113 research outputs found
IUE observations of young variables
A number of far UV spectrograms of young variable stars have become available through observations with the IUE satellite. Several T Tauri stars and Herbig type Be- and Ae-stars in dark nebulae were observed and spectrograms of a total of 17 stars, which were considered to be very young pre-main-sequence stars, were collected. In many of these cases only part of the spectral region available at the IUE is covered and there are examples where large spectral regions are severely underexposed. With a few exceptions, the stars have been observed with the low resolution cameras, providing spectra resolutions of 6 to 7 A
Periodic radial velocity variations in RU Lupi
Context. RU Lup is a Classical T Tauri star with unusually strong emission
lines, which has been interpreted as manifestations of accretion. Recently,
evidence has accumulated that this star might have a variable radial velocity.
Aims. We intended to investigate in more detail the possible variability in
radial velocity using a set of 68 high-resolution spectra taken at the VLT
(UVES), the AAT (UCLES) and the CTIO (echelle). Methods. Using standard
cross-correlation techniques, we determined the radial velocity of RU Lup. We
analysed these results with Phasedispersion minimization and the Lomb-Scargle
periodogram and searched for possible periodicities in the obtained radial
velocities. We also analysed changes in the absorption line shapes and the
photometric variability of RU Lup. Results. Our analysis indicated that RU Lup
exhibits variations in radial velocity with a periodicity of 3.71 days and an
amplitude of 2.17 km/s. These variations can be explained by the presence of
large spots, or groups of spots, on the surface of RU Lup. We also considered a
low-mass companion and stellar pulsations as alternative sources for these
variations but found these to be unlikely.Comment: 8 pages, 4 figures, Accepted by A&
Rosette Globulettes and Shells in the Infrared
Tiny, dense clumps of sub-solar mass called globulettes form in giant
galactic HII regions. The young central clusters compress the surrounding
molecular shells which break up into clumps, filaments, and elephant trunks
that interact with UV light from the central OB stars. We study the nature of
the infrared emission and extinction in the shell and globulettes in the
Rosette Nebula (RN) and search for associated newborn stars. We imaged the
northwestern quadrant of the RN in the near-infrared (NIR) through JHKs and
narrow-band H2 1-0 S(1), Pbeta and continuum filters. NIR images were used to
study the surface brightness of the globulettes and associated bright rims. NIR
photometry was used to create an extinction map and to search for NIR excess
objects. Archival images from Spitzer IRAC and MIPS 24 and Herschel PACS
observations were used to further study the region and its stellar population
and to examine the structure of the shell and trunks. The globulettes and
elephant trunks have bright rims in the Ks band on the sides facing the central
cluster. Analysis of 21 globulettes where surface brightness in the H2 1-0 S(1)
line is detected shows that about a third of the surface brightness observed in
Ks is due to this line: the observed average of the H2/Ks surface brightness is
0.26+-0.02 in the globulettes cores and 0.30+-0.01 in the rims. The estimated
H2 1-0 S(1) surface brightness of the rims is 3-8*10^{-8}
Wm^{-2}sr^{-1}um^{-1}. The H2/Ks surface brightness ratio supports fluorescence
as the H2 excitation mechanism. The globulettes have number densities of
n(H2)~10^{-4} cm^{-3} or higher. We confirm the results from previous optical
and CO surveys that the larger globulettes contain very dense cores and dense
envelopes, and that their masses are sub-solar. Two NIR protostellar objects
were found in an elephant trunk and one in the most massive globulette in our
study. (abridged)Comment: Accepted for publication in A&A. 24 pages, 27 figures. JHKs
photometry will be available electronicall
Rosette nebula globules: Seahorse giving birth to a star
The Rosette Nebula is an HII region ionized mainly by the stellar cluster NGC
2244. Elephant trunks, globules, and globulettes are seen at the interface
where the HII region and the surrounding molecular shell meet. We have observed
a field in the northwestern part of the Rosette Nebula where we study the small
globules protruding from the shell. Our aim is to measure their properties and
study their star formation history in continuation of our earlier study of the
features of the region. We imaged the region in broadband near-infrared (NIR)
JsHKs filters and narrowband H2 1-0 S(1), P, and continuum filters using
the SOFI camera at the ESO/NTT. The imaging was used to study the stellar
population and surface brightness, create visual extinction maps, and locate
star formation. Mid-infrared (MIR) Spitzer IRAC and WISE and optical NOT images
were used to further study the star formation and the structure of the
globules. The NIR and MIR observations indicate an outflow, which is confirmed
with CO observations made with APEX. The globules have mean number densities of
~. P is seen in absorption in the cores of
the globules where we measure visual extinctions of 11-16 mag. The shell and
the globules have bright rims in the observed bands. In the Ks band 20 to 40%
of the emission is due to fluorescent emission in the 2.12 m H2 line
similar to the tiny dense globulettes we studied earlier in a nearby region. We
identify several stellar NIR excess candidates and four of them are also
detected in the Spitzer IRAC 8.0 m image and studied further. We find an
outflow with a cavity wall bright in the 2.124 m H2 line and at 8.0 m
in one of the globules. The outflow originates from a Class I young stellar
object (YSO) embedded deep inside the globule. An H image suggests the
YSO drives a possible parsec-scale outflow. (abridged)Comment: 20 pages, 19 figures, accepted for publication in Astronomy and
Astrophysics, figures reduced for astro-p
Another deep dimming of the classical T Tauri star RW Aur A
Context. RW Aur A is a classical T Tauri star (CTTS) with an unusually rich
emission line spectrum. In 2014 the star faded by ~ 3 magnitudes in the V band
and went into a long-lasting minimum. In 2010 the star suffered from a similar
fading, although less deep. These events in RW Aur A are very unusual among the
CTTS, and have been attributed to occultations by passing dust clouds. Aims. We
want to find out if any spectral changes took place after the last fading of RW
Aur A with the intention to gather more information on the occulting body and
the cause of the phenomenon. Methods. We collected spectra of the two
components of RW Aur. Photometry was made before and during the minimum.
