Massive Stars: From the VLT to the ELT
_wlr_comp.ps
To appear in “Massive Stars: Fundamental Parameters and Circumstellar Interactions (2007)”RevMexAA(SC)
MASSIVE STARS: FROM THE VLT TO THE ELT
Chris Evans 1
RESUMEN
Nuestro conocimiento de las estrellas masivas ha aumentado significativamente en los últimos 30 años gracias a
las nuevas instalaciones y tecnoloǵıas. En esta contribución presento un gran survey de estrellas masivas que se
ha realizado recientemente mediante el uso de VLT-FLAMES, mostrando los campos observados y remarcando
la fracción de estrellas binarias que se ha encontrado. Estos datos se han utilizado para la primera comprobación
empirica de la dependencia de la intensidad de los vientos estelares con la metalicidad, encontrandose un buen
acuerdo con la teora – un resultado de gran importancia para los modelos de evolución estelar, que se utilizan
para la interpretaión de cúmulos lejanos, starburst y galaxias con formación estelar. Dando un paso más,
comentaré como en la actualidad se están dedicando grandes esfuerzos al avance de los planes de actuacin y
desarrollo de los Telescopios de Gran Tamaño, que serán una realidad en un futuro próximo; este hecho nos
ofrecerá una posibilidad más que interesante para obtener observaciones con resolución espacial de estrellas
masivas más allá del Grupo Local.
ABSTRACT
New facilities and technologies have advanced our understanding of massive stars significantly over the past 30
years. Here I introduce a new large survey of massive stars using VLT-FLAMES, noting the target fields and
observed binary fractions. These data have been used for the first empirical test of the metallicity dependence
of the intensity of stellar winds, finding good agreement with theory – an important result for the evolutionary
models that are used to interpret distant clusters, starbursts, and star-forming galaxies. Looking ahead,
plans for future Extremely Large Telescopes (ELTs) are now undergoing significant development, and offer the
exciting prospect of observing spatially-resolved massive stars well beyond the Local Group.
Key Words: GALAXIES: MAGELLANIC CLOUDS — INSTRUMENTATION — STARS: EARLY TYPE
1. INTRODUCTION
An excellent example of the progress enabled by
new observing capabilities is afforded by our knowl-
edge of the stellar content of NGC346 – the largest
H II region in the Small Magellanic Cloud (SMC).
The first photometric study of NGC 346 was pub-
lished by Niemela et al. (1986) using data from
the 1-m Yale telescope at the Cerro Tololo Inter-
American Observatory (CTIO). This paper also pre-
sented spectroscopy of some of the brighter mem-
bers, building on the pioneering work of Walborn
(1978). In the following years, 4-m class telescopes
were used to investigate the cluster initial mass
function (Massey et al., 1989), and to obtain high-
resolution echelle spectra for detailed atmospheric
analysis (Kudritzki et al. 1989; Walborn et al. 2000;
Bouret et al. 2003). More recently, the images of the
cluster from the Hubble Space Telescope Advanced
Camera for Surveys (e.g. Sabbi et al. 2007) offer a
dramatic illustration of the current ‘state-of-the-art’.
Understanding the role of environment and
1UK Astronomy Technology Centre, Royal Observatory,
Blackford Hill, Edinburgh, EH9 3HJ, UK (cje@roe.ac.uk).
metallicity on the evolution of massive stars has been
a key topic for the past two decades. However, un-
til now, we have lacked the facilities to obtain large,
homogenous sets of observations (in a sensible allo-
cation of telescope time) to provide robust empirical
constraints to evolutionary models. The delivery of
the Fibre Large Array Multi-Element Spectrograph
(FLAMES) to the Very Large Telescope (VLT) was
the catalyst for such a survey – to address questions
such as the metallicity dependence of stellar rota-
tional velocities and wind mass-loss rates.
2. THE VLT-FLAMES SURVEY
The FLAMES survey of massive stars has ob-
served over 800 targets in 7 fields, centered on stel-
lar clusters in the Galaxy and SMC/LMC, as listed
in Table 1; the inclusion of NGC 346 was an obvi-
ous choice. The full content of the survey has been
presented by Evans et al. (2005, 2006).
