664 research outputs found
Towards a better classification of unclear eruptive variables: the cases of V2492 Cyg, V350 Cep, and ASASSN-15qi
Eruptive variables are young stars that show episodic variations of
brightness: EXors/FUors variations are commonly associated with enhanced
accretion outbursts occurring at intermittent cadence of months/years (EXors)
and decades/centuries (FUors). Variations that can be ascribed to a variable
extinction along their line of sight are instead classified as UXors. We aim at
investigating the long-term photometric behaviour of three sources classified
as eruptive variables. We present data from the archival plates of the Asiago
Observatory relative to the fields where the targets are located. For the sake
of completeness we have also analysed the Harvard plates of the same regions
that cover a much longer historical period, albeit at a lower sensitivity,
however we are only able to provide upper limits. A total of 273 Asiago plates
were investigated, providing a total of more than 200 magnitudes for the three
stars, which cover a period of about 34 yr between 1958 and 1991. We have
compared our data with more recently collected literature data. Our plates
analysis of V2492 Cyg provides historical upper limits that seem not to be
compatible with the level of the activity monitored during the last decade.
Therefore, recently observed accretion phenomena could be associated with the
outbursting episodes, more than repetitive obscuration. While a pure extinction
does not seem the only mechanism responsible for the ASASSN-15qi fluctuations,
it can account quite reasonably for the recent V350 Cep variations.Comment: 12 pages, accepted by A&
KINEMATICS OF THE TRUNK AND THE SPINE DURING UNRESTRICTED AND RESTRICTED SQUATS – A PRELIMINARY ANALYSIS
The aim of the study was the assessment and comparison of trunk kinematics during restricted (knee not beyond toes) and unrestricted squats. Eight repetitions of restricted and unrestricted squats with 0, 25% and 50% bodyweight loading using a barbell were assessed with a 12 camera motion analysis system. A trunk marker set was developed and applied that allowed the measurement of the 3D kinematics of the trunk, divided into three segments (lumbar, thoracic and upper trunk) and the analysis of the sagittal curvature (lumbar and thoracic spine). The preliminary results of four subjects showed a larger range of sagittal motion between lumbar and pelvic segments for the restricted compared to the unrestricted squat. The lumbar curvature straightened with increasing load. The unrestricted execution seems to lead to higher stresses in the lower back
On the binarity of the classical Cepheid X Sgr from interferometric observations
Optical-infrared interferometry can provide direct geometrical measurements
of the radii of Cepheids and/or reveal unknown binary companions of these
stars. Such information is of great importance for a proper calibration of
Period-Luminosity relations and for determining binary fraction among Cepheids.
We observed the Cepheid X Sgr with VLTI/AMBER in order to confirm or disprove
the presence of the hypothesized binary companion and to directly measure the
mean stellar radius, possibly detecting its variation along the pulsation
cycle. From AMBER observations in MR mode we performed a binary model fitting
on the closure phase and a limb-darkened model fitting on the visibility. Our
analysis indicates the presence of a point-like companion at a separation of
10.7 mas and 5.6 magK fainter than the primary, whose flux and position are
sharply constrained by the data. The radius pulsation is not detected, whereas
the average limb-darkened diameter results to be 1.48+/-0.08 mas, corresponding
to 53+/-3 R_sun at a distance of 333.3 pc.Comment: 5 pages, 3 figures, research not
POISSON project - III - Investigating the evolution of the mass accretion rate
As part of the POISSON project (Protostellar Optical-Infrared Spectral Survey
on NTT), we present the results of the analysis of low-resolution NIR spectra
0.9-2.4 um) of two samples of YSOs in Lupus and Serpens (52 and 17 objects),
with masses 0.1-2.0 Msun and ages from 10^5 to a few 10^7 yr. After determining
the accretion parameters of the Lup and Ser targets by analysing their HI
near-IR emission features, we added the results to those from previous regions
(investigated in POISSON with the same methodology). We obtained a final
catalogue (143 objects) of mass accretion rates (Macc) derived in a homogeneous
fashion and analysed how Macc correlates with M* and how it evolves in time. We
derived the accretion luminosity (Lacc) and Macc for Lup and Ser objects from
the Br_gamma line by using relevant empirical relationships from the literature
that connect HI line luminosity and Lacc. To minimise the biases and also for
self-consistency, we re-derived mass and age for each source using the same set
of evolutionary tracks. We observe a correlation MaccM*^2.2, similarly to what
has previously been observed in several star-forming clouds. The time variation
of Macc is roughly consistent with the expected evolution in viscous disks,
with an asymptotic decay that behaves as t^-1.6. However, Macc values are
characterised by a large scatter at similar ages and are on average higher than
the predictions of viscous models. Although part of the scattering may be
related to the employed empirical relationship and to uncertainties on the
single measurements, the general distribution and decay trend of the Macc
points are real. These findings might be indicative of a large variation in the
initial mass of the disks, of fairly different viscous laws among disks, of
varying accretion regimes, and of other mechanisms that add to the dissipation
of the disks, such as photo-evaporation.Comment: 18 pages, 10 figures, accepted by A&
Pre-Main Sequence variables in the VMR-D : identification of T Tauri-like accreting protostars through Spitzer-IRAC variability
We present a study of the infrared variability of young stellar objects by
means of two Spitzer-IRAC images of the Vela Molecular Cloud D (VMR-D) obtained
in observations separated in time by about six months. By using the same
space-born IR instrumentation, this study eliminates all the unwanted effects
usually unavoidable when comparing catalogs obtained from different
instruments. The VMR-D map covers about 1.5 square deg. of a site where star
formation is actively ongoing. We are interested in accreting pre-main sequence
variables whose luminosity variations are due to intermittent events of disk
accretion (i.e. active T Tauri stars and EXor type objects). The variable
objects have been selected from a catalog of more than 170,000 sources detected
at a S/N ratio > 5. We searched the sample of variables for ones whose
photometric properties are close to those of known EXor's. These latter are
monitored in a more systematic way than T Tauri stars and the mechanisms that
regulate the observed phenomenology are exactly the same. Hence the modalities
of the EXor behavior is adopted as driving criterium for selecting variables in
general. We selected 19 bona fide candidates that constitute a well-defined
sample of new variable targets for further investigation. Out of these, 10
sources present a Spitzer MIPS 24 micron counterpart, and have been classified
as 3 Class I, 5 flat spectrum and 2 Class II objects, while the other 9 sources
have spectral energy distribution compatible with phases older than Class I.
