1,329 research outputs found
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&
When It Isn’t Always Lyme: Expanding the Differential Diagnosis for Acute-Onset Polyarthralgia in the West Virginia Eastern Panhandle
This case presentation discusses a 36 year-old female animal care worker presenting with an acute-onset polyarthropathy during the summer months in a Lyme endemic region. Though she appeared to be a good candidate for the diagnosis of Lyme borreliosis, her screening serology reported negative results and alternative diagnoses were considered. Her subsequent diagnosis with parvovirus B19 acts to remind the general practitioner to have confidence in the accuracy of a negative Lyme screen and, upon negative result, to expand the differential to include less common infections including parvovirus B19. It also highlights the need to remember parvovirus B19 in a similar patient who has the potential to be or to become pregnant during the course of her illness
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
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
On the 2015 outburst of the EXor variable star V1118 Ori
After a long-lasting period of quiescence of about a decade, the source V1118
Ori, one of the most representative members of the EXor variables, is now
outbursting. Since the initial increase of the near-infrared flux of about 1
mag (JHK bands) registered on 2015 September 22, the source brightness has
remained fairly stable. We estimate DeltaV about 3 mag with respect to the
quiescence phase. An optical/near-IR low-resolution spectrum has been obtained
with the Large Binocular Telescope instruments MODS and LUCI2, and compared
with a spectrum of similar spectral resolution and sensitivity level taken
during quiescence. Together with the enhancement of the continuum, the outburst
spectrum presents a definitely higher number of emission lines, in particular
HI recombination lines of the Balmer, Paschen, and Brackett series, along with
bright permitted lines of several species, forbidden atomic lines, and CO
ro-vibrational lines. Both mass accretion and mass loss rates have
significantly increased (by to about an order of magnitude, mass accretion rate
= 1.2-4.8 10^-8 M_sun/yr, mass loss rate = 0.8-2 10^-9 M_sun/yr) with respect
to the quiescence phase. If compared with previous outbursts, the present one
appears less energetic. Alternatively, it could already be in the fading phase
(with the maximum brightness level reached when the source was not visible),
or, viceversa, still in the rising phase.Comment: Accepted ApJ Letter
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&
On the nature of the EXor accretion events: an unfrequent manifestation of a common phenomenology ?
We present the results of a comparison between classical and newly identified
EXor based on literature data and aimed at recognizing possible differences or
similarities of both categories. Optical and near-IR two-color diagrams,
modalities of fluctuations, and derived values of the mass accretion rates are
indicative of strong similarities between the two samples. We demonstrate how
the difference between the outburst and the quiescence spectral energy
distribution of all the EXor can be well fitted with a single blackbody, as if
an additional thermal component appears during the outbursting phase.
Temperatures of this additional component span between 1000 and 4500 K, while
the radii of the emitting regions (assumed to be a uniform disk) span between
0.01 and 0.1 AU, sizes typical of the inner portions of the circumstellar disk.
Spots persisting up to 50% of the outburst duration, not exceeding the 10% of
the stellar surface, and with temperatures compatible with the EXor mass
accretion rates, are able to account for both the appearance of the additional
thermal component and the dust sublimation in the inner structures of the disk.
We also compare the EXor events with the most significant color and magnitude
fluctuations of active T Tauri stars finding that (i} burst accretion phenomena
should also be important for this latter class; (ii} EXor events could be more
frequent then those accidentally discovered. Remarkable is the case of the
source V2493 Cyg, a T Tauri star recently identified as a strong outbursting
object: new optical and near-IR photometric and spectroscopic data are
presented trying to clarify its EXor or FUor nature.Comment: Accepted for publication in Ap
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