571 research outputs found
Debris Disks in Nearby Young Moving Groups in the ALMA Era
Many members of nearby young moving groups exhibit infrared excess attributed
to circumstellar debris dust, formed via erosion of planetesimals. With their
proximity and well-dated ages, these groups are excellent laboratories for
studying the early evolution of debris dust and of planetesimal belts. ALMA can
spatially resolve the disk emission, revealing the location and extent of these
belts, putting constraints on planetesimal evolution models, and allowing us to
study planet-disk interactions. While the main trends of dust evolution in
debris disks are well-known, there is almost no information on the evolution of
gas. During the transition from protoplanetary to debris state, even the origin
of gas is dubious. Here we review the exciting new results ALMA provided by
observing young debris disks, and discuss possible future research directions.Comment: To appear in "Young Stars and Planets Near the Sun", Proceedings of
IAU Symposium No. 314 (Cambridge University Press), J.H. Kastner, B. Stelzer,
S.A. Metchev, ed
The intermediate-mass star-forming region Lynds 1340. An optical view
We have performed an optical spectroscopic and photometric search for young
stellar objects associated with the molecular cloud Lynds 1340, and examined
the structure of the cloud by constructing an extinction map, based on SDSS
data. The new extinction map suggests a shallow, strongly fragmented cloud,
having a mass of some 3700~Msun. Longslit spectroscopic observations of the
brightest stars over the area of L1340 revealed that the most massive star
associated with L1340 is a B4 type, about 5 solar mass star. The new
spectroscopic and photometric data of the intermediate mass members led to a
revised distance of 825 (+110 /-80) pc, and revealed seven members of the young
stellar population with M > 2 solar masses. Our search for H alpha emission
line stars, conducted with the Wide Field Grism Spectrograph 2 on the 2.2-meter
telescope of the University of Hawaii and covering a 30 arcmin x 40 arcmin
area, resulted in the detection of 75 candidate low-mass pre-main sequence
stars, 58 of which are new. We constructed spectral energy distributions of our
target stars, based on SDSS, 2MASS, Spitzer, and WISE photometric data, derived
their spectral types, extinctions, and luminosities from BVRIJ fluxes,
estimated masses by means of pre-main sequence evolutionary models, and
examined the disk properties utilizing the 2-24 micron interval of the spectral
energy distribution. We measured the equivalent width of the H alpha lines and
derived accretion rates. The optically selected sample of pre-main sequence
stars has a median effective temperature of 3970 K, stellar mass 0.7 Msun, and
accretion rate of 7.6 10^{-9} Msun/yr.Comment: 47 pages, 15 figures, 10 tables; accepted for publication in ApJ,
typos correcte
„Pontállandósítás” az égen - milyen kvazárok alkalmasak az égi vonatkoztatási rendszer kijelölésére?
Röviden bemutatjuk a nemzetközi égi vonatkoztatási rendszert (ICRF) és összefoglaljuk, hogy miért
a rádiósugárzó aktív galaxismagok (kvazárok) a legalkalmasabbak a rendszer definiálására. Szólunk
a kvazárok pozíciós stabilitásáról. Megvizsgáljuk, hogy az összefüggésben lehet-e a kvazárok
ezred-ívmásodperces szögskálán, nagyon hosszú bázisvonalú rádió-interferometriás (VLBI) mérésekkel
megfigyelt szerkezetével
Cold CO gas in the envelopes of FU Orionis-type young eruptive stars
FUors are young stellar objects experiencing large optical outbursts due to
highly enhanced accretion from the circumstellar disk onto the star. FUors are
often surrounded by massive envelopes, which play a significant role in the
outburst mechanism. Conversely, the subsequent eruptions might gradually clear
up the obscuring envelope material and drive the protostar on its way to become
a disk-only T Tauri star. Here we present an APEX CO and CO
survey of eight southern and equatorial FUors. We measure the mass of the
gaseous material surrounding our targets. We locate the source of the CO
emission and derive physical parameters for the envelopes and outflows, where
detected. Our results support the evolutionary scenario where FUors represent a
transition phase from envelope-surrounded protostars to classical T Tauri
stars.Comment: 5 pages, 3 figures, accepted for publication in the Ap
PACS photometer calibration block analysis
The absolute stability of the PACS bolometer response over the entire mission
lifetime without applying any corrections is about 0.5% (standard deviation) or
about 8% peak-to-peak. This fantastic stability allows us to calibrate all
scientific measurements by a fixed and time-independent response file, without
using any information from the PACS internal calibration sources. However, the
analysis of calibration block observations revealed clear correlations of the
internal source signals with the evaporator temperature and a signal drift
during the first half hour after the cooler recycling. These effects are small,
but can be seen in repeated measurements of standard stars. From our analysis
we established corrections for both effects which push the stability of the
PACS bolometer response to about 0.2% (stdev) or 2% in the blue, 3% in the
green and 5% in the red channel (peak-to-peak). After both corrections we still
see a correlation of the signals with PACS FPU temperatures, possibly caused by
parasitic heat influences via the Kevlar wires which connect the bolometers
with the PACS Focal Plane Unit. No aging effect or degradation of the
photometric system during the mission lifetime has been found.Comment: 15 pages, accepted for publication in Experimental Astronom
Long-term photometry of three active red giants in close binary systems: V2253 Oph, IT Com and IS Vir
We present and analyze long-term optical photometric measurements of the
three active stars V2253 Oph, IT Com and IS Vir. All three systems are
