92 research outputs found
The central density of R136 in 30 Doradus
The central density rho_0 of a stellar cluster is an important physical
parameter for determining its evolutionary and dynamical state. How much mass
segregation there is, or whether the cluster has undergone core collapse both
depends on rho_0. We reanalyze the results of a previous paper that gives the
mass density profile of R136 and combine them with both a conservative upper
limit for the core parameter and a more uncertain recent measurement. We thus
place a lower limit on rho_0 under reasonable and defensible assumptions about
the IMF, finding rho_0 >~ 1.5x10^4 Msun/pc^3 for the conservative assumption a
< 0.4 pc for the cluster core parameter. If we use the lower, but more
uncertain value a = 0.025 pc, the central density estimate becomes greater than
10^7 Msun/pc^3. A mechanism based on the destruction of a large number of
circumstellar disks is posited to explain the hitherto unexplained increase in
reddening presented in that same work.Comment: 6 pages, 2 figure
Unveiling the 3D structure of nova shells with MUSE -- The case of RR Pic
Nova eruptions occur in cataclysmic variables when enough material has been
accreted onto the surface of the white dwarf primary. As a consequence, the
material that has been accumulated until then is expelled into the interstellar
medium, forming an expanding nova shell around the system. Understanding the
physical process that shapes the morphology of nova shells is essential to
fully comprehend how the ejection mechanism operates during nova eruptions.
Because of its closeness and age, the nova shell around the classical nova RR
Pic (Nova Pic 1925) is an ideal target for studying the evolving morphology of
nova shells. In this work, we present an IFS study of the RR Pic nova shell,
with a particular emphasis on the extraction of the 3D morphology of the shell.
The nova shell was observed by the Multi-Unit Spectroscopic Explorer (MUSE)
instrument placed at the ESO-VLT. The MUSE datacube confirms the presence of
the nova shell in H, H and [OIII], and very faintly in
[NII]. A comparison with previous observations suggests that the shell
continues in its free-expansion phase but with the different parts of the shell
apparently expanding at different rates. The data analysis corroborates the
previous vision that the shell is composed of an equatorial ring and polar
filaments. At the same time, the new data also reveal that [OIII] is confined
in gaps located in the tropical regions of the shell where no Hydrogen is
observed. The flux measurements indicate that ~99% of the shell flux is
confined to the equatorial ring, while the polar filaments show a flux
asymmetry between the NE and SW filaments. We have estimated the mass of the
shell to be ~5x10M. From the analysis of the 3D-extracted data,
we determine that the ring structure extends ~8,000 au from the central binary,
and has a position angle of ~155 deg and an inclination of ~74 deg.Comment: 18 pages, 12 figures. Accepted for publication in A&A. Waiting for
production offic
Revisiting the Impact of Atmospheric Dispersion and Differential Refraction on Widefield Multiobject Spectroscopic Observations. From VLT/VIMOS to Next Generation Instruments
(Abridged) Atmospheric dispersion and field differential refraction impose
severe constraints on widefield MOS observations. Flux reduction and spectral
distortions must be minimised by a careful planning of the observations --
which is especially true for instruments that use slits instead of fibres. This
is the case of VIMOS at the VLT, where MOS observations have been restricted,
since the start of operations, to a narrow two-hour range from the meridian to
minimise slit losses. We revisit in detail the impact of atmospheric effects on
the quality of VIMOS-MOS spectra. We model slit losses across the entire VIMOS
FOV as a function of target declination. We explore two different slit
orientations at the meridian: along the parallactic angle (North-South), and
perpendicular to it (East-West). We show that, for fields culminating at zenith
distances larger than 20 deg, slit losses are minimised with slits oriented
along the parallactic angle at the meridian. The two-hour angle rule holds for
these observations using N-S orientations. Conversely, for fields with zenith
angles smaller than 20 deg at culmination, losses are minimised with slits
oriented perpendicular to the parallactic angle at the meridian. MOS
observations can be effectively extended to plus/minus three hours from the
meridian in these cases. In general, night-long observations of a single field
will benefit from using the E-W orientation. All-sky or service mode
observations, however, require a more elaborate planning that depends on the
target declination, and the hour angle of the observations. We establish
general rules for the alignment of slits in MOS observations that will increase
target observability, enhance the efficiency of operations, and speed up the
completion of programmes -- a particularly relevant aspect for the forthcoming
spectroscopic public surveys with VIMOS.Comment: Accepted to A&A. 11 pages, 15 figures. This paper presents the new
recommendations for optimal slit alignment in VLT/VIMOS observation
Towards DIB mapping in galaxies beyond 100 Mpc. A radial profile of the 5780.5 diffuse interstellar band in AM 1353-272 B
Diffuse Interstellar Bands (DIBs) are non-stellar weak absorption features of
unknown origin found in the spectra of stars viewed through one or several
clouds of Interstellar Medium (ISM). Research of DIBs outside the Milky Way is
currently very limited. Specifically spatially resolved investigations of DIBs
outside of the Local Group is, to our knowledge, inexistent. Here, we explore
the capability of the high sensitivity Integral Field Spectrograph, MUSE, as a
tool to map diffuse interstellar bands at distances larger than 100 Mpc. We use
MUSE commissioning data for AM 1353-272 B, the member with highest extinction
of the "The Dentist's Chair", an interacting system of two spiral galaxies.
