Search

The details of the processes responsible for heating the chromospheres of evolved cool stars remain uncertain. While most spectroscopic diagnostics measure spatially-integrated emission, here we examine diagnostics sensitive to localized atmospheric regions that are specific to cool evolved stars in binary systems with hot main-sequence companions: emission from Si i and C i resulting from the ultraviolet irradiation of the evolved star’s atmosphere. HST Space Telescope Imaging Spectrograph (STIS) high-spectral-resolution near-ultraviolet observations of ζ Aurigae A+B (K4 Ib + B5 V) were obtained at three orbital phases, including total eclipse, to search for Si i and the corresponding C i line emission. Si i 2987.645 Å emission was detected at phases ϕ = .101 and .448 (from periastron) in-line with predictions from a previous study of optical Si i 3905 Å and Si i 4102 Å emission lines. No other Si i line emission is apparent, and the analogous C i lines at 2478.561 Å and 2582.901 Å also are not detected. High-spectral-resolution HST STIS and Goddard High Resolution Spectrograph spectra confirm the results of a previous study that showed that the intrinsic chromospheric fluxes on the visible hemisphere of ζ Aur A, observed during total eclipse, are representative of the single K supergiant λ Vel (K4 Ib). Furthermore, the HST spectra show that the chromospheric turbulent velocities are very close to those in this spectral-type proxy. These combined results highlight the importance of detailed spatially-resolved chromospheric models of ζ Aur systems, based on sequences of atmospheric eclipse spectra, to help constrain the poorly understood mechanisms that heat the atmospheres and drive stellar winds in cool evolved stars.

In this paper we aim to constrain the wind temperature, outflow and turbulent velocities, ionization state, and mass-loss rate of the single red giant Arcturus (α Boo K1.5 III) using high spectral resolution Hubble Space Telescope Space Telescope Imaging Spectrograph profiles of Si iii 1206.5 Å , O i 1304 Å and 1306 Å, C ii 1334 Å and 1335 Å, and Mg ii h 2802 Å. The use of the E140-H setting for α Boo allows the Si iii 1206.5 Å line to be cleanly extracted from the echelle format for the first time. The ratios of the wind optical depths of lines from different species constrain the temperature at the base of the wind to T wind ∼ 15,400 K. The mass-loss rate derived is 2.5 × 10−11 M ⊙ yr − 1 for Epoch 2018–2019, smaller than previous semiempirical estimates. These results can be reconciled with multiwavelength Very Large Array radio continuum fluxes for Epoch 2011–2012 by increasing the temperature to T wind ∼ 18,000 K, or increasing the mass-loss rate to 4.0 × 10−11 M ⊙ yr − 1 . Interpreting the wind acceleration and turbulence in terms of a steady WKB Alfvén wave–driven wind reveals that the wave energy damping length increases with increasing radius, opposite to the trend expected for ion-neutral damping of monochromatic waves, confirming a previous result by Kuin and Ahmad derived for ζ Aur binaries. This implies that a spectrum of waves is required in this framework with wave periods in the range of hours to days, consistent with the photospheric granulation timescale. Constraints on a radial magnetic field (B) at 1.2 R * are an upper limit of B ≤ 2 G from the implied wave heating, and B ≥ 0.3 G to avoid excessive wave amplitudes.

