Nils Ryde

Detection of Water Vapor in the Photosphere of Arcturus

We report detections of pure rotation lines of OH and H2O in the K1.5 III red-giant star Arcturus (α Bootis) using high-resolution, infrared spectra covering the regions 806 −822cm −1 (12.2 −12.4 µm) and 884 −923cm −1 (10.8 −11.3 µm). Arcturus is the hottest star yet to show water-vapor features in its disk-averaged spectrum. We argue that the water vapor lines originate from the photosphere, albeit in the outer layers. We are able to predict the observed strengths of OH and H2O lines satisfactorily after lowering the temperature structure of the very outer parts of the photosphere (log τ500 = −3.8 and beyond) compared to a flux- constant, hydrostatic, standard marcs model photosphere. Our new model is consistently calculated including chemical equilibrium and radiative transfer from the given temperature structure. Possible reasons for a temperature decrease in the outer-most parts of the photosphere and the assumed break-down of the assumptions made in classical model-atmosphere codes are discussed.

Probing the mass loss history of carbon stars using CO line and dust continuum emission

We have modelled radio and far-infrared rotational lines of (CO)-C-12 and (CO)-C-13 Obtained from the circumstellar envelope of the infrared-bright carbon star IRAS 15194-5115. Eleven rotational lines between J=1-0 and 21-20 and nine rotational lines betw

On the Galactic chemical evolution of sulfur

Department of Astronomy, University of Texas, Austin, TX 78712-1083, USAe-mail: ryde@astro.uu.seReceived ; acceptedAbstract. Sulfur abundances have been determined for ten stars to resolve a debate in the literature on theGalactic chemical evolution of sulfur in the halo phase of the Milky Way. Our analysis is based on observationsof the Si lines at 9212.9, 9228.1, and 9237.5 ˚A for stars for which the S abundance was obtained previously frommuch weaker Si lines at 8694.0 and 8694.6 ˚A. In contrast to the previous results showing [S/Fe] to rise steadilywith decreasing [Fe/H], our results show that [S/Fe] is approximately constant for metal-poor stars ([Fe/H]<∼−1)at [S/Fe] ≃ +0.3. Thus, sulfur behaves in a similar way to the other α elements, with an approximately constant[S/Fe] for metallicities lower than [Fe/H] ≃ −1. We suggest that the reason for the earlier claims of a rise of [S/Fe]is partly due to the use of the weak Si 8694.0 and 8694.6˚A lines and partly uncertainties in the determinationof the metallicity when using Fei lines. The Si 9212.9, 9228.1, and 9237.5˚A lines are preferred for an abundanceanalysis of sulfur for metal-poor 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.

The Zeeman-sensitive emission lines of Mg I at 12 microns in Procyon

Emission lines of magnesium at 12 mum have been observed in the spectrum of Procyon. We reproduce the observed disk-averaged line flux from Procyon ( as well as the observed intensity profiles from ...

The 3 micron spectrum of R Doradus observed with the ISO-SWS

We have modeled the 2:6{3:7m spectrum of the red semiregular variable R Doradus observed with the Short-Wavelength Spectrometer on board the Infrared Space Observatory. The wavelength resolution of the observations varies between R 2000{2500. We have calculated a synthetic spectrum using a hydrostatic model photosphere in spherical geometry. The agreement between the synthetic spectrum and the ISO observations is encouraging, especially in the wavelength region of 2:8{3:7m, suggesting that a hydrostatic model photosphere is adequate for the calculation of synthetic spectra in the near infrared for this moderately varying red giant star. However, an additional absorption component is needed at 2:6{2:8m and this discrepancy is discussed. The spectral signatures are dominated by water vapour in the stellar photosphere, but several photospheric OH, CO, and SiO features are also present. The eective temperature and surface gravity derived for R Dor, based on the 2:6{3:7m ISO spectrum and the modeling of it with a hydrostatic model photosphere, are 3000 100 K and logg =0 1 (cgs), respectively. The spectral region observed is found to be temperature sensitive. The eective temperature given here is slightly higher than those reported in the literature. We also discuss possible reasons for this.

Nonthermal Mg I Emission At 12 Micron From Procyon

We report on stellar Mg I emission at 12 μm from α CMi (Procyon), a star slightly hotter than the Sun. Solar Mg I emission is well known, and its formation was successfully explained in detail by Carlsson et al. Here, for the first time, we successfully model and compare synthetic spectra of the emission lines at 12 μm with observations of a star other than the Sun. The use of these lines as stellar diagnostics has been anticipated for 10 years or more (see, e.g., Carlsson et al.). We find that the model reproduces the observed emission in Procyon quite well. We expect that high-resolution spectrographs on 8-10 m telescopes will finally be able to exploit these new diagnostics.

Water Vapor in the Outer Atmospheres of Red Giants Probed by High-Resolution, Mid-Infrared Observations

Here, we report on our detection of water vapor in the mid-infrared (11-12 microns) spectrum of the Kl.5 giant Arcturus (α Boo) obtained with the high-resolution, infrared spectrograph TEXES. The detection is presented in detail in Ryde et al. 2002 ApJ, submitted. We are successful in modeling several resolved, rotational lines of water vapor and vibration-rotational (1.5 microns) and rotational lines of OH with a modified MARCS photosphere

An analysis of the 2.6 - 3.7 micron spectrum of R Dor

Here, I present our modelling (Ryde & Eriksson, A&A 2002, 386, 874) of the 2.6 – 3.7 microns spectrum of the red semiregular variable R Doradus observed with the Short-Wavelength Spectrometer on board the Infrared Space Observatory.

The Mass-Loss History of Carbon Stars Probed by Co Line and Dust Continuum Emission

An extensive modelling of CO line emission from the circumstellar envelopes around a number of carbon stars is performed. By combining radio observations and infrared observations obtained by ISO the circumstellar envelope characteristics are probed over a large radial range. The combined data set gives better determined envelope parameters, and puts constraints on the mass loss history of these carbon stars. Moreover, analysis of the dust emission puts further constraints on the mass-loss rate history. The stars presented here are not likely to have experienced any drastic long-term, mass-loss rate modulations, at least less than a factor of about 5, over the past thousands of years