High signal-to-noise spectra were created by co-adding the signal of many
spatial elements distributed in a geometry of concentric elliptical half-rings.
We derived decreasing radial profiles for the equivalent width of the
5780.5 DIB both in the receding and approaching side of the companion
galaxy up to distances of 4.6 kpc from the center of the galaxy.
Likewise, interstellar extinction, as derived from the Halpha/Hbeta line ratio
displays a similar trend, with decreasing values towards the external parts.
This translates into an intrinsic correlation between the strength of the DIB
and the extinction within AM 1353-272 B consistent with the current existing
global trend between these quantities when using measurements for both Galactic
and extragalactic sight lines. Mapping of DIB strength in the Local Universe as
up to now only done for the Milky Way seems feasible. This offers a new
approach to study the relationship between DIBs and other characteristics and
species of the ISM in different conditions as those found in our Galaxy to the
use of galaxies in the Local Group and/or single sightlines towards supernovae,
quasars and galaxies outside the Local Group.Comment: 4 pages, 4 figures, accepted for publication as a Letter in Astronomy
and Astrophysics; Received 10 February 2015 / Accepted 20 February 2015 ;
English corrections include
X-shooter, NACO, and AMBER observations of the LBV Pistol Star \footnote{Based on ESO runs 85.D-0182A, 085.D-0625AC}
We present multi-instruments and multi-wavelengths observations of the famous
LBV star Pistol Star. These observations are part of a larger program about
early O stars at different metallicities. The Pistol star has been claimed as
the most massive star known, with 250 solar masses. We present the preliminary
results based on X-Shooter spectra, as well as the observations performed with
the VLTI-AMBER and the VLT-NACO adaptive optics. The X-shooter spectrograph
allows to obtain simultaneously a spectrum from the UV to the K-band with a
resolving power of 15000. The preliminary results obtained indicate that
Pistol Star has similar properties of Eta Car, including shells of matter, but
also the binarity. Other objects of the program, here briefly presented, were
selected for their particular nature: early O stars with mass discrepancies
between stellar evolution models and observations, discrepancies with the wind
momentum luminosity relation.Comment: Poster at the 39th LIAC, submitted version of the proceeding
The scale-free character of the cluster mass function and the universality of the stellar IMF
Our recent determination of a Salpeter slope for the IMF in the field of 30
Doradus (Selman and Melnick 2005) appears to be in conflict with simple
probabilistic counting arguments advanced in the past to support observational
claims of a steeper IMF in the LMC field. In this paper we re-examine these
arguments and show by explicit construction that, contrary to these claims, the
field IMF is expected to be exactly the same as the stellar IMF of the clusters
out of which the field was presumably formed. We show that the current data on
the mass distribution of clusters themselves is in excellent agreement with our
model, and is consistent with a single spectrum {\it by number of stars} of the
type with beta between -1.8 and -2.2 down to the smallest clusters
without any preferred mass scale for cluster formation. We also use the random
sampling model to estimate the statistics of the maximal mass star in clusters,
and confirm the discrepancy with observations found by Weidner and Kroupa
(2006). We argue that rather than signaling the violation of the random
sampling model these observations reflect the gravitationally unstable nature
of systems with one very large mass star. We stress the importance of the
random sampling model as a \emph{null hypothesis} whose violation would signal
the presence of interesting physics.Comment: 9 pages emulateap
A MUSE map of the central Orion Nebula (M 42)
We present a new integral-field spectroscopic dataset of the central part of
the Orion Nebula (M 42), observed with the MUSE instrument at the ESO VLT. We
reduced the data with the public MUSE pipeline. The output products are two
FITS cubes with a spatial size of ~5.9'x4.9' (corresponding to ~0.76 pc x 0.63
pc) and a contiguous wavelength coverage of 4595...9366 Angstrom, spatially
sampled at 0.2". We provide two versions with a sampling of 1.25 Angstrom and
0.85 Angstrom in dispersion direction. Together with variance cubes these files
have a size of 75 and 110 GiB on disk. They represent one of the largest
