208 research outputs found
XMM-Newton imaging of V1818 Ori: a young stellar group on the eastern edge of the Kappa Ori ring
We present the results of a 40 ks XMM-Newton observation centered on the
variable star V1818 Ori. Using a combination of the XMM-Newton and AllWISE
catalog data, we identify a group of about 31 young stellar objects around
V1818 Ori. This group is coincident with the eastern edge of the dust ring
surrounding Kappa Ori. Previously, we concluded that the young stellar objects
on the western side of ring were formed in an episode of star formation that
started 3-5 Myr ago, and are at a distance similar to that of kappa Ori
(250-280 pc) and in the foreground to the Orion A cloud. Here we use the
XMM-Newton observation to calculate X-ray fluxes and luminosities of the young
stars around V1818 Ori. We find that their X-ray luminosity function (XLF),
calculated for a distance of ~270 pc, matches the XLF of the YSOs west of Kappa
Ori. We rule out that this group of young stars is associated to Mon R2 as
assumed in the literature, but rather they are part of the same Kappa Ori's
ring stellar population.Comment: 4 pages, 4 figures, 1 table. Accepted for publication to A&
The stellar population of Sco OB2 revealed by Gaia DR2 data
Sco OB2 is the nearest OB association, extending over approximately 2000
sq.deg. on the sky. Only its brightest members are already known (from
Hipparcos) across its entire size, while studies of its lower-mass population
refer only to small portions of its extent. In this work we exploit the
capabilities of Gaia DR2 measurements to search for Sco OB2 members across its
entire size and down to the lowest stellar masses. We use both Gaia astrometric
and photometric data to select association members, using minimal assumptions
derived mostly from the Hipparcos studies. Gaia resolves small details in both
the kinematics of individual Sco OB2 subgroups and their distances from the
Sun. We develop methods to explore the 3D kinematics of stellar populations
covering large sky areas. We find ~11000 pre-main sequence (PMS) Sco OB2
members (with <3% contamination), plus ~3600 MS candidate members with a larger
(10-30%) field-star contamination. A higher-confidence subsample of ~9200 PMS
(and ~1340 MS) members is also selected (<1% contamination for the PMS),
affected however by larger (~15%) incompleteness. We classify separately stars
in compact and diffuse populations. Most members belong to a few kinematically
distinct diffuse populations, whose ensemble outlines the association shape.
Upper Sco is the densest part of Sco OB2, with a complex spatial and
kinematical structure, and no global pattern of motion. Other dense subclusters
are found in Upper Centaurus-Lupus and in Lower Centaurus-Crux. Most clustered
stars appear to be younger than the diffuse PMS population, suggesting star
formation in small groups which rapidly disperse and dilute, while keeping
memory of their original kinematics. We also find that the open cluster IC 2602
has a similar dynamics to Sco OB2, and its PMS members are evaporating and
forming a ~10 deg halo around its double-peaked core.Comment: 27 pages, 37 figures. Accepted for publication in Astronomy and
Astrophysic
X-ray flares on the UV Ceti-type star CC Eridani: a "peculiar" time-evolution of spectral parameters
Context: Weak flares are supposed to be an important heating agent of the
outer layers of stellar atmospheres. However, due to instrumental limitations,
only large X-ray flares have been studied in detail until now.
Aims: We used an XMM-Newton observation of the very active BY-Dra type binary
star CC Eri in order to investigate the properties of two flares that are
weaker than those typically studied in the literature.
Methods: We performed time-resolved spectroscopy of the data taken with the
EPIC-PN CCD camera. A multi-temperature model was used to fit the spectra. We
inferred the size of the flaring loops using the density-temperature diagram.
The loop scaling laws were applied for deriving physical parameters of the
flaring plasma. We also estimated the number of loops involved in the observed
flares.
Results: A large X-ray variability was found. Spectral analysis showed that
all the regions in the light curve, including the flare segments, are
well-described by a 3-T model with variable emission measures but,
surprisingly, with constant temperatures (values of 3, 10 and 22 MK). The
analysed flares lasted ~ 3.4 and 7.1 ks, with flux increases of factors
1.5-1.9. They occurred in arcades made of a few tens of similar coronal loops.