Our observations provide lower limits to the bi-
nary fraction of the O- and early B-type targets,
finding ∼25% in NGC346 and ∼35% in N11. Al-
though the programme was not originally concerned
with binary detection, the FLAMES spectra are of
http://arxiv.org/abs/0704.1119v1
2 CHRIS EVANS
TABLE 1
SUMMARY OF VLT-FLAMES FIELDS
‘Young’ clusters ‘Old’ clusters
(<5 Myrs) (10-20 Myr)
Milky Way NGC 6611 NGC 3293 & 4755
LMC N11 NGC 2004
SMC NGC 346 NGC 330
sufficient quality that the nature and properties of
many of the newly discovered systems can be deter-
mined. Specific systems will be the subject of future
papers, and we are seeking further monitoring of our
fields to better constrain the binary fraction.
The first science papers from the survey have
now been published, presenting analyses of the O-
and early B-type spectra (Mokiem et al. 2006, 2007;
Hunter et al. 2007). One of our primary motivations
for the FLAMES survey was to test the theoretical
prediction that wind mass-loss rates are dependent
on metallicity (Kudritzki et al. 1987; Vink et al.
2001). Figure 1 shows the wind momentum – lumi-
nosity relations (WLR) obtained from our LMC and
SMC targets, compared with those from contempo-
rary Galactic results. This is the first comprehensive
empirical test of the metallicity dependence. For lu-
minosities greater than ∼105.2 L⊙, the relative off-
sets between different metallicity regimes are in good
agreement with theory.
3. ELT SCIENCE CASE
With instruments such as the FOcal-Reducer
low-dispersion Spectrograph (FORS) on the VLT,
detailed analysis of stars in galaxies at 1-2 Mpc is
currently possible (e.g. Urbaneja et al., 2003; Evans
et al., 2007). But ideally we want to reach out to
more distant systems (thereby sampling different en-
vironments – elliptical galaxies, lower metallicities
etc.), or we need spectral resolutions that are greater
than those available from FORS.
Moving into 2007, the European Southern Ob-
servatory (ESO) is starting a full phase A design of
its Extremely Large Telescope (ELT). The current
baseline reference design is a 42-m primary, with a
novel 5-mirror solution that provides correction for
ground-layer atmospheric turbulence as its minimum
operating mode. An ELT will allow us to observe the
resolved massive-star populations in a wide range of
systems beyond the Local Group, exploring chemical
abundances, stellar kinematics and so on. An ELT
will also be able to probe distant clusters and star-
4.5 5.0 5.5 6.0
log(L/Lsun)
Fig. 1. The observed wind momentum – luminosity rela-
tions (solid lines) compared with theoretical predictions
(Vink et al., 2001; dotted lines). The top, middle and
bottom lines correspond, respectively, to Galactic, LMC
and SMC results (Mokiem et al., submitted).
bursts at new levels of detail to study their initial
mass functions and kinematic structure.
There will be many exciting new opportunities in
the ELT era, but it is worth noting for our commu-
nity that the most significant observational advances
will almost certainly arise in the near-infrared (be-
cause of the demands of adaptive optics). To fully
exploit an ELT, a large effort will be required to de-
velop further diagnostic tools in this wavelength re-
gion – both in terms of atomic data (e.g. Przybilla,
2005), and comparison studies in the Milky Way and
Magellanic Clouds.
REFERENCES
Bouret, J.-C., et al. 2003, ApJ, 595, 1182
Evans, C. J., et al. 2005, A&A, 437, 467
Evans, C. J., et al. 2006, A&A, 456, 623
Evans, C. J., et al. 2007, ApJ, astro-ph/0612114
Hunter, I., et al. 2007, A&A, astro-ph/0609710
Kudritzki, R.-P., Pauldrach, A. W. A. & Puls, J. 1987,
A&A, 173, 293
Kudritzki, R.-P., et al. 1989, A&A, 226, 235
Massey, P., Parker, J. W. & Garmany, C. D., 1989, AJ,
98, 1305
Mokiem, R. M., et al. 2006, A&A, 456, 1131
Mokiem, R. M., et al. 2007, A&A, accepted
Niemela, V. S., Marraco, H. G., & Cabanne, M. L. 1986,
PASP, 98, 1133
Przybilla, N. 2005, A&A, 443, 293
Sabbi, E., et al. 2007, AJ, 133, 44
Urbaneja, M. A. et al. 2003, ApJ, 584, 73
Vink, J. S., de Koter, A. & Lamers, H. J. G. L. M. 2001,
A&A, 369, 574
Walborn, N. R. 1978, ApJ, 224, L133
Walborn, N. R. et al. 2000, PASP, 112, 1243
http://arxiv.org/abs/astro-ph/0612114
http://arxiv.org/abs/astro-ph/0609710