integral field mosaics to date in terms of information content. We make them
available for use in the community. To validate this dataset, we compare world
coordinates, reconstructed magnitudes, velocities, and absolute and relative
emission line fluxes to the literature and find excellent agreement. We derive
a two-dimensional map of extinction and present de-reddened flux maps of
several individual emission lines and of diagnostic line ratios. We estimate
physical properties of the Orion Nebula, using the emission line ratios [N II]
and [S III] (for the electron temperature ) and [S II] and [Cl III] (for
the electron density ), and show two-dimensional images of the velocity
measured from several bright emission lines.Comment: Resubmitted to A&A after incorporating referee comments; access to
full dataset via http://muse-vlt.eu/science/data-release
MUSE library of stellar spectra
Context. Empirical stellar spectral libraries have applications in both extragalactic and stellar studies, and they confer an advantage over theoretical libraries because they naturally include all relevant chemical species and physical processes. In recent years we have seen a stream of new sets of high-quality spectra, but increasing the spectral resolution and widening the wavelength coverage means resorting to multi-order echelle spectrographs. Assembling the spectra from many pieces results in lower fidelity of their shapes. Aims: We aim to offer the community a library of high-signal-to-noise spectra with reliable continuum shapes. Furthermore, the use of an integral field unit (IFU) alleviates the issue of slit losses. Methods: Our library was built with the MUSE (Multi-Unit Spectroscopic Explorer) IFU instrument. We obtained spectra over nearly the entire visual band (lambda ~ 4800-9300 AA). Results: We assembled a library of 35 high-quality MUSE spectra for a subset of the stars from the X-shooter Spectral Library. We verified the continuum shape of these spectra with synthetic broadband colors derived from the spectra. We also report some spectral indices from the Lick system, derived from the new observations. Conclusions: We offer a high-fidelity set of stellar spectra covering the Hertzsprung-Russell diagram. These can be used for both extragalactic and stellar studies and demonstrate that the IFUs are excellent tools for building reliable spectral libraries
High-angular resolution observations of the Pistol Star
First results of near-IR adaptive optics (AO)-assisted imaging,
interferometry, and spectroscopy of this Luminous Blue Variable (LBV) are
presented. They suggest that the Pistol Star is at least double. If the
association is physical, it would reinforce questions concerning the importance
of multiplicity for the formation and evolution of extremely massive stars.Comment: poster at IAUS27
Luminous blue variables: An imaging perspective on their binarity and near environment
Luminous blue variables (LBVs) are rare massive stars with very high
luminosity. They are characterized by strong photo-metric and spectroscopic
variability related to transient eruptions. The mechanisms at the origin of
these eruptions is not well known. In addition, their formation is still
problematic and the presence of a companion could help to explain how they
form. Aims. This article presents a study of seven LBVs (about 20% of the known
Galactic population), some Wolf-Rayet stars, and massive binaries. We probe the
environments that surround these massive stars with near-, mid-, and
far-infrared images, investigating potential nebula/shells and the companion
stars. Methods. To investigate large spatial scales, we used seeing-limited and
near diffraction-limited adaptive optics images to obtain a differential
diagnostic on the presence of circumstellar matter and to determine their
extent. From those images, we also looked for the presence of binary companions
on a wide orbit. Once a companion was detected, its gravitational binding to
the central star was tested. Tests include the chance projection probability,
the proper motion estimates with multi-epoch observations, flux ratio, and star
separations. Results. We find that two out of seven of LBVs may have a wide
orbit companion. Most of the LBVs display a large circumstellar envelope or
several shells. In particular, HD168625, known for its rings, possesses several
shells with possibly a large cold shell at the edge of which the rings are
formed. For the first time, we have directly imaged the companion of LBV stars
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