85 research outputs found
GR 290 (Romano's Star): 2. Light history and evolutionary state
We have built the historical light curve of the luminous variable GR 290 back
to 1901, from old observations of the star found in several archival plates of
M 33. These old recordings together with published and new data show that for
at least half a century the star was in a low luminosity state, with B ~18.
After 1960, five large variability cycles of visual luminosity were recorded.
The amplitude of the oscillations was seen increasing towards the 1992-1994
maximum, then decreasing during the last maxima. The recent light curve
indicates that the photometric variations have been quite similar in all the
bands, and that the B-V color index has been constant within +/-0.1 m despite
the 1.5m change of the visual luminosity. The spectrum of GR 290 at the large
maximum of 1992-94, was equivalent to late-B type, while, during 2002-2014, it
has varied between WN10h-11h near the visual maxima to WN8h-9h at the
luminosity minima. We have detected, during this same period, a clear
anti-correlation between the visual luminosity, the strength of the HeII 4686 A
emission line, the strength of the 4600-4700 A lines blend and the spectral
type. From a model analysis of the spectra collected during the whole 2002-2014
period we find that the Rosseland radius R_{2/3}, changed between the minimum
and maximum luminosity phases by a factor of 3, while T_eff varied between
about 33,000 K and 23,000 K. The bolometric luminosity of the star was not
constant, but increased by a factor of ~1.5 between minimum and maximum
luminosity, in phase with the apparent luminosity variations. In the light of
current evolutionary models of very massive stars, we find that GR 290 has
evolved from a ~60 M_Sun progenitor star and should have an age of about 4
million years. We argue that it has left the LBV stage and is moving to a
Wolf-Rayet stage of late nitrogen spectral type.Comment: Accepted on The Astronomical Journal, 10 figures. Replaced because
the previous uploaded file was that without the final small corrections
requested by the refere
An HST/WFPC2 survey of bright young clusters in M31. I. VdB0, a massive star cluster seen at t â 25 Myr
Aims. We introduce our imaging survey of possible young massive globular clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). We obtained shallow (to B ~ 25) photometry of individual stars in 20 candidate clusters. We present here details of the data reduction pipeline that is being applied to all the survey data and describe its application to the brightest among our targets, van den Bergh 0 (VdB0), taken as a test case.
Methods. Point spread function fitting photometry of individual stars was obtained for all the WFPC2 images of VdB0 and the completeness of the final samples was estimated using an extensive set of artificial stars experiments. The reddening, the age and the metallicity of the cluster were estimated by comparing the observed color magnitude diagram (CMD) with theoretical isochrones. Structural parameters were obtained from model-fitting to the intensity profiles measured within circular apertures on the WFPC2 images.
Results. Under the most conservative assumptions, the stellar mass of VdB0 is M> 2.4 x 10^4 M_â , but our best estimates lie in the range â4-9 x 10^4 M_â. The CMD of VdB0 is best reproduced by models having solar metallicity and age â25 Myr. Ages less than â12 Myr and greater than â60 Myr are clearly ruled out by the available data. The cluster has a remarkable number of red super giants (âł18) and a CMD very similar to Large Magellanic Cloud clusters usually classified as young globulars such as NGC 1850, for example.
Conclusions. VdB0 is significantly brighter (âł1 mag) than Galactic open clusters of similar age. Its present-day mass and half-light radius ((r_h = 7.4 pc) are more typical of faint globular clusters than of open clusters. However, given its position within the disk of M31, it is expected to be destroyed by dynamical effects, in particular by encounters with giant molecular clouds, within the next ~4 Gyr
The Gaia spectrophotometric standard stars survey -II. Instrumental effects of six ground-based observing campaigns
The Gaia SpectroPhotometric Standard Stars (SPSS) survey started in 2006, it
was awarded almost 450 observing nights, and accumulated almost 100,000 raw
data frames, with both photometric and spectroscopic observations. Such large
observational effort requires careful, homogeneous, and automated data
reduction and quality control procedures. In this paper, we quantitatively
evaluate instrumental effects that might have a significant (i.e.,1%)
impact on the Gaia SPSS flux calibration. The measurements involve six
different instruments, monitored over the eight years of observations dedicated
to the Gaia flux standards campaigns: DOLORES@TNG in La Palma, EFOSC2@NTT and
ROSS@REM in La Silla, [email protected] in Calar Alto, BFOSC@Cassini in Loiano, and
[email protected] in San Pedro Martir. We examine and quantitatively evaluate the
following effects: CCD linearity and shutter times, calibration frames
stability, lamp flexures, second order contamination, light polarization, and
fringing. We present methods to correct for the relevant effects, which can be
applied to a wide range of observational projects at similar instruments.Comment: 16 pages, 13 figures, accepted for publication in Astron. Nach
Age as the Second Parameter in NGC 288 / NGC 362? I. Turnoff Ages: a Purely Differential Comparison
