J-type Carbon Stars in the Large Magellanic Cloud

A sample of 1497 carbon stars in the Large Magellanic Cloud has been observed in the red part of the spectrum with the 2dF facility on the AAT. Of these, 156 have been identified as J-type (i.e. 13C-rich) carbon stars using a technique which provides a clear distinction between J stars and the normal N-type carbon stars that comprise the bulk of the sample, and yields few borderline cases. A simple 2-D classification of the spectra, based on their spectral slopes in different wavelength regions, has been constructed and found to be related to the more conventional c- and j-indices, modified to suit the spectral regions observed. Most of the J stars form a photometric sequence in the K - (J-K) colour magnitude diagram, parallel to and 0.6 mag fainter than the N star sequence. A subset of the J stars (about 13 per cent) are brighter than this J star sequence; most of these are spectroscopically different from the other J stars. The bright J stars have stronger CN bands than the other J stars and are found strongly concentrated in the central regions of the LMC. Most of the rather few stars in common with Hartwick and Cowley's sample of suspected CH stars are J stars. Overall, the proportion of carbon stars identified as J stars is somewhat lower than has been found in the Galaxy. The Na D lines are weaker in the LMC J stars than in either the Galactic J stars or the LMC N stars, and do not seem to depend on temperature.Comment: 19 pages, 21 figures, Latex; in press, MNRA

Rotational spectroscopy of AlO: Low N transitions of astronomical interest in the X^2 Sigma^+ state

The detection of rotational transitions of the AlO radical at millimeter wavelengths from an astronomical source has recently been reported. In view of this, rotational transitions in the ground X^2 Sigma^+ state of AlO have been reinvestigated. Comparisons between Fourier transform and microwave data indicate a discrepancy regarding the derived value of gamma_D in the v = 0 level of the ground state. This discrepancy is discussed in the light of comparisons between experimental data and synthesized rotational spectra in the v = 0, 1 and 2 levels of X^2 Sigma^+. A list of calculated rotational lines in v = 0, 1 and 2 of the ground state up to N' = 11 is presented which should aid astronomers in analysis and interpretation of observed AlO data and also facilitate future searches for this radical.Comment: To appear in Astronomy and Astrophysic

Further detections of OH masers in carbon stars with silicate features

A sample of J-type carbon stars was searched for OH maser emission. The new detection of three OH lines towards two silicate carbon stars is reported. In V778 Cyg, previously known as the main-lines (1665 and 1667 MHz) maser source, the satellite 1612 MHz emission was discovered while in NSV 2814 the main OH lines were detected. The presence of OH maser lines confirms the former suggestion that oxygen-rich material is located in the vicinity ($\approx$ $10^{15-16}$ cm) of silicate carbon stars.Comment: LaTeX2e, 4 pages with 2 figure

Lifetime of OH masers at the tip of the asymptotic giant branch

Context: A large fraction of otherwise similar asymptotic giant branch stars (AGB) do not show OH maser emission. As shown recently, a restricted lifetime may give a natural explanation as to why only part of any sample emits maser emission at a given epoch. Aims: We wish to probe the lifetime of 1612 MHz OH masers in circumstellar shells of AGB stars. Methods: We reobserved a sample of OH/IR stars discovered more than 28 years ago to determine the number of stars that may have since lost their masers. Results: We redetected all 114 OH masers. The minimum lifetime inferred is 2800 years (1 sigma). This maser lifetime applies to AGB stars with strong mass loss leading to very red infrared colors. The velocities and mean flux density levels have not changed since their discovery. As the minimum lifetime is of the same order as the wind crossing time, strong variations in the mass-loss process affecting the excitation conditions on timescales of ~3000 years or less are unlikely. Keywords: OH masers -- Stars: AGB and post-AGB -- circumstellar matterComment: 8 pages, 5 figures, accepted for publication in Astronomy & Astrophysic

The Temperature Scale of Metal-Rich M Giants Based on TiO Bands: Population Synthesis in the Near Infrared

