Alex de Koter

THE IONIZATION IN THE WINDS OF O STARS AND THE DETERMINATION OF MASS-LOSS RATES FROM ULTRAVIOLET LINES

Empirical ionization fractions of C IV ,N V ,S iIV, and empirical ionization plus excitation fractions of C III* and N IV* in the winds of 34 O stars and one B star have been derived. We combine the mass-loss rates derived from radio measurements and Ha with the line —tting of ultraviolet resonance lines and subordinate lines using the Sobolev plus exact integration (SEI) method. The dependence of the empiri- cal ionization fractions, SqT, on the stellar eUective temperature and on the mean wind density is dis- cussed. This sets constraints for the models of ionization in the winds of hot stars. The ionization and excitation fractions can be expressed in terms of an empirical radiation temperature. This radiation tem- perature scales with and we derive empirical relations for as a function of The radiation T eff , T rad T eff . temperatures are on the order of 0.5¨0.9 with signi—cant diUerences between the ions. The derived T eff , relations between the ionization fractions and the stellar parameters have an uncertainty of 0.2 dex for Si IV ,N V, and C III*, and about 0.26 dex for N IV*. For C IV, we can only derive an expression for the mean ionization fraction in the wind if the mass-loss rate is small, yr~1, because the C IV M 0\ 10~6 M _ lines are usually saturated for higher mass-loss rates. The resulting expressions for can be used to T rad derive the mass-loss rates from studies of ultraviolet P Cygni pro—les in the range of stellar parameters studied here: 30,000 K, and ( 7.5 K ( T eff ( 50,500 5.2 ( log L /L * ( 6.4, M _ ( log M0 ( (4.6 M _ yr~1. An accuracy of about a factor of 2 or better can be reached, depending on the lines that are used and the accuracy of the line —ts and the stellar parameters. The Si IV lines give the most reliable mass- loss rates, because the abundance is about the same for all O stars, the lines saturate only for high mass-loss rates, the doublet lines only partly overlap, and the mass-loss rate is proportional to the square root of the column density. The radiation temperature of N V shows a surprisingly strict relation with with a scatter of only The mass-loss rate cannot be derived from the N V T eff , *T rad /T eff \ 0.01. lines, because the column density of the N V ions in the wind is independent of A consistency check M0 . and a test of the method for the stars HD 14749 and HD 190429 show that the mass-loss rate derived from the UV lines with the ionization fractions of this paper agree very well with the mass-loss rate derived from new radio —ux measurements. Subject headings: stars: atmospheresstars: early-typestars: mass lossultraviolet: stars

The ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). I. Project description, survey sample, and quality assessment

The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the [Formula: see text] fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort to systematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution (R ~ 70 000 - 100 000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305-1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided.

Disks formed by rotation induced bi-stability

We discuss the evidence for the existence of bi-stable stellar winds of early type stars, both theoretically and observationally. The ratio between the terminal wind velocity and the escape velocity drops steeply from about 2.6 for stars with T eff>21 000 K to about 1.3 at T eff<21 000 K. This is the bi-stability jump, which is due to a change in the ionization of the wind and in the wind driving lines. The mass loss rate increases across the jump by about a factor 2 to 5 from the hotter to the cooler stars. The mass flux from rapidly rotating stars can also show the bi-stability jump at some lattitude between the pole and the equator, with a slow high density wind in the equatorial region and a faster low density wind from the poles. This might explain the disks of rapidly rotating B[e] stars, formed by the Rotation Induced Bi-stability mechanism. We discuss the RIB mechanism and its properties. We also describe some future improvements of the model.

CUBESPEC: low-cost space-based astronomical spectroscopy

CubeSats are routinely used for low-cost photometry from space. Space-borne spectroscopy, however, is still the exclusive domain of much larger platforms. Key astrophysical questions in e.g. stellar physics and exoplanet research require uninterrupted spectral monitoring from space over weeks or months. Such monitoring of individual sources is unfortunately not affordable with these large platforms. With CUBESPEC we plan to offer the astronomical community a low-cost CubeSat solution for near-UV/optical/near-IR spectroscopy that enables this type of observations. CUBESPEC is a generic spectrograph that can be configured with minimal hardware changes to deliver both low resolution (R = 100) with very large spectral coverage (200 - 1000 nm), as well as high resolution (R = 30 000) over a selected wavelength range. It is built around an off-axis Cassegrain telescope and a slit spectrograph with configurable dispersion elements. CUBESPEC will use a compact attitude determination and control system for coarse pointing of the entire spacecraft, supplemented with a fine-guidance system using a fast steering mirror to center the source on the spectrograph slit and to cancel out satellite jitter. An extremely compact optical design allows us to house this instrument in a 6U CubeSat with a volume of only 10 × 20 × 30 cm3 , while preserving a maximized entrance pupil of ca. 9 × 19 cm2 . In this contribution, we give an overview of the CUBESPEC project, discuss its most relevant science cases, and present the design of the instrument.

The ESO Diffuse Interstellar Band Large Exploration Survey (EDIBLES)

The ESO Diffuse Interstellar Band Large Exploration Survey (EDIBLES) is a Large Programme that is collecting high-signal-to-noise (S/N) spectra with UVES of a large sample of O and B-type stars covering a large spectral range. The goal of the programme is to extract a unique sample of high-quality interstellar spectra from these data, representing different physical and chemical environments, and to characterise these environments in great detail. An important component of interstellar spectra is the diffuse interstellar bands (DIBs), a set of hundreds of unidentified interstellar absorption lines. With the detailed line-of- sight information and the high-quality spectra, EDIBLES will derive strong constraints on the potential DIB carrier molecules. EDIBLES will thus guide the laboratory experiments necessary to identify these interstellar “mystery molecules”, and turn DIBs into powerful diagnostics of their environments in our Milky Way Galaxy and beyond. We present some preliminary results showing the unique capabilities of the EDIBLES programme.

The Metallicity Dependence of the Mass Loss of Early-Type Massive Stars

We report on a comprehensive study of the wind properties of 115 Oand early B-type stars in the Galaxy and the Large Magellanic Clouds. This work is part of the VLT/FLAMES Survey of Massive Stars. The data is used to construct the empirical dependence of the massloss in stellar winds on the metal content of their atmospheres. The metal content of early-type stars in the Magellanic Clouds is discussed. Assuming a power-law dependence of mass loss on metal content, Ṁ ∝ Z , we find m = 0.83 ± 0.16 from an analysis of the wind momentum luminosity relation (Mokiem et al. 2007b). This result is in good agreement with the prediction m = 0.69 ± 0.10 by Vink et al. (2001). Though the scaling agrees, the absolute empirical value of mass loss is found to be a factor of two higher than predictions. This may be explained by a modest amount of clumping in the outflows of the objects studied.

Mass-loss predictions and stellar masses of early-type stars

We show that the stellar masses implied by our predictions of the wind properties of massive stars are in agreement with masses derived from evolution theory and from direct measurements using spectroscopic binaries, contrary to previous attempts to derive masses from wind theory.

The winds of Luminous Blue Variables and the mass of AG Carinae

We present radiation-driven wind models for Luminous Blue Variables (LBVs) and predict their mass-loss rates. A comparison between our predictions and the observations of AG Car shows that the variable mass loss behaviour of LBVs is due the recombination/ionisation of Fe IV/III and Fe III/II. We also derive a present-day mass of 35 Msun for AG Car.