We present deep V,I photometry of the globular clusters NGC 288, NGC 362 and
NGC 1851 obtained during a single observational run under strictly homogeneous
conditions. We use the bimodal horizontal branch (HB) of NGC 1851 as a
``bridge'' to obtain the optimum relative match between the HBs of NGC 288 and
NGC 362. In this way we can effectively remove the uncertainties associated
with distance, reddening and inhomogeneities in the absolute calibration, thus
obtaining a very robust, purely differential estimate of the age difference
between these two clusters. According to the bridge test, NGC 288 is found to
be older than NGC 362 by 2 +- 1 Gyr. This result is fully confirmed also by all
classical differential age diagnostics, either based on the luminosity or color
of the main sequence turnoff point.Comment: Accepted by The Astronomical Journal - 30 pages, 6 tables, 14
figures, LaTeX, the emulateapj5.sty macro is used. Fig. 2 and Fig. 3 are
provided in reduced resolution. Full resolution versions available upon
request to the first author ([email protected]). Minor changes due to
modifications in the companion paper (Pap II
The first two transient supersoft X-ray sources in M 31 globular clusters and the connection to classical novae
Classical novae (CNe) have been found to represent the major class of
supersoft X-ray sources (SSS) in our neighbour galaxy M 31. We determine
properties and evolution of the two first SSSs ever discovered in the M 31
globular cluster (GC) system. We have used XMM-Newton, Chandra and Swift
observations of the centre region of M 31 to discover both SSS and to determine
their X-ray light curves and spectra. We performed detailed analysis of
XMM-Newton EPIC PN spectra of the source in Bol 111 (SS1) using blackbody and
NLTE white dwarf (WD) atmosphere models. For the SSS in Bol 194 (SS2) we used
optical monitoring data to search for an optical counterpart. Both GC X-ray
sources were classified as SSS. We identify SS1 with the CN M31N 2007-06b
recently discovered in the M 31 GC Bol 111. For SS2 we did not find evidence
for a recent nova outburst and can only provide useful constraints on the time
of the outburst of a hypothetical nova. The only known CN in a M 31 GC can be
identified with the first SSS found in a M31 GC. We discuss the impact of our
observations on the nova rate for the M 31 GC system.Comment: 11 pages, 9 figures, 4 tables, submitted to A&A; v2: accepted versio
Gaia data release 1: Principles of the photometric calibration of the G band
Context. Gaia is an ESA cornerstone mission launched on 19 December 2013
aiming to obtain the most complete and precise 3D map of our Galaxy by
observing more than one billion sources. This paper is part of a series of
documents explaining the data processing and its results for Gaia Data Release
1, focussing on the G band photometry. Aims. This paper describes the
calibration model of the Gaia photometric passband for Gaia Data Release 1.
Methods. The overall principle of splitting the process into internal and
external calibrations is outlined. In the internal calibration, a
self-consistent photometric system is generated. Then, the external calibration
provides the link to the absolute photometric flux scales. Results. The Gaia
photometric calibration pipeline explained here was applied to the first data
release with good results. Details are given of the various calibration
elements including the mathematical formulation of the models used and of the
extraction and preparation of the required input parameters (e.g. colour
terms). The external calibration in this first release provides the absolute
zero point and photometric transformations from the Gaia G passband to other
common photometric systems. Conclusions. This paper describes the photometric
calibration implemented for the first Gaia data release and the instrumental
effects taken into account. For this first release no aperture losses,
radiation damage, and other second-order effects have not yet been implemented
in the calibration.Comment: 15 pages, 16 figures, Gaia data release 1 documentation special
volum
Time variability of X-ray sources in the M 31 centre field
We present an extension to our XMM-Newton X-ray source catalogue of M 31,
containing 39 newly found sources. In order to classify and identify more of
the sources we search for X-ray time variability in XMM-Newton archival data of
the M 31 centre field.
As a source list we used our extended catalogue based on observations
covering the time span from June 2000 to July 2004. We then determined the flux
or at least an upper limit at the source positions for each observation.
Deriving the flux ratios for the different observations and searching for the
maximum flux difference we determined variability factors. We also calculated
the significance of the flux ratios.
Using hardness ratios, X-ray variability and cross correlations with
catalogues in the X-ray, optical, infrared and radio regimes, we detected three
super soft source candidates, one supernova remnant and six supernova remnant
candidates, one globular cluster candidate, three X-ray binaries and four X-ray
binary candidates. Additionally we identified one foreground star candidate and
classified fifteen sources with hard spectra, which may either be X-ray
binaries or Crab-like supernova remnants in M 31 or background active galactic
nuclei. The remaining five sources stay unidentified or without classification.