This is consistent with what is known about the small sample of known EXor's,
and suggests that the accretion flaring or EXor stage might come as a Class
I/II transition. We present also new prescriptions that can be useful in future
searches for accretion variables in large IR databases.Comment: 35 pages, 12 figures To appear in Ap
Simultaneous monitoring of the photometric and polarimetric activity of the young star PV Cep in the optical/near-infrared bands
We present the results of a simultaneous monitoring, lasting more than 2
years, of the optical and near-infrared photometric and polarimetric activity
of the variable protostar PV Cep. During the monitoring period, an outburst has
occurred in all the photometric bands, whose declining phase (J
3 mag) lasted about 120 days. A time lag of 30 days between
optical and infrared light curves has been measured and interpreted in the
framework of an accretion event. This latter is directly recognizable in the
significant variations of the near-infrared colors, that appear bluer in the
outburst phase, when the star dominates the emission, and redder in declining
phase, when the disk emission prevails. All the observational data have been
combined to derive a coherent picture of the complex morphology of the whole PV
Cep system, that, in addition to the star and the accretion disk, is composed
also by a variable biconical nebula. In particular, the mutual interaction
between all these components is the cause of the high value of the polarization
( 20%) and of its fluctuations. The observational data concur to
indicate that PV Cep is not a genuine EXor star, but rather a more complex
object; moreover the case of PV Cep leads to argue about the classification of
other recently discovered young sources in outburst, that have been considered,
maybe over-simplifying, as EXor.Comment: Accepted for publication on Ap
The HH34 outflow as seen in [FeII]1.64um by LBT-LUCI
Dense atomic jets from young stars copiously emit in [FeII] IR lines, which
can, therefore, be used to trace the immediate environments of embedded
protostars. We want to investigate the morphology of the bright [FeII] 1.64um
line in the jet of the source HH34 IRS and compare it with the most commonly
used optical tracer [SII]. We analyse a 1.64um narrow-band filter image
obtained with the Large Binocular Telescope (LBT) LUCI instrument, which covers
the HH34 jet and counterjet. A Point Spread Function (PSF) deconvolution
algorithm was applied to enhance spatial resolution and make the IR image
directly comparable to a [SII] HST image of the same source. The [FeII]
emission is detected from both the jet, the (weak) counter-jet, and from the
HH34-S and HH34-N bow shocks. The deconvolved image allows us to resolve jet
knots close to about 1\arcsec from the central source. The morphology of the
[FeII] emission is remarkably similar to that of the [SII] emission, and the
relative positions of [FeII] and [SII] peaks are shifted according to proper
motion measurements, which were previously derived from HST images. An analysis
of the [FeII]/[SII] emission ratio shows that Fe gas abundance is much lower
than the solar value with up to 90% of Fe depletion in the inner jet knots.
This confirms previous findings on dusty jets, where shocks are not efficient
enough to remove refractory species from grains.Comment: 5 pages, 4 figures, note accepted by A&
The 2016-2017 peak luminosity of the pre-main sequence variable V2492 Cyg
V2492 Cyg is a young pre-main sequence star presenting repetitive brightness
variations of significant amplitude (Delta R > 5 mag) whose physical origin has
been ascribed to both extinction (UXor-type) and accretion (EXor-type)
variability, although their mutual proportion has not been clarified yet.
Recently, V2492 Cyg has reached a level of brightness ever registered in the
period of its documented activity. Optical and near-infrared photometry and
spectroscopy have been obtained in October 2016 and between March and July
2017. The source has remained bright until the end of May 2017, then it started
to rapidly fade since the beginning of June at a rate of about 0.08 mag/day. On
mid-July 2017 the source has reached the same low-brightness level as two years
before. Extinction and mass accretion rate were derived by means of the
luminosity of the brightest lines, in particular Halpha and Hbeta. A couple of
optical high-resolution spectra are also presented to derive information on the
gas kinematics. Visual extinction variations do not exceed a few magnitudes,
while the mass accretion rate is estimated to vary from less than 10^-8 up to a
few 10^-7 M_sun/yr. This latter is comparable to that estimated on the previous
high-state in 2010, likely occurred under more severe extinction conditions.
The combined analysis of the optical and near-infrared (NIR) observations
extends to the present event the original suggestion that the V2492 Cyg
variability is a combination of changing extinction and accretion.Comment: Accepted by A&
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