Results. The overall spectral signatures reflecting emission from accretion
flows from disk to star did not change after the fading. However, blue-shifted
absorption components related to the stellar wind had increased in strength in
certain resonance lines, and the profiles and strengths, but not fluxes, of
forbidden lines had become drastically different. Conclusions. The extinction
through the obscuring cloud is grey indicating the presence of large dust
grains. At the same time, there are no traces of related absorbing gas. The
cloud occults the star and the interior part of the stellar wind, but not the
wind/jet further out. The dimming in 2014 was not accompanied by changes in the
accretion flows at the stellar surface. There is evidence that the structure
and velocity pattern of the stellar wind did change significantly. The dimmings
could be related to passing condensations in a tidally disrupted disk, as
proposed earlier, but we also speculate that large dust grains have been
stirred up from the inclined disk into the line-of-sight through the
interaction with an enhanced wind.Comment: 5 pages, 5 figures, Accepted for publication in A&
Unveiling extremely veiled T Tauri stars
Photospheric absorption lines in classical T Tauri stars (CTTS) are weak
compared to normal stars. This so-called veiling is normally identified with an
excess continuous emission formed in shock-heated gas at the stellar surface
below the accretion streams. We have selected four stars (RW Aur A, RU Lup, S
CrA NW and S CrA SE) with unusually strong veiling to make a detailed
investigation of veiling versus stellar brightness and emission line strengths
for comparisons to standard accretion models. We have monitored the stars
photometrically and spectroscopically at several epochs. In standard accretion
models a variable accretion rate will lead to a variable excess emission.
Consequently, the stellar brightness should vary accordingly. We find that the
veiling of absorption lines in these stars is strongly variable and usually so
large that it would require the release of several stellar luminosities of
potential energy. At states of very large line dilution, the correspondingly
large veiling factors derived correlate only weakly with brightness. Moreover,
the emission line strengths violate the expected trend of veiling versus line
strength. The veiling can change dramatically in one night, and is not
correlated with the phase of the rotation periods found for two stars. We show
that in at least three of the stars, when the veiling becomes high, the
photospheric lines become filled-in by line emission, which produces large
veiling factors unrelated to changes in any continuous emission from shocked
regions. We also consider to what extent extinction by dust and electron
scattering in the accretion stream may affect veiling measures in CTTS. We
conclude that the degree of veiling cannot be used as a measure of accretion
rates in CTTS with rich emission line spectra.Comment: Accepted for publication in A&A Letters. New language-edited version.