We report NASA-DLR SOFIA upGREAT circumstellar [O i] 63.2 μm and [C ii] 157.7 μm emission profiles and FIFI-LS [O i] 63.2 μm, [O i] 145.5 μm, and [C ii] 157.7 μm fluxes obtained shortly after Betelgeuse’s 2019/2020 Great Dimming event. Haas et al. noted a potential correlation between the [O i] 63.2 μm flux and V magnitude based on three Kuiper Airborne Observatory observations made with the CGS and FIFI instruments. The FIFI observation was obtained when V ≃ 0.88 and revealed a 3σ non-detection at a quarter of the previous CGS flux measurement made when V ≃ 0.35. A potential explanation could be a change in dust-gas drag heating by circumstellar silicates caused by variations in the photospheric radiation field. SOFIA observations provide a unique test of this correlation because the V-band brightness went to its lowest value on record, V ≃ 1.61, with the SOFIA observations being made when V FIFI−LS ≃ 1.51 and V upGREAT ≃ 1.36. The upGREAT spectra show a [O i] 63.2 μm flux larger than previous space observatory measurements obtained when V ≃ 0.58. The profile is consistent with formation in the slower, more turbulent inner S1 outflow, while the [C ii] 157.7 μm profile is consistent with formation farther out in the faster S2 outflow. Modeling of dust-gas drag heating, combined with 25 yr of Wing three-filter and V photometry, reveals that it is unlikely that the S1 circumstellar envelope and [O i] 63.2 μm fluxes are dominated by the dust-gas drag heating and that another heating source is also active. The [O i] 63.2 μm profile is hard to reconcile with existing outflow velocity models.

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 distance to the M supergiant Betelgeuse is poorly known, with the Hipparcos parallax having a significant uncertainty. For detailed numerical studies of M supergiant atmospheres and winds, accurate distances are a prerequisite to obtaining reliable estimates for many stellar parameters. New high spatial resolution, multiwavelength, NRAO33The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Very Large Array (VLA) radio positions of Betelgeuse have been obtained and then combined with Hipparcos Catalogue Intermediate Astrometric Data to derive new astrometric solutions. These new solutions indicate a smaller parallax, and hence greater distance (197 ± 45 pc), than that given in the original Hipparcos Catalogue (131 ± 30 pc) and in the revised Hipparcos reduction. They also confirm smaller proper motions in both right ascension and declination, as found by previous radio observations. We examine the consequences of the revised astrometric solution on Betelgeuse's interaction with its local environment, on its stellar properties, and its kinematics. We find that the most likely star-formation scenario for Betelgeuse is that it is a runaway star from the Ori OB1 association and was originally a member of a high-mass multiple system within Ori OB1a.

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.

Ultraviolet spectra ofTau (K5 III) obtained by the Hubble Space Telescope (HST) show many emission lines affectedbybroad absorptionfrom thestrongwindof thisred giant star. For the Mgii handklines there isalsoa narrow absorptionfeature in the midst of the windabsorptionthathas beeninterpreted as being fromTau's windYinterstellar medium (ISM) interaction region (i.e., its ''astrosphere''). We try to reproduce this absorption using hydrodynamic models of theTau astrosphere, which show that stellar wind material heated, compressed, and decelerated at the wind's termination shock (TS) can produce significant absorption at about the right velocity. By experimenting with different model input parameters, we findthatthe parameter that the absorptionismostsensitive to isthe ISM pressure, which determines the location of and therefore the density at the TS. However, the models underestimate both the amountofdecelerationatthe TSand the amountof absorptionfor realistic input parameters. We demonstrate thatthese problems can in principle be resolved by modeling the TS as a radiative shock. However, a cooling timescale short enough to affect the postshock flow is only attainable ifTau's wind speed is increased from the 27Y30 km s � 1 values derivedfromfitstowindabsorptiontoatleast35kms � 1 . The modelsalsoseemto require averyhighISM pressure of P/k � 30;000 cm � 3 K toinducedensitiesattheTShighenoughtoyieldsufficient radiativecooling.Thispressureis at least a factor of 2 higher than other estimates of ISM thermal pressure within the Local Bubble. Subject headingg circumstellar matter — stars: individual (� Tauri) — stars: winds, outflows — ultraviolet: stars

The Chandra X-Ray Observatory has detected coronal (T 106 K) emission from the red giant γ Draconis (HD 164058: K5 III; d = 45 pc), now fully resolved from a stronger source 21'' to the SE that had confused earlier measurements by Rontgensatellit (ROSAT). The second source is coincident with the 13th magnitude visual component ADS 10923B (γ Dra B), possibly a dM star in a wide orbit around the red giant. The 0.2-2 keV luminosity of γ Dra is LX ~ 1.2 × 1027 ergs s-1 (1 σ confidence interval), assuming log(Tcor) 6.5 K, while that of the faint optical companion is 3.2 × 1027 ergs s-1, assuming the same distance. Both sources have an intermediate spectral hardness within the range displayed by coronal stars. γ Dra has LX/Lbol an order of magnitude brighter than the other red giants previously imaged by Chandra, Arcturus (α Boo: K1.5 III) and Aldebaran (α Tau: K5 III), despite having an only 2 × elevated LC iv/Lbol (T ~ 105 K).

