95 research outputs found
Analysis of the stellar population in the central area of the HII region Sh 2-284
There is a lack of state-of-the-art information on very young open clusters,
with implications for determining the structure of the Galaxy. Our main
objective is to study the timing and location of the star formation processes
which yielded the generation of the giant HII region Sh 2-284. The analysis is
based on UBVRcIc CCD measurements and JHKs photometry in the central part of
the HII region, where the cluster Dolidze 25 is located.The determination of
cluster distance, reddening and age is carried out through comparison with
ZAMS, post-MS and PMS isochrones. Reference lines for metallicity Z=0.004 are
used, in agreement with spectroscopic metallicity determination published for
several cluster members. The results are: E(B-V)=0.78+-0.02, M=12.8+-0.2,
LogAge(yr)=6.51+-0.07. A PMS member sequence is proposed, coeval within the
errors with the post-MS cluster age (LogAge(yr)=6.7+-0.2). The mass function
for this PMS population in the mass range above 1.3-3.5 Msun is well fitted by
a Salpeter mass function.The presence of a different star generation in the
cluster with a distinctly older age, around 40 Myr, is suggested. The NIR
results indicate a large number of sources with H-Ks excess, practically
distinct from the optical PMS candidate members.
The distance determined for the cluster is distinctly lower than previously
published values. This result originates in the consistent use of low
metallicity models for ZAMS fitting, applying published metallicity values for
the cluster.Comment: gzipped tar file: 1 aa.cls, 1 tex file, 1 bbl file, 13 figures.
Accepted in Astronomy & Astrophysics (9.11.2009
Multiple protostellar systems. II. A high resolution near-infrared imaging survey in nearby star-forming regions
(abridged) Our project endeavors to obtain a robust view of multiplicity
among embedded Class I and Flat Spectrum protostars in a wide array of nearby
molecular clouds to disentangle ``universal'' from cloud-dependent processes.
We have used near-infrared adaptive optics observations at the VLT through the
H, Ks and L' filters to search for tight companions to 45 Class I and Flat
Spectrum protostars located in 4 different molecular clouds (Taurus-Auriga,
Ophiuchus, Serpens and L1641 in Orion). We complemented these observations with
published high-resolution surveys of 13 additional objects in Taurus and
Ophiuchus. We found multiplicity rates of 32+/-6% and 47+/-8% over the 45-1400
AU and 14-1400 AU separation ranges, respectively. These rates are in excellent
agreement with those previously found among T Tauri stars in Taurus and
Ophiuchus, and represent an excess of a factor ~1.7 over the multiplicity rate
of solar-type field stars. We found no non-hierarchical triple systems, nor any
quadruple or higher-order systems. No significant cloud-to-cloud difference has
been found, except for the fact that all companions to low-mass Orion
protostars are found within 100 AU of their primaries whereas companions found
in other clouds span the whole range probed here. Based on this survey, we
conclude that core fragmentation always yields a high initial multiplicity
rate, even in giant molecular clouds such as the Orion cloud or in clustered
stellar populations as in Serpens, in contrast with predictions of numerical
simulations. The lower multiplicity rate observed in clustered Class II and
Class III populations can be accounted for by a universal set of properties for
young systems and subsequent ejections through close encounters with unrelated
cluster members.Comment: 15 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Not a galaxy: IRAS 04186+5143, a new young stellar cluster in the outer Galaxy
We report the discovery of a new young stellar cluster in the outer Galaxy
located at the position of an IRAS PSC source that has been previously
mis-identified as an external galaxy. The cluster is seen in our near-infrared
imaging towards IRAS 04186+5143 and in archive Spitzer images confirming the
young stellar nature of the sources detected. There is also evidence of
sub-clustering seen in the spatial distributions of young stars and of gas and
dust.
Near- and mid-infrared photometry indicates that the stars exhibit colours
compatible with reddening by interstellar and circumstellar dust and are likely
to be low- and intermediate-mass YSOs with a large proportion of Class I YSOs.
Ammonia and CO lines were detected, with the CO emission well centred near
the position of the richest part of the cluster. The velocity of the CO and
NH lines indicates that the gas is Galactic and located at a distance of
about 5.5 kpc, in the outer Galaxy.
Herschel data of this region characterise the dust environment of this
molecular cloud core where the young cluster is embedded. We derive masses,
luminosities and temperatures of the molecular clumps where the young stars
reside and discuss their evolutionary stages.Comment: 14 pages, 15 figure
The extended population associated with W40
W40 is a heavily obscured bipolar HII region projected in the direction of
the Aquila Rift and ionized by hot stars in a central, partly embedded cluster.
