Graham M. Harper

A Spatially Resolved, Semiempirical Model for the Extended Atmosphere of α Orionis (M2 Iab)

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.

A NEW VLA-HIPPARCOS DISTANCE TO BETELGEUSE AND ITS IMPLICATIONS

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 Betelgeuses 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.

Far-Ultraviolet Emissions of the Sun in Time: Probing Solar Magnetic Activity and Effects on Evolution of Paleoplanetary Atmospheres*

We present and analyze Far Ultraviolet Spectroscopic Explorer (FUSE) observations of six solar analogs. These are single main-sequence G0-G5 stars selected as proxies for the Sun at several stages of its main-sequence lifetime from ~130 Myr to ~9 Gyr. The emission features in the FUSE 920-1180 ? wavelength range allow for a critical probe of the hot plasma over three decades in temperature: from ~104 K for the H I Lyman series to ~6 ? 106 K for the coronal Fe XVIII ?975 line. Using the flux ratio C III ?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 ~105 K plasma decreases by a factor of ~70 between the young fast-rotating (Prot = 2.7 days) magnetically active star and the old, slow-rotating (Prot ~ 35 days) inactive star. We also study the variations in the total surface flux for specific emission features that trace the hot gas in the stellar chromosphere (C II), transition region (C III, O VI), and corona (Fe XVIII). The observations indicate that the average surface fluxes of the analyzed 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 (~104 K) to hotter coronal (~107 K) plasma. The relationship for the integrated (920-1180 ?) FUSE flux indicates that the solar far-ultraviolet (FUV) 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. Some examples of the effects of the early Suns enhanced FUV irradiance on the atmospheres of Earth and Mars are also discussed.

Buried Alive in the Coronal Graveyard

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

Water Vapor on Betelgeuse as Revealed by TEXES High-Resolution 12 μm Spectra

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 MOLsphere 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 that 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 nonclassical. Our data put new, strong constraints on atmospheric models of Betelgeuse, and we conclude that continued investigation requires consideration of nonclassical 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.

From Radio to X-Ray: The Quiescent Atmosphere of the dMe Flare Star EV Lacertae

We report on multiwavelength observations spanning radio to X-ray wavelengths of the M dwarf flare star EV Lacertae and 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 appears 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 2 orders of magnitude in density (ne ~ 1013 cm-3 at log T = 6.9). X-ray, EUV, FUV, and NUV spectra constrain the differential emission measure (DEM) from the upper chromosphere through the corona. The coronal pressures are inconsistent with the assumption of hydrostatic equilibrium, either through emission measure (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 Lacs 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.

Electron Density and Turbulence Gradients within the Extended Atmosphere of the M Supergiant Betelgeuse (α Orionis)

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.

Identification of Fe II emission lines in FUSE stellar spectra

We identify two complexes of Fe II emission lines in far-ultraviolet spectra of the stars α TrA and HD 104237. Using spectra from both the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST), we show that these emission lines, which represent the majority of previously unidentifed emission features in cool star spectra between 912 and 1180 A, are fluorescent decays in Fe II following excitation by H Lyα. Specifically, following photoexcitation from the third term (4s a 4D) of Fe II, subsequent decays are observed to the two lowest terms (4s a 6D and 3d7 a 4F) which are observed near 1100 and 1135 A, respectively. Decays to higher terms, and hence longer wavelengths, also are clearly seen in the STIS spectra. Differences in the fluorescent Fe II spectra of α TrA and HD 104237 are tentatively identified as resulting from differences in the intrinsic width of the density-weighted H Lyα radiation fields. The additional Fe II lines observed in α TrA result from a broadened H Lyα profile. Two features near 1060 A appear to be fluorescent lines of Cr II, also excited by H Lyα.

ALMA sub-mm maser and dust distribution of VY Canis Majoris

Aims. Cool, evolved stars have copious, enriched winds. Observations have so far not fully constrained models for the shaping and acceleration of these winds. We need to understand the dynamics better, from the pulsating stellar surface to similar to 10 stellar radii, where radiation pressure on dust is fully effective. Asymmetric nebulae around some red supergiants imply the action of additional forces. Methods. 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 accuracy and to resolve the dusty continuum. Results. 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 overlap but avoid each other on scales of up to 10 au. Their distribution is broadly consistent with excitation models but the conditions and kinematics are complicated by wind collisions, clumping, and asymmetries.

GHRS Observations of Cool, Low-Gravity Stars. V. The Outer Atmosphere and Wind of the Nearby K Supergiant λ Velorum*

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.