We have computed a grid of high resolution synthetic spectra for cool stars (2500<Teff<6000 K) in the wavelength range 6000 -- 10200A, by employing an updated line list of atomic and molecular lines, together with state-of-the-art model atmospheres. As a by-product, by fitting TiO bandheads in spectra of well-known M giants, we have derived the electronic oscillator strengths of the TiO gamma prime, delta, epsilon and phi systems. The derived oscillator strenghts for the gamma prime, epsilon and phi systems differ from the laboratory and ab initio values found in the literature, but are consistent with the model atmospheres and line lists employed, resulting in a good match to the observed spectra of M giants of known parameters. The behavior of TiO bands as a function of the stellar parameters Teff, log g and [Fe/H] is presented and the use of TiO spectral indices in stellar population studies is discussed.Comment: ApJ accepted, 27 pages + 11 figures, AASLatex v4.

Reprocessing the Hipparcos data for evolved giant stars II. Absolute magnitudes for the R-type carbon stars

The Hipparcos Intermediate Astrometric Data for carbon stars have been reprocessed using an algorithm which provides an objective criterion for rejecting anomalous data points and constrains the parallax to be positive. New parallax solutions have been derived for 317 cool carbon stars, mostly of types R and N. In this paper we discuss the results for the R stars. The most important result is that the early R stars (i.e., R0 - R3) have absolute magnitudes and V-K colors locating them among red clump giants in the Hertzsprung-Russell diagram. Stars with subtypes R4 - R9 tend to be cooler and have similar luminosity to the N-type carbon stars, as confirmed by their position in the (J-H, H-K) color-color diagram. The sample of early R-type stars selected from the Hipparcos Catalogue appears to be approximately complete to magnitude K_0 ~ 7, translating into a completeness distance of 600 pc if all R stars had M_K= -2 (400 pc if M_K= -1). With about 30 early R-type stars in that volume, they comprise about 0.04% (0.14% for M_K= -1) of the red clump stars in the solar neighborhood. Identification with the red clump locates these stars at the helium core burning stage of stellar evolution, while the N stars are on the asymptotic giant branch, where helium shell burning occurs. The present analysis suggests that for a small fraction of the helium core burning stars (far lower than the fraction of helium shell-burning stars), carbon produced in the interior is mixed to the atmosphere in sufficient quantities to form a carbon star.Comment: 11 pages, 6 figures, A&A Latex. To appear in A&

Dust temperature and density profiles of AGB and post-AGB stars from mid-infrared observations

First mid-infrared images of a sample of AGB and post-AGB carbon stars (V Hya, IRC +10216, CIT 6 and Roberts 22) obtained at La Silla Observatory (ESO, Chile) are reported. CIT 6 presents a cometary-like feature clearly seen in the 9.7$\mu$m image, Roberts 22 shows an envelope slightly elongated in the north-east direction while images of V Hya and IRC+10216 are roughly spherically symmetric. Using inversion technique, the dust emissivity was derived from the observed intensity profiles, allowing a determination of the grain temperature and density distributions inside the envelope for these stars. Dust masses and mass-loss rates were estimated for V Hya and IRC +10216. Our results are comparable to those obtained in previous studies if dust grains have dimensions in the range $\sim$ 0.01 - 0.2 $\mu$m. Color maps suggest the presence of temperature inhomogeneities in the central regions of the dust envelopes. In the case of V Hya, an eccentric hot point, which direction coincides with the jet previously seen in [SII] emission, suggest that we are observing a material ejected in a previous mass-loss event. Bipolar lobes are clearly seen in the color maps of Roberts 22 and IRC +10216.Comment: accepted for publication in Astronomy and Astrophysic