Based on the time variability results we suggest six sources, which were
formerly classified as "hard", to be X-ray binary candidates. The
classification of one other source (XMMM31 J004236.7+411349) as a supernova
remnant, has to be rejected due to the distinct time variability we found. We
now classify this source as a foreground star.Comment: 13 pages, 6 figures, accepted for publication in A&
The deep XMM-Newton Survey of M 31
Aims. M 31 has been completely imaged for the first time obtaining a
luminosity lower limit ~10^35 erg s-1 in the 0.2-4.5 keV band. Our XMM-Newton
EPIC survey combines archival observations with observations taken between June
2006 and February 2008. The main goal of the paper is a study of the X-ray
source population of M 31. Methods. An X-ray catalogue of 1897 sources was
created; 914 were detected for the first time. Source classification and
identification were based on X-ray hardness ratios, spatial extent of the
sources, and by cross correlating with catalogues in the X-ray, optical,
infrared and radio wavelengths. We also analysed the long-term variability of
the X-ray sources. Results. Fourteen of the 30 supersoft source candidates
represent supersoft emission of optical novae. Many of the 25 supernova
remnants (SNRs) and 31 SNR candidates lie within the 10 kpc dust ring and other
star forming regions in M 31; this connection between SNRs and star forming
regions implies that most of the remnants originate in type II supernovae. The
brightest sources in X-rays in M 31 belong to the class of X-ray binaries
(XRBs). Ten low mass XRBs (LMXBs) and 26 LMXB candidates were identified based
on their temporal variability. In addition 36 LMXBs and 17 LMXB candidates were
identified due to correlations with globular clusters and globular cluster
candidates. From optical and X-ray colour-colour diagrams, possible high mass
XRB (HMXB) candidates were selected. Two of these candidates have an X-ray
spectrum as is expected for an HMXB containing a neutron star primary.
Conclusions. While our survey has greatly improved our understanding of the
X-ray source populations in M 31, at this point 65% of the sources can still
only be classified as "hard" sources. Deeper observations in X-ray and at other
wavelengths would help classify these sources. ABRIDGEDComment: 51 pages, 36 figures, submitted to A&
A HST/WFPC2 survey of bright young clusters in M31. I. VdB0, a massive star cluster seen at ~= 25 Myr
{Aims.} We introduce our imaging survey of possible young massive globular
clusters in M31 performed with the Wide Field and Planetary Camera 2 (WFPC2) on
the Hubble Space Telescope (HST). We present here details of the data reduction
pipeline that is being applied to all the survey data and describe its
application to the brightest among our targets, van den Bergh 0 (VdB0), taken
as a test case. {Methods.} The reddening, the age and the metallicity of the
cluster are estimated by comparison of the observed Color Magnitude Diagram
(CMD) with theoretical isochrones. {Results.} Under the most conservative
assumptions the stellar mass of VdB0 is M > 2.4 x 10^4 M_sun, but our best
estimates lie in the range ~ 4-9 x 10^4 M_sun. The CMD of VdB0 is best
reproduced by models having solar metallicity and age = 25 Myr. Ages smaller
than = 12 Myr and larger than = 60 Myr are clearly ruled out by the available
data. The cluster has a remarkable number of Red Super Giants (> 18) and a CMD
very similar to Large Magellanic Cloud clusters usually classified as young
globulars such as NGC 1850, for example. {Conclusions.} VdB0 is significantly
brighter (>~ 1 mag) than Galactic open clusters of similar age. Its present-day
mass and half-light radius (r_h=7.4 pc) are more typical of faint globular
clusters than of open clusters. However, given its position within the disk of
M31 it is expected to be destroyed by dynamical effects, in particular by
encounters with giant molecular clouds, within the next ~ 4 Gyr.Comment: 18 pages, 16 figures (quality of Figures 1,2,3 and 4 reduced).
Accepted for publication in Astronomy & Astrophysics. Minor revisions to
sect. 1.
Comparison of different delivery systems of DNA vaccination for the induction of protection against tuberculosis in mice and guinea pigs
The great challenges for researchers working in the field of vaccinology are optimizing DNA vaccines for use in humans or large animals and creating effective single-dose vaccines using appropriated controlled delivery systems. Plasmid DNA encoding the heat-shock protein 65 (hsp65) (DNAhsp65) has been shown to induce protective and therapeutic immune responses in a murine model of tuberculosis (TB). Despite the success of naked DNAhsp65-based vaccine to protect mice against TB, it requires multiple doses of high amounts of DNA for effective immunization. In order to optimize this DNA vaccine and simplify the vaccination schedule, we coencapsulated DNAhsp65 and the adjuvant trehalose dimycolate (TDM) into biodegradable poly (DL-lactide-co-glycolide) (PLGA) microspheres for a single dose administration. Moreover, a single-shot prime-boost vaccine formulation based on a mixture of two different PLGA microspheres, presenting faster and slower release of, respectively, DNAhsp65 and the recombinant hsp65 protein was also developed. These formulations were tested in mice as well as in guinea pigs by comparison with the efficacy and toxicity induced by the naked DNA preparation or BCG. The single-shot prime-boost formulation clearly presented good efficacy and diminished lung pathology in both mice and guinea pigs
- âŠ