Donald G. Luttermoser

Chromospheric structure of cool carbon stars

A semiempirical chromospheric model is proposed for TX Psc which is a prototype for the N-type carbon stars. Observational data imply that the chromospheric temperature rise must begin at a low density, that the temperature gradient in the lower chromosphere must be steep, that partial redistribution must be employed in the Mg II calculation, and that the lower chromosphere is expanding away from the photosphere with a velocity of close to 50 km/s. The present model also shows that the microturbulent velocity is about 7 km/s at the temperature minimum region, dropping to 5 km/s in the chromosphere, and that the Lyman lines are optically thick in the chromosphere. 55 refs.

Outer layers of a carbon star: The view from the Hubble Space Telescope

To advance our understanding of the relationship between stellar chromospheres and mass loss, which is a common property of carbon stars and other asymptotic giant branch stars, we have obtained ultraviolet spectra of the nearby N-type carbon star UU Aur using the Hubble Space Telescope (HST). In this paper we describe the HST observations, identify spectral features in both absorption and emission, and attempt to infer the velocity field in the chromosphere, upper troposphere, and circumstellar envelope from spectral line shifts. A mechanism for producing fluoresced emission to explain a previously unobserved emission line is proposed. Some related ground-based observations are also described.

Ionization and excitation in cool giant stars. I - Hydrogen and helium

The influence that non-LTE radiative transfer has on the electron density, ionization equilibrium, and excitation equilibrium in model atmospheres representative of both oxygen-rich and carbon-rich red giant stars is demonstrated. The radiative transfer and statistical equilibrium equations are solved self-consistently for H, H(-), H2, He I, C I, C II, Na I, Mg I, Mg II, Ca I, and Ca II in a plane-parallel static medium. Calculations are made for both radiative-equilibrium model photospheres alone and model photospheres with attached chromospheric models as determined semiempirically with IUE spectra of g Her (M6 III) and TX Psc (C6, 2). The excitation and ionization results for hydrogen and helium are reported.

Line profile variations in M giants - Clues to mass-loss and chromospheric heating mechanisms

Analysis is presented of time-series, high dispersion spectra of the Mg II, k, Ca II H, and K lines of the semiregular giants Rho Per (M4 II-III, periodicity of about 50 days), R Lyr (M5 III, period of about 46 days), and g Her (M6 III, period of about 90 days). The fine error sensor on the IUE satellite and ground based UBV photometry was used to relate line profile variations to photospheric variations. The above mentioned stars were selected to study the relative importance of convective motions and global stellar pulsations in determining the structure of the outer atmospheres. Small amplitude changes, but substantial changes in the profiles of Mg II and Ca II lines were detected. It is contended that the observed variability is due to changes in chromospheric conditions and not variations within the circumstellar shell. The picture of a steady state chromosphere, which is modulated on long time scales, is corroborated by these observations. Localized heating is found in g Her.

Ultraviolet spectra and chromospheres of cool carbon stars

The authors assemble and discuss all available low-resolution IUE spectra of N-type carbon stars - including TW Hor, BL Ori, UU Aur, NP Pup, U Hya, T Ind, and TX Psc. Identification of spectral features is aided by a composite spectrum. Shortward of 2850 A only emission lines of C II, Mg II, Al II, and Fe II are seen, while the spectrum longward of 2850 A appears to be a photospheric absorption spectrum with a few superposed emission lines of Fe II. The most prominent absorption features are due to Fe I, CH, and CaCl. The emission feature at 2325 A, second only to Mg II in strength, is conclusively identified as C II (UV 0.01). Ultraviolet spectra of N-type carbon stars are similar to, though the emission-line fluxes are generally weaker than, those of the coolest M-giant stars available, such as HD 18191 (M6 III).

The Atmosphere of Mira Variables: A View with the Hubble Space Telescope

Ultraviolet spectra obtained with Hubble Space Telescope (HST ) of two Mira-type variable stars, R Leo and R Hya, are presented, along with analysis providing information on their outer atmospheres. These high-dispersion spectra were taken with the Goddard High Resolution Spectrograph (HRS) in two spectral regions: 2320¨2368 to record the C II) (UV0.01) multiplet and 2785¨2835 to obtain the Mg II Ae Ae h and k lines. The R Hya spectrum was obtained at visual light phase 0.26 and shows a Mg II spec- trum that is very clean, showing clear evidence for the overlying circumstellar absorption from Fe I (UV3) and Mn I (UV1) over the k line. The —uoresced Fe I (UV44) feature at 2824 is plainly visible in Ae this spectrum, whereas past International Ultraviolet Explorer (IUE) observations of Mira variables at high dispersion were unable to record this feature. Remarkably, the newly identi—ed —uoresced Fe I (UV45) feature near 2807 is seen in this spectrum. Until now, this line has been seen only in cool Ae carbon stars with HST /HRS. This line is pumped by the thin C II) (UV0.01) emission line at 2325.5 Ae . Two of the strongest C II) (UV0.01) lines near 2325 are plainly seen in this spectrum. This region of Ae the spectrum, however, is dominated by the Si II) (UV0.01) line near 2335 in contrast to that observed Ae , in the carbon stars and the non-Mira oxygen-rich red giant stars. Very weak Mg II lines are seen in the R Leo spectrum at phase 0.12. At this phase, these lines are typically absent in IUE spectra. Velocity shifts of emission features in the UV spectra of Mira variables are consistent with previously published hydrodynamic models of these stars. These velocities indicate, however, that the C II) (UV0.01) emission lines are not formed in the same atmospheric layers as the Mg II emission. The electron density deduced from the C II) (UV0.01) multiplet is D109 cm~3. Finally, the temperature-density structure of the semi- regular variable carbon stars is similar to the oxygen-rich Mira variablesboth are hydrodynamic in nature; however, the carbon stars macroscopic velocity —elds are not identical to the Mira stars in the atmosphere layers between the Mg II emission region and the circumstellar shell. Subject headings: stars: abundancesstars: AGB and post-AGB ¨ stars: variables: other (long-period variables) ¨ ultraviolet: stars