The study of the cluster and its surroundings has been greatly hampered thus
far by the strong extinction in the region. We use the Gaia eDR3 catalog to
establish astrometric membership criteria based on the population of the W40
central cluster, reassess the distance of the region, and identify in this way
new members, both inside and outside the cluster. We obtain visible
spectroscopy in the red spectral region to classify both known and new members,
complemented with Gaia and Spitzer photometry to assess the evolutionary status
of the stellar population. We derive a high-confidence geometric distance to
the W40 region of 502 pc 4 pc and confirm the presence of a comoving
extended population of stars at the same distance, spreading over the whole
projected area of the HII region and beyond. Spectral classifications are
presented for 21 members of the W40 region, 10 of them belonging to the central
cluster. One of the newly identified B stars in the extended population is
clearly interacting with the shell surrounding the HII region, giving rise to a
small arc-shaped nebula that traces a bow shock. The infrared excess properties
suggest that the extended population is significantly older ( Myr) than
the W40 central cluster ( Myr). The area currently occupied by the W40 HII
region and its surroundings has a history of star formation extending at least
several million years in the past, of which the formation of the W40 central
cluster and the subsequent HII region is one of the latest episodes. The newly
determined distance suggests that W40 is behind, and physically detached from,
a pervasive large dust layer which is some 60 pc foreground to it as determined
by previous studies.Comment: Submitted to Astronomy and Astrophysic
The historical record of massive star formation in Cygnus
The Cygnus region, which dominates the local spiral arm of the Galaxy, is one
of the nearest complexes of massive star formation. Its massive stellar
content, regions of ongoing star formation, and molecular gas have been studied
in detail. However, little is known of the history of the region beyond the
past 10 Myr. The brightness and spectroscopic characteristics of red
supergiants make it easy to identify them and build up a virtually complete
sample of such stars at the distance of the Cygnus region, thus providing a
record of massive star formation extending several tens of Myr into the past, a
period inaccessible through the O and early B stars observable at present. We
have made a selection of a sample of bright, red stars in an area of 84 square
degrees covering the whole present extension of the Cygnus region. We have
obtained spectroscopy in the red visible range allowing an accurate,
homogeneous spectral classification as well as a reliable separation between
supergiants and other cool stars. Our data are complemented with Gaia Data
Release 2 astrometric data. We have identified 29 red supergiants in the area,
17 of which had not been previously classified as supergiants. Twenty-four of
the 29 most likely belong to the Cygnus region and four of the remaining to the
Perseus arm. We have used their derived luminosities and masses to infer the
star formation history of the region. Intense massive star formation activity
is found to have started approximately 15 Myr ago, and we find evidence for two
other episodes, one taking place between 20 and 30 Myr ago and another one
having ended approximately 40 Myr ago. There are small but significant
differences between the kinematic properties of red supergiants younger or
older then 20 Myr, hinting that stars of the older group were formed outside
the precursor of the present Cygnus complex, possibly in the Sagittarius-Carina
arm.Comment: Accepted for publication in Astronomy and Astrophysic
Another deep dimming of the classical T Tauri star RW Aur A
Context. RW Aur A is a classical T Tauri star (CTTS) with an unusually rich
emission line spectrum. In 2014 the star faded by ~ 3 magnitudes in the V band
and went into a long-lasting minimum. In 2010 the star suffered from a similar
fading, although less deep. These events in RW Aur A are very unusual among the
CTTS, and have been attributed to occultations by passing dust clouds. Aims. We
want to find out if any spectral changes took place after the last fading of RW
Aur A with the intention to gather more information on the occulting body and
the cause of the phenomenon. Methods. We collected spectra of the two
components of RW Aur. Photometry was made before and during the minimum.
Results. The overall spectral signatures reflecting emission from accretion
flows from disk to star did not change after the fading. However, blue-shifted
absorption components related to the stellar wind had increased in strength in
certain resonance lines, and the profiles and strengths, but not fluxes, of
forbidden lines had become drastically different. Conclusions. The extinction
through the obscuring cloud is grey indicating the presence of large dust
grains. At the same time, there are no traces of related absorbing gas. The
cloud occults the star and the interior part of the stellar wind, but not the
wind/jet further out. The dimming in 2014 was not accompanied by changes in the
accretion flows at the stellar surface. There is evidence that the structure
and velocity pattern of the stellar wind did change significantly. The dimmings
could be related to passing condensations in a tidally disrupted disk, as
proposed earlier, but we also speculate that large dust grains have been
stirred up from the inclined disk into the line-of-sight through the
interaction with an enhanced wind.Comment: 5 pages, 5 figures, Accepted for publication in A&
First Light of Engineered Diffusers at the Nordic Optical Telescope Reveal Time Variability in the Optical Eclipse Depth of WASP-12b
We present the characterization of two engineered diffusers mounted on the
2.5 meter Nordic Optical Telescope, located at Roque de Los Muchachos, Spain.