The size of the flaring loops is much smaller than the distance between the
stellar surfaces in the binary system, and even smaller than the radius of each
of the stars. The obtained results are consistent with the following ideas: (i)
the whole X-ray light curve of CC Eri could be the result of a superposition of
multiple low-energy flares, and (ii) stellar flares can be scaled-up versions
of solar flares.Comment: 14 pages, 12 figures. Accepted for publication in Astronomy &
Astrophysic
Symbiotic stars in X-rays II: faint sources detected with XMM-Newton and Chandra
We report the detection, with and XMM-, of
faint, soft X-ray emission from four symbiotics stars that were not known to be
X-ray sources. These four object show a -type X-ray spectrum, i.e. their
spectra can be modeled with an absorbed optically thin thermal emission with
temperatures of a few million degrees. Photometric series obtained with the
Optical Monitor on board XMM- from V2416 Sgr and NSV 25735
support the proposed scenario where the X-ray emission is produced in a
shock-heated region inside the symbiotic nebulae.Comment: 5 pages, 1 figure, 2 tables; A&A publishe
Detection of magnetic field in the B2 star Oph A with ESO FORS2
Circumstantial evidence suggests that magnetism and enhanced X-ray emission
are likely correlated in early B-type stars: similar fractions of them (
10 %) are strong and hard X-ray sources and possess strong magnetic fields. It
is also known that some B-type stars have spots on their surface. Yet up to now
no X-ray activity associated with spots on early-type stars was detected. In
this Letter we report the detection of a magnetic field on the B2V star
Oph A. Previously, we assessed that the X-ray activity of this star is
associated with a surface spot, herewith we establish its magnetic origin. We
analyzed FORS2 ESO VLT spectra of Oph A taken at two epochs and detected
a longitudinal component of the magnetic field of order of G in one
of the datasets. The detection of the magnetic field only at one epoch can be
explained by stellar rotation which is also invoked to explain observed
periodic X-ray activity. From archival HARPS ESO VLT high resolution spectra we
derived the fundamental stellar parameters of Oph A and further
constrained its age. We conclude that Oph A provides strong evidence for
the presence of active X-ray emitting regions on young magnetized early type
stars.Comment: 4 pages, 1 figure, 2 tables, accepted as a "Letter to the Editor" to
Astronomy & Astrophysic
FUV variability of HD 189733. Is the star accreting material from its hot Jupiter?
Hot Jupiters are subject to strong irradiation from the host stars and, as a
consequence, they do evaporate. They can also interact with the parent stars by
means of tides and magnetic fields. Both phenomena have strong implications for
the evolution of these systems. Here we present time resolved spectroscopy of
HD~189733 observed with the Cosmic Origin Spectrograph (COS) on board to HST.
The star has been observed during five consecutive HST orbits, starting at a
secondary transit of the planet ( ~0.50-0.63). Two main episodes of
variability of ion lines of Si, C, N and O are detected, with an increase of
line fluxes. Si IV lines show the highest degree of variability. The FUV
variability is a signature of enhanced activity in phase with the planet
motion, occurring after the planet egress, as already observed three times in
X-rays. With the support of MHD simulations, we propose the following
interpretation: a stream of gas evaporating from the planet is actively and
almost steadily accreting onto the stellar surface, impacting at
ahead of the sub-planetary point.Comment: 35 pages, 19 Figures. Accepted for publication to Ap
VLT/Flames observations of the star forming region NGC 6530
Mechanisms regulating the evolution of pre-main sequence stars can be understood by studying stellar properties such as rotation, disk accretion, internal mixing and binarity. To investigate such properties, we studied a sample of 332 candidate members of the massive and populous star forming region NGC 6530. We want to select cluster members by using different membership criteria,to study the properties of pre-main sequence stars with or without circumstellar disks. We use intermediate resolution spectra including the Li I 6707.8 Angstroms line to derive radial and rotational velocities, binarity and to measure the Equivalent Width of the lithium line; these results are combined with X-ray data to study the cluster membership. Optical-IR data and Halpha spectra, these latter available for a subsample of our targets, are used to classify CTTS and WTTS and to compare the properties of stars with and without disks. We find a total of 237 certain members including 53 binaries. The rotational velocity distributions of stars with IR excesses are statistically different from that of stars without IR excesses, while the fraction of binaries with disks is significantly smaller than that of single stars. Stars with evidence for accretion show circumstellar disks; youth of cluster members is confirmed by the lithium abundance consistent with the initial content. As indicated by the disk-locking picture, stars with disks have in general rotational velocities lower than stars without disks. Binaries in NGC 6530 seem have undergone a significant disk evolution
XMM-Newton observations of HD189733 during planetary transits
We report on two XMM-Newton observations of the planetary host star HD189733.