single-lined spectroscopic binaries with an early K giant as primary component
but in different stages of orbital-rotational synchronization. Our photometry
is supplemented by 2MASS and WISE near-IR and mid-IR magnitudes and then used
to obtain more accurate effective temperatures and extinctions. For V2253 Oph
and IT Com, we found their spectral energy distributions consistent with pure
photospheric emission. For IS Vir, we detect a marginal mid-IR excess which
hints towards a dust disk. The orbital and rotational planes of IT Com appear
to be coplanar, contrary to previous findings in the literature. We apply a
multiple frequency analysis technique to determine photometric periods, and
possibly changes of periods, ranging from days to decades. New rotational
periods of 21.55+-0.03d, 65.1+-0.3d, and 23.50+-0.04d were determined for V2253
Oph, IT Com, and IS Vir, respectively. Splitting of these periods led to
tentative detections of differential surface rotations of delta P/P ~0.02 for
V2253 Oph and 0.07 for IT Com. Using a time-frequency technique based on
short-term Fourier transforms we present evidence of cyclic light variations of
length ~10yrs for V2253 Oph and 5-6yrs for IS Vir. A single flip-flop event has
been observed for IT Com of duration 2-3yrs. Its exchange of the dominant
active longitude had happened close to a time of periastron passage, suggesting
some response of the magnetic activity from the orbital dynamics. The 21.55-d
rotational modulation of V2253 Oph showed phase coherence also with the orbital
period, which is 15 times longer than the rotational period, thus also
indicating a tidal feedback with the stellar magnetic activity.Comment: 13 pages, 14 figures, accepted to A
Long-term evolution of FU Orionis objects at infrared wavelengths
We investigate the brightness evolution of 7 FU Orionis systems in the 1-100
micrometer wavelength range using data from the Infrared Space Observatory
(ISO). The ISO measurements were supplemented with 2MASS and MSX observations
performed in the same years as the ISO mission (1995-98). The spectral energy
distributions (SEDs) based on these data points were compared with earlier ones
derived from the IRAS photometry as well as from ground-based observations
carried out around the epoch 1983. In 3 cases (Z CMa, Parsamian 21, V1331 Cyg)
no difference between the two epochs was seen within the measurement
uncertainties. V1057 Cyg, V1515 Cyg and V1735 Cyg have become fainter at
near-infrared wavelengths while V346 Nor has become slightly brighter. V1057
Cyg exhibits a similar flux change also in the mid-infrared. At lambda >= 60
micrometer most of the sources remained constant; only V346 Nor seems to fade.
Our data on the long-term evolution of V1057 Cyg agree with the model
predictions of Kenyon & Hartmann (1991) and Turner et al. (1997) at near- and
mid-infrared wavelengths, but disagree at lambda > 25 micrometer. We discuss if
this observational result at far-infrared wavelengths could be understood in
the framework of the existing models.Comment: 9 pages, 3 figures, to be published in Astronomy & Astrophysic
Dense cores in the dark cloud complex LDN1188
We present a molecular line emission study of the LDN1188 dark cloud complex
located in Cepheus. In this work we focused on the densest parts of the cloud
and on the close neighbourhood of infrared point sources. We made ammonia
mapping with the Effelsberg 100-m radio telescope and identified 3 dense cores.
CS(1--0), CS(2--1) and HCO(1--0) measurements performed with the Onsala
20\,m telescope revealed the distribution of dense molecular material. The
molecular line measurements were supplemented by mapping the dust emission at
1.2\,mm in some selected directions using the IRAM 30\,m telescope. With these
data we could work out a likely evolutionary sequence in this dark clould
complex.Comment: YouResAstro2012 conference presentation; accepted to Astronomishen
Nachrichten (25-July-2013
Magnitude-range brightness variations of overactive K giants
We study three representative, overactive spotted K giants (IL Hya, XX Tri,
and DM UMa) known to exhibit V-band light variations between 0.65-1.05 mags.
Our aim is to find the origin of their large brightness variation. We employ
long-term phase-resolved multicolor photometry, mostly from automatic
telescopes, covering 42 yr for IL Hya, 28 yr for XX Tri, and 34 yr for DM UMa.
For one target, IL Hya, we present a new Doppler image from NSO data taken in
late 1996. Effective temperatures for our targets are determined from all
well-sampled observing epochs and are based on a V-I_C color-index calibration.
The effective temperature change between the extrema of the rotational
modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux
during maximum of the rotational modulation, i.e., the least spotted states,
varied by up to 39% in IL Hya and up to 54% in XX Tri over the course of our
observations. We emphasize that for IL Hya this is just about half of the total
luminosity variation that can be explained by the photospheric temperature
(spots/faculae) changes, while for XX Tri it is even about one third. The
long-term, 0.6 mag V-band variation of DM UMa is more difficult to explain
because little or no B-V color index change is observed on the same timescale.
Placing the three stars with their light and color variations into H-R
diagrams, we find that their overall luminosities are generally too low
compared to predictions from current evolutionary tracks. A change in the
stellar radius due to strong and variable magnetic fields during activity
cycles likely plays a role in explaining the anomalous brightness and
luminosity of our three targets. At least for IL Hya, a radius change of about
9% is suggested from m_bol and T_eff, and is supported by independent vsin(i)
measurements.Comment: 13 pages, 8 figures, accepted in A&
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