To assess the reliability and the efficiency of the diffusers, we carried out
several test observations of two photometric standard stars, along with
observations of one primary transit observation of TrES-3b in the red (R-band),
one of CoRoT-1b in the blue (B-band), and three secondary eclipses of WASP-12b
in V-band. The achieved photometric precision is in all cases within the
sub-millimagnitude level for exposures between 25 and 180 seconds. Along a
detailed analysis of the functionality of the diffusers, we add a new transit
depth measurement in the blue (B-band) to the already observed transmission
spectrum of CoRoT-1b, disfavouring a Rayleigh slope. We also report variability
of the eclipse depth of WASP-12b in the V-band. For the WASP-12b secondary
eclipses, we observe a secondary-depth deviation of about 5-sigma, and a
difference of 6-sigma and 2.5-sigma when compared to the values reported by
other authors in similar wavelength range determined from Hubble Space
Telescope data. We further speculate about the potential physical processes or
causes responsible for this observed variabilityComment: 11 pages, 9 figure
Expanding shells around young clusters - S 171/Be 59
Context. Some HII regions that surround young stellar clusters are bordered by molecular shells that appear to expand at a rate inconsistent with our current model simulations. In this study we focus on the dynamics of Sharpless 171 (including NGC 7822), which surrounds the cluster Berkeley 59. Aims. We aim to compare the velocity pattern over the molecular shell with the mean radial velocity of the cluster for estimates of the expansion velocities of different shell structures, and to match the observed properties with model simulations. Methods. Optical spectra of 27 stars located in Berkeley 59 were collected at the Nordic Optical Telescope, and a number of molecular structures scattered over the entire region were mapped in 13CO(1- 0) at Onsala Space Observatory. Results. We obtained radial velocities and MK classes for the clustera\u27s stars. At least four of the O stars are found to be spectroscopic binaries, in addition to one triplet system. From these data we obtain the mean radial velocity of the cluster. From the 13CO spectra we identify three shell structures, expanding relative to the cluster at moderate velocity (4 km s- 1), high velocity (12 km s- 1), and in between. The high-velocity cloudlets extend over a larger radius and are less massive than the low-velocity cloudlets. We performed a model simulation to understand the evolution of this complex. Conclusions. Our simulation of the Sharpless 171 complex and Berkeley 59 cluster demonstrates that the individual components can be explained as a shell driven by stellar winds from the massive cluster members. However, our relatively simple model produces a single component. Modelling of the propagation of shell fragments through a uniform interstellar medium demonstrates that dense cloudlets detached from the shell are decelerated less efficiently than the shell itself. They can reach greater distances and retain higher velocities than the shell
The Crab pulsar and its pulsar-wind nebula in the optical and infrared
We investigate the emission mechanism and evolution of pulsars that are
associated with supernova remnants.
We used imaging techniques in both the optical and near infrared, using
images with very good seeing (<0.6) to study the immediate surroundings of the
Crab pulsar. In the case of the infrared, we took two data sets with a time
window of 75 days, to check for variability in the inner part of the Crab
nebula. We also measure the spectral indices of all these wisps, the nearby
knot, and the interwisp medium, using our optical and infrared data. We then
compared the observational results with the existing theoretical models.
We report variability in the three nearby wisps located to the northwest of
the pulsar and also in a nearby anvil wisp in terms of their structure,
position, and emissivity within the time window of 75 days. All the wisps and
the inner knot display red spectra with similar spectral indices. Similarly,
the interwisp medium regions also show red spectra similar to those of the
wisps. Also, based on archival HST data and our IR data, we find that the inner
knot remains stationary for a time period of 13.5 years. The projected average
velocity relative to the pulsar for this period is < 8 km/s.
By comparing the spectral indices of the structures in the inner Crab with
the current theoretical models, we find that the Del Zanna et al. (2006) model
for the synchrotron emission fits our observations, although the spectral index
is at the flatter end of their modelled spectra.Comment: 8 pages, 5 figure
Density streams in the disc winds of Classical T Tauri stars
Spectral and photometric variability of the Classical T Tauri stars RY Tau
and SU Aur from 2013 to 2022 is analyzed. We find that in SU Aur the H-alpha
line's flux at radial velocity RV = -50 +- 7 km/s varies with a period P = 255
+- 5 days. A similar effect previously discovered in RY Tau is confirmed with
these new data: P = 21.6 days at RV = -95 +- 5 km/s. In both stars, the radial
velocity of these variations, the period, and the mass of the star turn out to
be related by Kepler's law, suggesting structural features on the disc plane
orbiting at radii of 0.2 AU in RY Tau and 0.9 AU in SU Aur, respectively. Both
stars have a large inclination of the accretion disc to the line of sight - so
that the line of sight passes through the region of the disc wind. We propose
there is an azimuthal asymmetry in the disc wind, presumably in the form of
'density streams', caused by substructures of the accretion disc surface. These
streams cannot dissipate until they go beyond the Alfven surface in the disc's
magnetic field. These findings open up the possibility to learn about the
structure of the inner accretion disc of CTTS on scales less than 1 AU and to
reveal the orbital distances related to the planet's formation.Comment: 9 pages, 10 figure
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