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.

A Far Ultraviolet Spectroscopic Explorer (FUSE) pointing on the ultrafast rotating yellow giant FK Comae Berenices (HD 117555; v sin i ~ 163 km s-1) recorded emission profiles of C III λ977 (T ~ 8 × 104 K) and O VI λ1031 (T ~ 3 × 105 K) that are exceptionally broad and asymmetric, but nearly identical in shape, aside from a blueward absorption component in the latter (identified as interstellar O I, rather than, say, a C III outflow feature). The FWHMs exceed 500 km s-1, twice the broadest far-UV line shape of any normal late-type star observed to date, but similar to the Hα profiles of FK Com, and following the trend of other fast spinning early G giants that often display "superrotational" broadening of their UV "hot" lines. Although the red-asymmetric O VI λ1031 profile is suggestive of an outflow at ~3 × 105 K, the weaker member of the doublet, λ1037, does not display the differential absorption pattern expected from a warm wind. Furthermore, at times the chromospheric Mg II λ2796 + λ2803 composite profile, from a collection of International Ultraviolet Explorer (IUE) echellegrams obtained two decades earlier, is nearly identical in shape to red-asymmetric O VI λ1031. A contemporaneous optical Doppler map places the photospheric dark spots mainly in the polar regions of the approaching hemisphere. The dominantly redward biased profiles of C III and O VI could be explained if the associated emission zones were leading the starspots in phase and partially rooted in lower latitudes.

We report Chandra detections of coronal X-rays from the early G supergiants α Aquarii (HD 209750: G2 Ib) and β Aquarii (HD 204867: G0 Ib). Previous ROSAT observations of these archetypical "hybrid chromosphere" stars were inconclusive, in the case of α Aqr owing to a 38' mispointing, and for β Aqr because of a small positional discrepancy of the apparent source. The Chandra High Resolution Camera (HRC-I), with its superior spatial resolution and sensitivity, has obtained a positive detection of α Aqr and recovered faint emission at the location of β Aqr, now well separated from the stronger source to the southeast that dominated the earlier ROSAT image. The coronal LX/LC IV luminosity ratios of both supergiants are extremely depressed relative to early G main-sequence stars, continuing the "X-ray deficiency syndrome" originally identified in late F/early G luminosity class III giants of the Hertzsprung gap.

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.

We have used the High Resolution Camera (HRC-I) of the Chandra X-Ray Observatory to search for coronal (T � 10 6 K) emission from the archetype ‘‘ noncoronal ’’ red giants Arcturus (� Bootis=HD 124897, K1 III) and Aldebaran (� Tauri=HD 29139, K5 III). Our program follows up previous detections of ultraviolet coronal proxies such as C iv � 1548 (T � 1 � 10 5 K) and O vi � 1031 (T � 3 � 10 5 K). The deep (� 19 ks) HRC-I pointings obtained a tentative 3 � detection of Arcturus, with fXð0:2 2 keV Þ¼ 1:0 þ1:8 � 0:8 � 10 � 15 ergs cm � 2 s � 1 (95% confidence limits [CLs]), but failed to record Aldebaran, with an upper limit of d1:5 � 10 � 15 ergs cm � 2 s � 1 (also at 95% CL). The corresponding LX=Lbol ratios are a factor of ten thousand less than the Sun, a low-activity coronal dwarf. At the same time, Hubble Space Telescope Imaging Spectrograph far-ultraviolet spectra suggest the presence of a ‘‘ cool absorber,’’ probably near the base of the red giant chromosphere, imprinting discrete low-excitation absorptions on top of highly ionized features such as Si iv � 1393. The hot emission zones thus are at least partially buried under a large column of chromospheric material, which would severely attenuate any soft X-rays that might be emitted. The submerged hot structures presumably are magnetic because of their high temperatures and broad C iv profiles (FWHM � 130 km s � 1 ). Perhaps these structures are analogous to small-scale ephemeral bipolar regions seen ubiquitously on the Sun throughout the sunspot cycle and thought to be of direct convective origin. If small-scale magnetic fields indeed are present in the lower atmospheres of red giants such as Arcturus and Aldebaran, they might play a role in initiating the cool winds of such stars, perhaps through a mechanism similar to solar spicules. Subject headings: stars: coronae — stars: individual (Aldebaran, Arcturus) — ultraviolet: stars — X-rays: stars