Evidence for a companion to BM Gem, a silicate carbon star

Balmer and Paschen continuum emission as well as Balmer series lines of P Cygni-type profile from H_gamma through H_23 are revealed in the violet spectra of BM Gem, a carbon star associated with an oxygen-rich circumstellar shell (`silicate carbon star') observed with the high dispersion spectrograph (HDS) on the Subaru telescope. The blue-shifted absorption in the Balmer lines indicates the presence of an outflow, the line of sight velocity of which is at least 400 km s^-1, which is the highest outflow velocity observed to date in a carbon star. We argue that the observed unusual features in BM Gem are strong evidence for the presence of a companion, which should form an accretion disk that gives rise to both an ionized gas region and a high velocity, variable outflow. The estimated luminosity of ~0.2 (0.03-0.6) L_sun for the ionized gas can be maintained by a mass accretion rate to a dwarf companion of ~10^-8 M_sun yr^-1, while ~10^-10 M_sun yr^-1 is sufficient for accretion to a white dwarf companion. These accretion rates are feasible for some detached binary configurations on the basis of the Bond-Hoyle type accretion process. We concluded that the carbon star BM Gem is in a detached binary system with a companion of low mass and low luminosity. However, we are unable to determine whether this companion object is a dwarf or a white dwarf. The upper limits for binary separation are 210 AU and 930 AU for a dwarf and a white dwarf, respectively. We also note that the observed features of BM Gem mimic those of Mira (omi Cet), which may suggest actual similarities in their binary configurations and circumstellar structures.Comment: 11 pages, 2 figures, 1 table, accepted for publication in Ap

High angular resolution N-band observation of the silicate carbon star IRAS08002-3803 with the VLTI/MIDI instrument

We present the results of N-band spectro-interferometric observations of the silicate carbon star IRAS08002-3803 with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory (ESO). The observations were carried out using two unit telescopes (UT2 and UT3) with projected baseline lengths ranging from 39 to 47 m. Our observations of IRAS08002-3803 have spatially resolved the dusty environment of a silicate carbon star for the first time and revealed an unexpected wavelength dependence of the angular size in the N band: the uniform-disk diameter is found to be constant and ~36 mas (72 Rstar) between 8 and 10 micron, while it steeply increases longward of 10 micron to reach ~53 mas (106 Rstar) at 13 micron. Model calculations with our Monte Carlo radiative transfer code show that neither spherical shell models nor axisymmetric disk models consisting of silicate grains alone can simultaneously explain the observed wavelength dependence of the visibility and the spectral energy distribution (SED). We propose that the circumstellar environment of IRAS08002-3803 may consist of two grain species coexisting in the disk: silicate and a second grain species, for which we consider amorphous carbon, large silicate grains, and metallic iron grains. Comparison of the observed visibilities and SED with our models shows that such disk models can fairly -- though not entirely satisfactorily -- reproduce the observed SED and N-band visibilities. Our MIDI observations and the radiative transfer calculations lend support to the picture where oxygen-rich material around IRAS08002-3803 is stored in a circumbinary disk surrounding the carbon-rich primary star and its putative low-luminosity companion.Comment: 15 pages, 8 figures, accepted for publication in A&

Should Research Ethics Encourage the Production of Cost-Effective Interventions?

This project considers whether and how research ethics can contribute to the provision of cost-effective medical interventions. Clinical research ethics represents an underexplored context for the promotion of cost-effectiveness. In particular, although scholars have recently argued that research on less-expensive, less-effective interventions can be ethical, there has been little or no discussion of whether ethical considerations justify curtailing research on more expensive, more effective interventions. Yet considering cost-effectiveness at the research stage can help ensure that scarce resources such as tissue samples or limited subject popula- tions are employed where they do the most good; can support parallel efforts by providers and insurers to promote cost-effectiveness; and can ensure that research has social value and benefits subjects. I discuss and rebut potential objections to the consideration of cost-effectiveness in research, including the difficulty of predicting effectiveness and cost at the research stage, concerns about limitations in cost-effectiveness analysis, and worries about overly limiting researchers’ freedom. I then consider the advantages and disadvantages of having certain participants in the research enterprise, including IRBs, advisory committees, sponsors, investigators, and subjects, consider cost-effectiveness. The project concludes by qualifiedly endorsing the consideration of cost-effectiveness at the research stage. While incorporating cost-effectiveness considerations into the ethical evaluation of human subjects research will not on its own ensure that the health care system realizes cost-effectiveness goals, doing so nonetheless represents an important part of a broader effort to control rising medical costs