nearby giants with well-determined stellar parameters. We use the 2.3 $\mu$m vibrational-rotational HF line and explore a pure rotational HF line at 12.2 $\mu$m. The latter has never been used before for an abundance analysis. To be able to do this we have calculated a line list for pure rotational HF lines. We find that the abundances derived from the two diagnostics agree. Our derived abundances are well reproduced by chemical evolution models only including fluorine production in AGB-stars and therefore we draw the conclusion that this might be the main production site of fluorine in the solar neighborhood. Furthermore, we highlight the advantages of using the 12 $\mu$m HF lines to determine the possible contribution of the $\nu$-process to the fluorine budget at low metallicities where the difference between models including and excluding this process is dramatic.

Multi-wavelength centimeter continuum observations of non-dusty, non-pulsating K spectral-type red giants directly sample their chromospheres and wind acceleration zones. Such stars are feeble emitters at these wavelengths however, and previous observations have provided only a small number of modest S/N measurements slowly accumulated over three decades. We present multi-wavelength Karl G. Jansky Very Large Array thermal continuum observations of the wind acceleration zones of two dust-free red giants, Arcturus (Alpha Boo: K2 III) and Aldebaran (Alpha Tau: K5 III). Importantly, most of our observations of each star were carried out over just a few days, so that we obtained a snapshot of the different stellar atmospheric layers sampled at different wavelengths, independent of any long-term variability. We report the first detections at several wavelengths for each star including a detection at 10 cm (3.0 GHz: S band) for both stars and a 20 cm (1.5 GHz: L band) detection for Alpha Boo. This is the first time single luminosity class III red giants have been detected at these continuum wavelengths. Our long-wavelength data sample the outer layers of Alpha Boo's atmosphere where its wind velocity is approaching its terminal value and the ionization balance is becoming frozen-in. For Alpha Tau, however, our long-wavelength data are still sampling its inner atmosphere, where the wind is still accelerating probably due to its lower mass-loss rate. We compare our data with published semi-empirical models based on ultraviolet data, and the marked deviations highlight the need for new atmospheric models to be developed. Spectral indices are used to discuss the possible properties of the stellar atmospheres, and we find evidence for a rapidly cooling wind in the case of Alpha Boo. Finally, we develop a simple analytical wind model for Alpha Boo based on our new long-wavelength flux measurements.

We exploit the high sensitivity and moderate spectral resolution of the Hubble Space Telescope Cosmic Origins Spectrograph to detect far-ultraviolet (UV) spectral features of carbon monoxide (CO) present in the inner regions of protoplanetary disks for the first time. We present spectra of the classical T Tauri stars HN Tau, RECX-11, and V4046 Sgr, representative of a range of CO radiative processes. HN Tau shows CO bands in absorption against the accretion continuum. The CO absorption most likely arises in warm inner disk gas. We measure a CO column density and rotational excitation temperature of N(CO) = (2 ± 1) × 1017 cm−2 and T rot(CO) 500 ± 200 K for the absorbing gas. We also detect CO A–X band emission in RECX-11 and V4046 Sgr, excited by UV line photons, predominantly H i Lyα. All three objects show emission from CO bands at λ > 1560 Å, which may be excited by a combination of UV photons and collisions with non-thermal electrons. In previous observations these emission processes were not accounted for due to blending with emission from the accretion shock, collisionally excited H2, and photo-excited H2, all of which appeared as a “continuum” whose components could not be separated. The CO emission spectrum is strongly dependent upon the shape of the incident stellar Lyα emission profile. We find CO parameters in the range: N(CO) ∼ 1018–1019 cm−2, T rot(CO) ≳ 300 K for the Lyα-pumped emission. We combine these results with recent work on photo-excited and collisionally excited H2 emission, concluding that the observations of UV-emitting CO and H2 are consistent with a common spatial origin. We suggest that the CO/H2 ratio (≡ N(CO)/N(H2)) in the inner disk is ∼1, a transition between the much lower interstellar value and the higher value observed in solar system comets today, a result that will require future observational and theoretical study to confirm.

Two red giant stars, A31 and A59, in the globular cluster NGC 6752 exhibit Mg II (2800 A) emission with surface fluxes comparable to those observed among metal-deficient halo field giants, and among low-activity Population I giants. Optical echelle spectra of these cluster giants reveal emission in the core of the Ca II K (3933.7 A) line, and in the wing of the H-alpha (6562.8 A) profile. Asymmetries exist both in the emission profiles and the line cores. These observations demonstrate unequivocally the existence of chromospheres among old halo population giants, and the presence of mass outflow in their atmospheres. Maintenance of a relatively constant level of chromospheric activity on the red giant branch contrasts with the decay of magnetic dynamo activity exhibited by dwarf stars and younger giants. A purely hydrodynamic phenomenon may be responsible for heating the outer atmospheres of these stars, enhancing chromospheric emission, thus extending the atmospheres and facilitating mass loss.

The mechanisms responsible for heating the extended atmospheres of early-M spectral-type supergiants are poorly understood. So too is the subsequent role these mechanisms play in driving the large mass-loss rates of these stars. Here we present ALMA long (i.e., $\sim$16 km) baseline 338 GHz (0.89 mm) continuum observations of the free-free emission in the extended atmosphere of the M2 spectral-type supergiant Betelgeuse. The spatial resolution of 14 mas exquisitely resolves the atmosphere, revealing it to have a mean temperature of 2760 K at $\sim$1.3 R$_{\star}$, which is below both the photospheric effective temperature ($T_{\textrm{eff}} = 3690$ K) and the temperatures at $\sim$$2 $R$_{\star}$. This is unambiguous proof for the existence of an inversion of the mean temperature in the atmosphere of a red supergiant. The emission is clearly not spherically symmetric with two notable deviations from a uniform disk detected in both the images and visibilities. The most prominent asymmetry is located in the north-east quadrant of the disk and is spatially resolved showing it to be highly elongated with an axis-ratio of 2.4 and occupying $\sim$$5\%$ of the disk projected area. Its temperature is approximately 1000 K above the measured mean temperature at 1.3 R$_{\star}$. The other main asymmetry is located on the disk limb almost due east of the disk center and occupies $\sim$$3\%$ of the disk projected area. Both emission asymmetries are clear evidence for localized heating taking place in the atmosphere of Betelgeuse. We suggest that the detected localized heating is related to magnetic activity generated by large-scale photospheric convection.