We describe an extensive search with the Space Telescope Imaging Spectrograph (STIS) for ultraviolet coronal (T > 10 6 K) forbidden lines in a sample of 29 F-M dwarfs, giants, and supergiants. Measuring coronal lines in the 1150-1700 Aband with STIS has important advantages of superior velocity resolution and an absolute wavelength calibration compared with using the Chandra or XMM-Newton grating spectrometers to observe permitted transitions of the same ion stages in the kilovolt X-ray region. Fe xii �� 1242, 1349 (T � 2 � 10 6 K) and Fe xxi � 1354 (10 7 K) are well known from solar studies and have been reported in previous stellar work. A search for other coronal forbidden lines in the 1200-1600 Aregion was largely negative. The few candidate identifications (e.g., Ar xiii � 1330 and Ca xv � 1375) are too faint to be diagnostically useful. We add new dwarfs to the list of Fe xii detections, including the nearby solar twin � Cen A (G2 V). Clear detections of Fe xxi were obtained in dMe stars, active giants, a short-period RS CVn binary, and possibly in active solar-type dwarfs. We developed a semiempirical method for removing the C i blend that partially affects the Fe xxi � 1354 profile. As discussed recently by Johnson et al., Capella (� Aur; G8 III+G1 III) displays clear Fe xxi variability between Goddard High-Resolution Spectograph (GHRS) and STIS observations 4 yr apart, which is apparently due to a substantial decline in the contribution from the G8 primary. We present an alternative model of the GHRS and STIS era profiles using information in the two sets of line shapes jointly, as well as knowledge of the behavior of Fexxi profiles of other late-G '' clump '' giants similar to Capella G8. The full survey sample also provides a context for the apparent variability: the Fe xxi flux of the G8 star in the GHRS spectrum is nearly identical (in LFe XXI=Lbol) to other clump giants of similar LX=Lbol, but it had dropped at least a factor of 6 in the STIS measurement. The He ii � 1640 B� feature—which is thought to be responsive to coronal irradiation—also showed significant changes between the GHRS and STIS epochs, but the decrease in the G8 star was much smaller than Fe xxi. The Fe xii flux displays a correlation with the ROSAT 0.2-2 keV X-ray flux that can be described by an � ¼ 0:5 power law. Fe xxi exhibits a steeper, perhaps linear (� ¼ 1), correlation with the ROSAT flux down to an activity level of LX=Lbol � 10� 5, below which detections of the coronal forbidden line are rare. There is no evidence of large, systematic Doppler shifts in either Fe xii � 1242 or Fe xxi � 1354. This suggests that the emissions arise dominantly in confined structures, analogous to magnetic loops on the Sun, rather than, say, in a hot wind. The Fe xii and Fe xxi line widths generally are close to thermal (FWHM � 40 90 km s � 1 at T � 10 6:2 10 7:0 K), except for the Hertzsprung-gap giants 31 Comae (G0 III) and Capella G1 and the K1 subgiant primary of HR 1099, all of which show evidence for excess broadening in Fe xxi (Fe xii is obscured in these objects by broad N v � 1242 features). If the excess broadening is rotational, it implies that the hot coronae of '' X-ray- deficient '' 31 Com and Capella G1 are highly extended, contrary to the compact structures suggested by recent density estimates in a number of active coronal sources. Subject headings: stars: coronae — ultraviolet: stars — X-rays: stars

