556 research outputs found
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
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
Evidence for T Tauri-like emission in the EXor V1118 Ori from near-IR and X-ray data
We present a near-IR study of the EXor variable V1118 Ori, performed by
following a slightly declining phase after a recent outburst. In particular,
the near-IR (0.8 - 2.3 micron) spectrum, obtained for the first time, shows a
large variety of emission features of the HI and HeI recombination and CO
overtone. By comparing the observed spectrum with a wind model, a mass loss
rate value is derived along with other parameters whose values are typical of
an accreting T Tauri star. In addition, we have used X-ray data from the XMM
archive, taken in two different epochs during the declining phase monitored in
IR. X-ray emission (in the range 0.5 - 10 keV) permits to derive several
parameters which confirm the T Tauri nature of the source. In the near-IR the
object maintains a low visual extinction during all the activity phases,
confirming that variable extinction does not contribute to brightness
variations. The lack of both a significant amount of circumstellar material and
any evidence of IR cooling from collimated jet/outflow driven by the source,
indicates that, at least this member of the EXor class, is in a late stage of
the Pre-Main Sequence evolution. In the X-ray regime, an evident fading is
present, detected in the post-outburst phase, that cannot be reconciled with
the presence of any absorbing material. This circumstance, combined with the
persistence (in the pre- and post-outburst phases) of a temperature component
at about 10 MK, suggests that accretion has some influence in regulating the
coronal activity
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
Application of double-pulse micro-LIBS 3D compositional mapping to the analysis of ceramics
We developed a new Laser-Induced Breakdown Spectroscopy (LIBS) instrument for 3D compositional mappings of archaeological objects. The system, based on the Modì double-pulse instrument, allows the reconstruction of maps with lateral resolution up to 20 microns and sub-micron depth resolution
Brain-derived neurotrophic factor association with amygdala response in major depressive disorder
Background: Brain-derived neurotrophic factor (BDNF) has an essential role in synaptic plasticity and neurogenesis. BDNF mediates amygdala-dependent learning for both aversive and appetitive emotional memories. The expression of BDNF in limbic regions is posited to contribute the development of depression, and amygdala responsivity is a potential marker of depressive state.
Methods: The present study examined the relationship between platelet BDNF levels and amygdala volume and function in major depressive disorder (MDD). Participants were 23 MDD (mean age 38.9 years) and 23 healthy controls (mean age 38.8 years). All participants were recruited from the community. MDD participants were in a current depressive episode of moderate severity and medication-free. Amygdala responses were acquired during a functional MRI task of implicit emotional processing with sad facial expressions.
Results: Significant correlation was observed between platelet BDNF levels and left amygdala responses, but no significant correlations were found with right amygdala responses or with amygdala volumes.
Limitations: Interactions with neuroprotective as well as neurotoxic metabolites in the kyneurenine pathway were not examined.
Conclusions: Relationship between BDNF levels and amygdala responsivity to emotionally salient stimuli in MDD could reflect the importance of BDNF in amygdala-dependent learning with clinical implications for potential pathways for treatment
A long-lasting quiescence phase of the eruptive variable V1118 Ori
V1118 Ori is an eruptive variable belonging to the EXor class of Pre-Main
Sequence stars whose episodic outbursts are attributed to disk accretion
events. Since 2006, V1118 Ori is in the longest quiescence stage ever observed
between two subsequent outbursts of its recent history. We present
near-infrared photometry of V1118 Ori carried out during the last eight years,
along with a complete spectroscopic coverage from 0.35 to 2.5 um. A longterm
sampling of V1118 Ori in quiescence has never been done, hence we can benefit
from the current circumstance to determine the lowest values (i.e. the zeroes)
of the parameters to be used as a reference for evaluating the physical changes
typical of more active phases. A quiescence mass accretion rate between 1--3
10 M_{\sun} yr can be derived and the difference with
previous determinations is discussed. From line emission and IR colors analysis
a visual extinction of 1-2 mag is consistently derived, confirming that V1118
Ori (at least in quiescence) is a low-extinction T Tauri star with a bolometric
luminosity of about 2.1 L_{\sun}. An anti-correlation exists between the
equivalent width of the emission lines and the underlying continuum. We
searched the literature for evaluating whether or not such a behaviour is a
common feature of the whole class. The anti-correlation is clearly recognizable
for all the available EXors in the optical range (H and H
lines), while it is not as much evident in the infrared (Pa and
Br lines). The observed anti-correlation supports the accretion-driven
mechanism as the most likely to account for continuum variations.Comment: 6 figures, 5 tables, accepted on Ap
- …