1,233 research outputs found
Line Structure in the Spectrum of FU Orionis
New high-resolution spectra of FU Ori, obtained with the HIRES spectrograph
at the Keck I telescope in 2003-2006, make it possible to compare the optical
line profiles with those predicted by the self-luminous accretion disk model. A
dependence of line width on excitation potential and on wavelength, expected
for a Keplerian disk, is definitely not present in the optical region, nor is
the line duplicity due to velocity splitting. The absorption lines observed in
the optical region of FU Ori must originate in or near the central object, and
here their profiles are shown to be those expected of a rigidly rotating
object. They can be fitted by a rapidly rotating (v sin i = 70 km/s)
high-luminosity G-type star having a large dark polar spot, with axis inclined
toward the line of sight. Over these years, the radial velocity of FU Ori has
remained constant to within +/-0.3 km/s, so there is no indication that the
star is a spectroscopic binary. These results apply to the optical region
( \AA); more distant, cooler regions of the disk contribute in
the infrared.Comment: 14 pages, 11 figures, accepted by A
The large amplitude outburst of the young star HBC 722 in NGC 7000/IC 5070, a new FU Orionis candidate
We report the discovery of a large amplitude outburst from the young star HBC
722 (LkHA 188 G4) located in the region of NGC 7000/IC 5070. On the basis of
photometric and spectroscopic observations, we argue that this outburst is of
the FU Orionis type. We gathered photometric and spectroscopic observations of
the object both in the pre-outburst state and during a phase of increase in its
brightness. The photometric BVRI data (Johnson-Cousins system) that we present
were collected from April 2009 to September 2010. To facilitate transformation
from instrumental measurements to the standard system, fifteen comparison stars
in the field of HBC 722 were calibrated in the BVRI bands. Optical spectra of
HBC 722 were obtained with the 1.3-m telescope of Skinakas Observatory (Crete,
Greece) and the 0.6-m telescope of Schiaparelli Observatory in Varese (Italy).
The pre-outburst photometric and spectroscopic observations of HBC 722 show
both low amplitude photometric variations and an emission-line spectrum typical
of T Tau stars. The observed outburst started before May 2010 and reached its
maximum brightness in September 2010, with a recorded Delta V~4.7 mag.
amplitude. Simultaneously with the increase in brightness the color indices
changed significantly and the star became appreciably bluer. The light curve of
HBC 722 during the period of rise in brightness is similar to the light curves
of the classical FUors - FU Ori and V1057 Cyg. The spectral observations during
the time of increase in brightness showed significant changes in both the
profiles and intensity of the spectral lines. Only H alpha remained in
emission, while the H beta, Na I 5890/5896, Mg I triplet 5174, and Ba II
5854/6497 lines were in strong absorption.Comment: 4 pages, 6 figures, accepted for publication in A&
Optical spectroscopy of EX Lupi during quiescence and outburst: Infall, wind, and dynamics in the accretion flow
We explore the accretion mechanisms in EX Lupi, prototype of EXor variables,
during its quiescence and outburst phases. We analyse high-resolution optical
spectra taken before, during, and after its 2008 outburst. In quiescence and
outburst, the star presents many permitted emission lines, including typical
CTTS lines and numerous neutral and ionized metallic lines. During the
outburst, the number of emission lines increases to over a thousand, with
narrow plus broad component structure (NC+BC). The BC profile is highly
variable on short timescales (24-72h). An active chromosphere can explain the
metallic lines in quiescence and the outburst NC. The dynamics of the BC line
profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform
accretion column that suffers velocity variations along the line-of-sight on
timescales of days. Assuming Keplerian rotation, the emitting region would be
located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity
profiles of the lines reveal a combination of rotation and infall. Line ratios
of ions and neutrals can be reproduced with a temperature of T~6500 K for
electron densities of a few times 10cm in the line-emitting
region. The data confirm that the 2008 outburst was an episode of increased
accretion, albeit much stronger than previous EX Lupi and typical EXors
outbursts. The line profiles are consistent with the infall/rotation of a
non-axisymmetric structure that could be produced by clumpy accretion during
the outburst phase. A strong inner disk wind appears in the epochs of higher
accretion. The rapid recovery of the system after the outburst and the
similarity between the pre-outburst and post-outburst states suggest that the
accretion channels are similar during the whole period, and only the accretion
rate varies, providing a superb environment for studying the accretion
processes.Comment: 15 pages plus 26 pages online material, accepted by A&
Profiles of Strong Permitted Lines in Classical T Tauri Stars
We present a spectral analysis of 30 T Tauri stars observed with the Hamilton
echelle spectrograph over more than a decade. One goal is to test
magnetospheric accretion model predictions. Observational evidence previously
published supporting the model, such as emission line asymmetry and a high
frequency of redshifted absorption components, are considered. We also discuss
the relation between different line forming regions and search for good
accretion rate indicators.