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 present a NASA-DLR SOFIA-Echelon Cross Echelle Spectrograph (EXES) and NASA Infrared Telescope Facility-Texas Echelon Cross Echelle Spectrograph (TEXES) mid-IR R ≃ 50,000 spectral study of forbidden Fe II transitions in the early-type M supergiants, Betelgeuse (α Ori: M2 Iab) and Antares (α Sco: M1 Iab + B3 V). With EXES, we spectrally resolve the ground term [Fe II] 25.99 μm ( a 6DJ= 7/2-9/2: Eup = 540 K) emission from Betelgeuse. We find a small centroid blueshift of 1.9 ± 0.4 km s-1 that is a significant fraction (20%) of the current epoch wind speed, with a FWHM of 14.3 ± 0.1 km s-1. The TEXES observations of [Fe II] 17.94 μm (a 4FJ= - 7/2 9/2: Eup = 3400 K) show a broader FWHM of 19.1 ± 0.2 km s-1, consistent with previous observations, and a small redshift of 1.6 ± 0.6 km s-1 with respect to the adopted stellar center-ofmass velocity of VCoM = 20.9 ± 0.3 km s-1. To produce [Fe II] 25.99 μm blueshifts of 20% wind speed requires that the emission arises closer to the star than existing thermal models for α Ori's circumstellar envelope predict. This implies a more rapid wind cooling to below 500 K within 10R∗ (q∗ = 44 mas, dist = 200 pc) of the star, where the wind has also reached a significant fraction of the maximum wind speed. The line width is consistent with the turbulence in the outflow being close to the hydrogen sound speed. EXES observations of [Fe II] 22.90 μm ( a 4DJ= 5/2-7/2: Eup = 11,700 K) reveal no emission from either star. These findings confirm the dominance of cool plasma in the mixed region where hot chromospheric plasma emits copiously in the UV, and they also constrain the wind heating produced by the poorly understood mechanisms that drive stellar outflows from these low variability and weak-dust signature stars.

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.

UV spectra of λ Velorum taken with the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope are used to probe the structure of the outer atmospheric layers and wind and to estimate the mass-loss rate from this K5 Ib-II supergiant. VLA radio observations at λ = 3.6 cm are used to obtain an independent check on the wind velocity and mass-loss rate inferred from the UV observations. Parameters of the chromospheric structure are estimated from measurements of UV line widths, positions, and fluxes and from the UV continuum flux distribution. The ratios of optically thin C II] emission lines indicate a mean chromospheric electron density of log Ne ≈ 8.9 ± 0.2 cm-3. The profiles of these lines indicate a chromospheric turbulence (v0 ≈ 25-36 km s-1), which greatly exceeds that seen in either the photosphere or wind. The centroids of optically thin emission lines of Fe II and of the emission wings of self-reversed Fe II lines indicate that they are formed in plasma approximately at rest with respect to the photosphere of the star. This suggests that the acceleration of the wind occurs above the chromospheric regions in which these emission line photons are created. The UV continuum detected by the GHRS clearly traces the mean flux-formation temperature as it increases with height in the chromosphere from a well-defined temperature minimum of 3200 K up to about 4600 K. Emission seen in lines of C III] and Si III] provides evidence of material at higher than chromospheric temperatures in the outer atmosphere of this noncoronal star. The photon-scattering wind produces self-reversals in the strong chromospheric emission lines, which allow us to probe the velocity field of the wind. The velocities to which these self-absorptions extend increase with intrinsic line strength, and thus height in the wind, and therefore directly map the wind acceleration. The width and shape of these self-absorptions reflect a wind turbulence of ≈9-21 km s-1. We further characterize the wind by comparing the observations with synthetic profiles generated with the Lamers et al. Sobolev with Exact Integration (SEI) radiative transfer code, assuming simple models of the outer atmospheric structure. These comparisons indicate that the wind in 1994 can be described by a model with a wind acceleration parameter β ~ 0.9, a terminal velocity of 29-33 km s-1, and a mass-loss rate ~ 3 × 10-9 M☉ yr-1. Modeling of the 3.6 cm radio flux observed in 1997 suggests a more slowly accelerating wind (higher β) and/or a higher mass-loss rate than inferred from the UV line profiles. These differences may be due to temporal variations in the wind or from limitations in one or both of the models. The discrepancy is currently under investigation.