The system has a close in planet and it can potentially affect the coronal
structure via interactions with the magnetosphere. We have obtained X-ray
spectra and light curves from EPIC and RGS on board XMM-Newton which we have
analyzed and interpreted. We reduced X-ray data from primary transit and
secondary eclipse occurred in April 17th 2007 and May 18th 2009, respectively.
In the April 2007 observation only variability due to weak flares is
recognized. In 2009 HD189733 exhibited a X-ray flux always larger than in the
2007 observation. The average flux in 2009 was higher than in 2007 observation
by a factor of 45%. During the 2009 secondary eclipse we observed a softening
of the X-ray spectrum significant at level of ~3 sigma. Further, we observed
the most intense flare recorded at either epochs. This flare occurred 3 ks
after the end of the eclipse.The flare decay shows several minor ignitions
perhaps linked to the main event and hinting for secondary loops that emit
triggered by the main loop. Magneto-Hydro-Dynamical (MHD) simulations show that
the magnetic interaction between planet and star enhances the density and the
magnetic field in a region comprised between the planet and the star because of
their relative orbital/rotation motion. X-ray observations and model
predictions are globally found in agreement, despite the quite simple MHD model
and the lack of precise estimate of parameters including the alignment and the
intensity of stellar and planetary magnetic fields. Future observations should
confirm or disprove this hypothesis, by determining whether flares are
systematically recurring in the light curve at the same planetary phase.Comment: Accepted for publication on The Astrophysical Journa
Deciphering the large-scale environment of radio galaxies in the local Universe: where do they born, grow and die?
The role played by the large-scale environment on the nuclear activity of
radio galaxies (RGs), is still not completely understood. Accretion mode, jet
power and galaxy evolution are connected with their large-scale environment
from tens to hundreds of kpc. Here we present a detailed, statistical, analysis
of the large-scale environment for two samples of RGs up to redshifts
=0.15. The main advantages of our study, with respect to those
already present in the literature, are due to the extremely homogeneous
selection criteria of catalogs adopted to perform our investigation. This is
also coupled with the use of several clustering algorithms. We performed a
direct search of galaxy-rich environments around RGs using them as beacon. To
perform this study we also developed a new method that does not appear to
suffer by a strong dependence as other algorithms. We conclude
that, despite their radio morphological (FR\,I FR\,II) and/or their
optical (HERG LERG) classification, RGs in the local Universe tend to live
in galaxy-rich large-scale environments having similar characteristics and
richness. We highlight that the fraction of FR\,Is-LERG, inhabiting galaxy rich
environments, appears larger than that of FR\,IIs-LERG. We also found that 5
out of 7 FR\,II-HERGs, with 0.11, lie in groups/clusters of
galaxies. However, we recognize that, despite the high level of completeness of
our catalogs, when restricting to the local Universe, the low number of HERGs
(10\% of the total FR\,IIs investigated) prevent us to make a strong
statistical conclusion about this source class.Comment: 21 pages, 25 figures, accepted for publication on the Astrophysical
Journal Supplement Series - pre-proof versio
X-ray spectral and timing characteristics of the stars in the young open cluster IC 2391
We present X-ray spectral and timing analysis of members of the young open
cluster IC 2391 observed with the XMM-Newton observatory. We detected 99 X-ray
sources by analysing the summed data obtained from MOS1, MOS2 and pn detectors
of the EPIC camera; 24 of them are members, or probable members, of the
cluster. Stars of all spectral types have been detected, from the early-types
to the late-M dwarfs.
Despite the capability of the instrument to recognize up to 3 thermal
components, the X-ray spectra of the G, K and M members of the cluster are well
described with two thermal components (at kT 0.3-0.5 keV and kT 1.0-1.2 keV respectively) while the X-ray spectra of F members require
only a softer 1-T model.
The Kolmogorov-Smirnov test applied to the X-ray photon time series shows
that approximately 46% of the members of IC 2391 are variable with a confidence
level 99%. The comparison of our data with those obtained with ROSAT/PSPC,
nine years earlier, and ROSAT/HRI, seven years earlier, shows that there is no
evidence of significant variability on these time scales, suggesting that
long-term variations due to activity cycles similar to that on the Sun are not
common, if present at all, among these young stars.Comment: Astronomy & Astrophysics, in pres
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