In this work we confirm several important points of the models, such as the
correlation between accretion and outflow, broad emission components that are
mostly central or slightly blueshifted and only the occasional presence of
redshifted absorption. We also show, however, that the broad emission
components supposedly formed in the magnetospheric accretion flow only
partially support the models. Unlike the predictions, they are sometimes
redshifted, and are mostly found to be symmetric. The published theoretical
profiles do not have a strong resemblance to our observed ones. We emphasize
the need for accretion models to include a strong turbulent component before
their profiles will match the observations. The effects of rotation, and the
outflow components, will also be needed to complete the picture.Comment: 25 pages including 9 figures, 3 tables, accepted for publication in
the Astronomical Journa
EX Lupi in quiescence
EX Lup is the prototype of EXors, a subclass of low-mass pre-main sequence
stars whose episodic eruptions are attributed to temporarily increased
accretion. In quiescence the optical and near-infrared properties of EX Lup
cannot be distinguished from those of normal T Tau stars. Here we investigate
whether it is the circumstellar disk structure which makes EX Lup an atypical
Class II object. During outburst the disk might undergo structural changes. Our
characterization of the quiescent disk is intended to serve as a reference to
study the physical changes related to one of EX Lupi's strongest known
eruptions in 2008 Jan-Sep. We searched the literature for photometric and
spectroscopic observations including ground-based, IRAS, ISO and Spitzer data.
After constructing the optical-infrared spectral energy distribution (SED), we
compared it with the typical SEDs of other young stellar objects and modeled it
using the Monte Carlo radiative transfer code RADMC. A mineralogical
decomposition of the 10 micron silicate emission feature and also the
description of the optical and near-infrared spectra were performed. The SED is
in general similar to that of a typical T Tauri star, though above 7 micron EX
Lup emits higher flux. The quiescent phase data suggest low level variability
in the optical-mid-infrared domain. Integrating the optical and infrared fluxes
we derived a bolometric luminosity of 0.7 L_Sun. The 10 micron silicate profile
could be fitted by a mixture consisting of amorphous silicates, no crystalline
silicates were found. A modestly flaring disk model with a total mass of 0.025
M_Sun and an outer radius of 150 AU was able to reproduce the observed SED. The
derived inner radius of 0.2 AU is larger than the sublimation radius, and this
inner gap sets EX Lup aside from typical T Tauri stars.Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 6
figure
V1647 ORIONIS: Keck/Nirspec 2 MICRON Echelle Observations
We present new Keck II NIRSPEC high-spectral resolution 2 um echelle
observations of the young eruptive variable star V1647 Orionis. This star went
into outburst in late 2003 and faded to its pre-outburst brightness after
approximately 26 months. V1647 Orionis is the illuminating star of McNeil's
Nebula and is located near M 78 in the Lynds 1630 dark cloud. Our spectra have
a resolving power of approximately 18,000 and allow us to study in detail the
weak absorption features present on the strong near-IR veiled continuum. An
analysis of the echelle orders containing Mg I (2.1066 um) and Al I (2.1099
um), Br-gamma (2.1661 um), the Na I doublet (2.206 and 2.209 um), and the CO
overtone bandhead (2.2935 um) gives us considerable information on the physical
and geometric characteristics of the regions producing these spectral features.
We find that, at high-spectral resolution, V1647 Orionis in quiescence
resembles a significant number of FU Orionis type eruptive variables and does
not appear similar to the quiescent EX Lupi variables observed. This
correspondence is discussed and implications for the evolutionary state of the
star are considered.Comment: 37 pages, 3 Tables, 17 Figure
Observation of Feshbach-like resonances in collisions between ultracold molecules
We observe magnetically tuned collision resonances for ultracold Cs2
molecules stored in a CO2-laser trap. By magnetically levitating the molecules
against gravity, we precisely measure their magnetic moment. We find an avoided
level crossing which allows us to transfer the molecules into another state. In
the new state, two Feshbach-like collision resonances show up as strong
inelastic loss features. We interpret these resonances as being induced by Cs4
bound states near the molecular scattering continuum. The tunability of the
interactions between molecules opens up novel applications such as controlled
chemical reactions and synthesis of ultracold complex molecules
Ab initio explanation of disorder and off-stoichiometry in Fe-Mn-Al-C kappa carbides
Carbides play a central role for the strength and ductility in many
materials. Simulating the impact of these precipitates on the mechanical
performance requires the knowledge about their atomic configuration. In
particular, the C content is often observed to substantially deviate from the
ideal stoichiometric composition. In the present work, we focus on Fe-Mn-Al-C
steels, for which we determined the composition of the nano-sized kappa
carbides (Fe,Mn)3AlC by atom probe tomography (APT) in comparison to larger
precipitates located in grain boundaries. Combining density functional theory
with thermodynamic concepts, we first determine the critical temperatures for
the presence of chemical and magentic disorder in these carbides. Secondly, the
experimentally observed reduction of the C content is explained as a compromise
between the gain in chemical energy during partitioning and the elastic strains
emerging in coherent microstructures
- …