Soft X-ray detections of stellar coronae (T ~ 106 K) are rare in the giant branch redward of ~K1 III. We have conducted a less direct—but more sensitive—search using the Hubble Space Telescope Goddard High-Resolution Spectrograph to probe for far-ultraviolet proxies of the hot coronal gas in representative "noncoronal" red giants. In every target so far examined, we find weak but statistically significant Si IV emission, as well as, commonly, C IV and, in some cases, N V. Si IV is not affected by the CNO anomalies produced by the first dredge-up, which can deplete the carbon abundance and weaken C IV. In the low-activity giants, the λ1393 component of the Si IV doublet must be corrected for sharp absorptions, which we believe are caused by carbon monoxide in overlying cool material. The normalized flux ratios ( ≡ f/fbol) of Si IV and X-rays among the "coronal" yellow giants (lying just blueward of the "noncoronal" zone) fall on a uniform track, X ~2Si IV. In the noncoronal zone, however, the Si IV index is nearly constant (Si IV ~10−8), independent of X (which ranges from ~10-8 to 10-10). The mechanism that diminishes X-ray activity in the red giants is highly sensitive to an as yet unidentified stellar property. Photoelectric absorption by cool gas might play a more important role than previously suspected, particularly if hot magnetic loops are partly or completely buried in the chromosphere.

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.

PUBLISHED, The outer atmosphere of the M supergiant Betelgeuse is puzzling. Published observations of different kinds have shed light on different aspects of the atmosphere, but no unified picture has emerged. They have shown, for example, evidence of a water envelope (MOLsphere) that in some studies is found to be optically thick in the mid-infrared. In this paper, we present high-resolution, mid-infrared spectra of Betelgeuse recorded with the TEXES spectrograph. The spectra clearly show absorption features of water vapor and OH. We show that a spectrum based on a spherical, hydrostatic model photosphere with Teff = 3600 K, an effective temperature often assumed for Betelgeuse, fails to model the observed lines. Furthermore, we show that published MOLspheres scenarios are unable to explain our data. However, we are able to model the observed spectrum reasonably well by adopting a cooler outer photospheric structure corresponding to Tmod = 3250 K. The success of this model may indicate the observed mid-infrared lines are formed in cool photospheric surface regions. Given the uncertainties of the temperature structure and the likely presence of inhomogeneities, we cannot rule out the possibility that our spectrum could be mostly photospheric, albeit non-classical. Our data put new, strong constraints on atmospheric models of Betelgeuse and we conclude that continued investigation requires consideration of non-classical model photospheres as well as possible effects of a MOLsphere. We show that the mid-infrared water-vapor features have great diagnostic value for the environments of K and M (super-) giant star atmospheres., NR would like to thank B. Plez and E. Josselin for fruitful discussions and their hospitality during his stay at GRAAL in Montpellier. We gratefully acknowledge the suggestions made by Bengt Gustafsson and Kjell Eriksson for improvements to the paper. We are grateful for the help of the TEXES team as well as the IRTF staff. The use of TEXES is supported by the National Science Foundation (NSF) grant AST-0205518 and by the Texas Advanced Research Program. NR received some financial support from the Swedish Research Council and GMH?s contribution was supported by the NASA ADP Grant No. NNG04GD33G issued through the Office of Space Science and the NSF US-Sweden Cooperative Research Program grant INT-0318835 to the University of Colorado. MJR received some financial support from the NSF (grant AST-0307497) and NASA (grant NNG04GG92G). TG received support from the Lunar and Planetary Institute, which is operated by the Universities Space Research Association under NASA CAN-NCC5-679. JL was supported by NSF grant AST-0205518.