A VLA 3.6 centimeter survey of N-type carbon stars

The results are presented of a VLA-continuum survey of 7 N-type carbon stars at 3.6 cm. Evidence exists for hot plasma around such stars; the IUE satellite detected emission lines of singly ionized metals in the optically brightest carbon stars, which in solar-type stars indicate the existence of a chromosphere. In the past, these emission lines were used to constrain the lower portion of the archetypical chromospheric model of N-type carbon stars, that of TX Psc. Five of the survey stars are semiregular (1 SRa and 4 SRb) variables and two are irregular (Lb) variables. Upper limits of about 0.07 mJy are set of the SRb and Lb variables and the lone SRa (V Hya) was detected with a flux of 0.22 mJy. The upper limits for the six stars that are not detected indicate that the temperature in their winds is less than 10,000 K. Various scenarios for the emission from V Hya are proposed, and it is suggested that the radio continuum is shock-related (either due to pulsation or the suspected bipolar jet) and not due to a supposed accretion disk around an unseen companion.

The Chromospheres of Carbon Stars

Most oxygen-rich late-type giant stars show evidence for chromospheres in their visual spectra (e.g. Ca ii H & K emission features). Cool (i.e. N-type) non-Mira carbon stars, however, have never been observed to have chromospheric emission in their Ca ii H & K lines. However, faint Mg ii h & k lines were detected in emission in low-dispersion IUE spectra of the brightest cool carbon stars in the early 1980s. May 1984 saw the first (and only) successful high-dispersion IUE spectrum taken of a cool carbon star, TX Psc (N0; C6,2). Armed with this high-dispersion spectrum, as well as low-dispersion IUE and ground-based spectra, Luttermoser et al. (1989) made the first detailed attempt to semiempirically model the chromosphere of a cool carbon star. This model was successful in reproducing the Mg ii lines, but it was not well constrained due to the lack of other observed high-resolution chromospheric profiles for comparison. Modeling carbon star chromospheres can now be addressed more accurately with HST/GHRS high-resolution spectra. New fluoresced emission features have been discovered in the GHRS spectra of carbon stars that are not present in their oxygen-rich counterparts.

Blinded by the Lines: Mid-IR Spectra of Mira Variables Taken with Spitzer

We present preliminary analysis of mid-infrared spectra of M-type and C-type Mira variables. Due to the brightness of this sample, it is straightforward to monitor changes with phase in the infrared spectral features of these regular pulsators. We have spectra of 25 Mira variables, taken with phase, using the Spitzer Infrared Spectrograph (IRS) high-resolution module. Each star has multiple spectra obtained over a one-year period from 2008-09. This is a rich, unique data set due to multiple observations of each star and the high signal-to-noise ratio from quick exposure times to prevent saturation of the IRS instrument. This paper focuses on the 17.6 and 33.2 micron lines shared by M-types and C-types. These are mostly emission lines that change with phase. We discuss preliminary physical diagnostics for the atmospheres based on the lines, as well as possible line identi cations such as uorescence of metal species.

Chromospheres in Red Giants

Increasing evidence suggests that the mechanism(s) for heating the chromospheres of the coolest red giants may be quite different from those responsible for heating in solar-type stars, but little progress toward a quantitative test of this important idea has been made, both because of the scarcity of suitable observations and the difficulty of creating NLTE model chromospheres. We report here the results of NLTE calculations for 10 “classical” model chromospheres (differing mainly in the column mass density above the temperature minimum) and one “shock” chromosphere for a cool giant star and compare the results to observations of 30 g Her (M6 III), one of the coolest (Teff = 3250 K) SRb (semi-regular) variable stars. Observed chromospheric spectral features include Mg II h & k; C II] UV0.01; Mg I λ2852; Ca II H, K & IRT; Ca I λ4227 & λ6573; Al II] UV1; and Balmer α. The equations of statistical equilibrium and radiative transfer are solved self-consistently for H I, H-, H2, He I, C I, C II, Na I, Mg I, Mg II, Al I, Al II, Ca I, and Ca II with the equivalent two-level-atom technique in a one-dimensional, hydrostatic, plane-parallel atmosphere. Synthetic spectra from the classical models are compared in detail with observations of 30 g Her. However, we find that no single-component classical model in hydrostatic equilibrium is able to reproduce both the Mg II line profiles and the relative strengths of the C II] lines. Some non-classical feature — either departures from hydrostatic equilibrium, shocks, inhomogeneities, or unusual velocity fields — is called for. Surprisingly, however, synthetic spectra from our simple shock model reproduce both of these ionic multiplets, but only if we severely constrain the temperature and thickness of the chromosphere and the position of a shock between the chromosphere and photosphere.