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.

Over the past decade, the Secret Lives of Cepheids (SLiC) program has been carried out at Villanova University to study aspects and behaviors of classical Cepheids that are still not well-understood. In this, the first of several planned papers on program Cepheids, we report the current results for delta Cep, the Cepheid prototype. Ongoing photometry has been obtained to search for changes in the pulsation period, light curve morphology and amplitude. Combining our photometry with the times of maximum light compilation by Berdnikov 2000 returns a small period change of dP/dt ~ -0.1006 +/- 0.0002 sec yr^-1. There is also evidence for a gradual light amplitude increase of ~0.011-mag (V-band) and ~0.012-mag (B-band) per decade over the last ~50 years. In addition, HST-COS UV spectrophotometry and XMM-Newton X-ray data were carried out to investigate the high-temperature plasmas present above the Cepheid photospheres. In total, from the five visits (eight exposures) with XMM-Newton, delta Cep is found to be a soft X-ray source (Lx [0.3--2 keV] ~ 4.5-13x10^28 erg sec^-1) with peak flux at kT = 0.6-0.9 keV. The X-ray activity is found to vary, possibly in phase with the stellar pulsations. From 2010-2013, nine observations of delta Cep were carried out with HST-COS. The UV emissions are also variable, and well-phased with the stellar pulsations. Maximum UV line emissions occur near, or slightly before, maximum optical light, varying by as much as twenty times. This variability shows that pulsation-induced shock-heating plays a significant role in Cepheid atmospheres, possibly in addition to a quiescent, magnetic heating. The results of this study show Cepheid atmospheres to be rather complex and dynamic., Comment: 34 pages, 7 figures, 4 tables - To appear in Engle et al. 2014, ApJ, 794, 80

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.

Goddard High Resolution Spectrograph (GHRS) observations have revealed the presence of broad wings in the transition-region lines of AU Mic and Capella. It has been proposed that these wings are signatures of microflares in the transition regions of these stars and that the solar analog for this phenomenon might be the 'transition region explosive events' discussed by Dere, Bartoe, & Brueckner. We have analyzed GHRS observations of Procyon (F5 IV-V) and HR 1099 (K1 IV + G5 IV) to search for broad wings in the UV emission lines of these stars. We find that the transition-region lines of HR 1099, which are emitted almost entirely by the K1 star, do indeed have broad wings that are even more prominent than those of AU Mic and Capella. This is consistent with the association of the broad wings with microflaring since HR 1099 is a very active binary system. In contrast, the transition-region lines of Procyon, a relatively inactive star, do not show evidence for broad wings, with the possible exception of N v lambda1239. However, Procyon's lines do appear to have excess emission in their blue wings. Linsky et al. found no evidence for broad wings in Capella's chromospheric lines, but we find that the Mg II resonance lines of HR 1099 do have broad wings. The striking resemblance between HR 1099's Mg II and C iv lines suggests that the Mg II line profiles may be regulated by turbulent processes similar to those that control the transition-region line profiles. If this is the case, microflaring may be occurring in the K1 star's chromosphere as well as in its transition region. However, radiative transfer calculations suggest that the broad wings of the Mg II lines can also result from normal chromospheric opacity effects rather than pure turbulence. The prominence of broad wings in the transition region and perhaps even chromospheric lines of active stars suggests that microflaring is very prevalent in the outer atmospheres of active stars.

Between 1990 August and 1991 January the ROSAT/IUE All Sky Survey (RIASS) coordinated pointings by the International Ultraviolet Explorer (IUE) with the continuous X-ray/EUV mapping by the Roentgensatellit (ROSAT). The campaign provided an unprecedented multiwavelength view of a wide variety of cosmic sources. We report findings for F-K stars, a large proportion of the RIASS targets. Forty-eight of our 91 'Coronathon' candidates were observed by the IUE during the campaign. For stars missed by the IUE, we supplemented the ROSAT survey fluxes with archival UV spectra and/or follow-on observations.

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.