(4 pages, 3 figures
Modelling Circumbinary Gas Flows in Close T Tauri Binaries
Young close binaries open central gaps in the surrounding circumbinary
accretion disc, but the stellar components may still gain mass from gas
crossing through the gap. It is not well understood how this process operates
and how the stellar components are affected by such inflows. Our main goal is
to investigate how gas accretion takes place and evolves in close T Tauri
binary systems. In particular, we model the accretion flows around two close T
Tauri binaries, V4046 Sgr and DQ Tau, both showing periodic changes in emission
lines, although their orbital characteristics are very different. In order to
derive the density and velocity maps of the circumbinary material, we employ
two-dimensional hydrodynamic simulations with a locally isothermal equation of
state. The flow patterns become quasi-stable after a few orbits in the frame
co-rotating with the system. Gas flows across the circumbinary gap through the
co-rotating Lagrangian points, and local circumstellar discs develop around
both components. Spiral density patterns develop in the circumbinary disc that
transport angular momentum efficiently. Mass is preferentially channelled
towards the primary and its circumstellar disc is more massive than the disc
around the secondary. We also compare the derived density distribution to
observed line profile variability. The line profile variability tracing the gas
flows in the central cavity shows clear similarities with the corresponding
observed line profile variability in V4046 Sgr, but only when the local
circumstellar disc emission was excluded. Closer to the stars normal
magnetospheric accretion may dominate while further out the dynamic accretion
process outlined here dominates. Periodic changes in the accretion rates onto
the stars can explain the outbursts of line emission observed in eccentric
systems such as DQ Tau.Comment: Accepted for publication in MNRA
Radio observations of globulettes in the Carina nebula
Context. The Carina nebula hosts a large number of globulettes. An optical study of these tiny molecular clouds shows that the majority are of planetary mass, but there are also those with masses of several tens up to a few hundred Jupiter masses. Aims. We seek to search for, and hopefully detect, molecular line emission from some of the more massive objects; in case of successful detection we aim to map their motion in the Carina nebula complex and derive certain physical properties. Methods. We carried out radio observations of molecular line emission in (CO)-C-12 and (CO)-C-13 (2-1) and (3-2) of 12 globulettes in addition to positions in adjacent shell structures using APEX. Results. All selected objects were detected with radial velocities shifted relative to the emission from related shell structures and background molecular clouds. Globulettes along the western part of an extended dust shell show a small spread in velocity with small velocity shifts relative to the shell. This system of globulettes and shell structures in the foreground of the bright nebulosity surrounding the cluster Trumpler 14 is expanding with a few km s(-1) relative to the cluster. A couple of isolated globulettes in the area move at similar speed. Compared to similar studies of the molecular line emission from globulettes in the Rosette nebula, we find that the integrated line intensity ratios and line widths are very different. The results show that the Carina objects have a different density/temperature structure than those in the Rosette nebula. In comparison the apparent size of the Carina globulettes is smaller, owing to the larger distance, and the corresponding beam filling factors are small. For this reason we were unable to carry out a more detailed modelling of the structure of the Carina objects in the way as performed for the Rosette objects. Conclusions. The Carina globulettes observed are compact and denser than objects of similar mass in the Rosette nebula. The distribution and velocities of these globulettes suggest that they have originated from eroding shells and elephant trunks. Some globulettes in the Trumpler 14 region are quite isolated and located far from any shell structures. These objects move at a similar speed as the globulettes along the shell, suggesting that they once formed from cloud fragments related to the same foreground shell.Peer reviewe
Hubble Space Telescope STIS spectrum of RW Aurigae A: Evidence for an ionized beltlike structure and mass ejection in timescales of a few hours
Profiles of the UV semiforbidden lines of C III](1908) and Si III](1892) of RW Aur have been obtained with the Hubble Space Telescope (HST) STIS. The C III](1908) profile shows two high-velocity components at upsilon = +/-170 km s(-1) and a central one. The Si III](1892) profile is very broad (FWHM = 293 km s(-1)), and the high-velocity components are unresolved. The high-velocity components are most probably produced in a rotating belt similar to that detected in other sources of bipolar outflows. A radius between 2.7 R-* and the corotation radius (6.1 R-*) is derived, and a log T-e(K) similar or equal to 4.7 and log n(e)(cm(-3)) = 11.6 are estimated. The belt is clumpy, and the most likely source of heating is local X-ray radiation, probably associated with the release of magnetic energy. In addition, profiles of the optical lines of He I, Fe II, and Halpha retrieved from the HST archive have been analyzed. The spectra were obtained by shifting the STIS slit between exposures in the transverse direction to the flow. Two features vary from one exposure to another: a blueshifted emission feature (detected in the Fe II and He I lines) and a redshifted absorption feature (detected in Halpha), which are observed at the velocity of the blueshifted and redshifted components of the jet, respectively. There is a clear-cut correlation between the equivalent widths of these two components. These components are not associated with the flaring activity of the source, since they remain stable timescales of similar or equal to40 minutes. They are most likely associated with variations in the stellar/circumstellar environment in timescales of a few (similar or equal to5) hours, consistent with reports by other authors for variations of the veiling and the appearance of shell signatures in the optical spectrum
ALMA detection of the dusty object silhouetted against the S0 galaxy NGC 3269 in the Antlia cluster
Context. An intriguing silhouette of a small dust patch can be seen against the disk of the S0 galaxy NGC 3269 in the Antlia cluster in optical images. The images do not provide any clue as to whether the patch is a local Jupiter mass-scale cloudlet or a large extragalactic dust complex.Aims. We aim to resolve the nature of this object: is it a small Galactic cloudlet or an extragalactic dust complex?Methods. ALMA and APEX spectroscopy and Gemini GMOS long-slit spectroscopy were used to measure the velocity of the patch and the NGC 3269 disk radial velocity curve.Results. A weak 16 2.5 km s(-1) wide (CO)-C-12(2-1) T-MB 19 +/- 2.5. mK line in a 2 .('') . '' 2 by 2 .('') .'' 12 beam associated with the object was detected with ALMA. The observed heliocentric velocity, V-r,V- hel=3878 +/- 5.0 km s(-1), immediately establishes the extragalactic nature of the object. The patch velocity is consistent with the velocity of the nucleus of NGC 3269, but not with the radial velocity of the NGC 3269 disk of the galaxy at its position. The similar to 4 '' angular size of the patch corresponds to a linear size of similar to 1 kpc at the galaxy's Hubble distance of 50.7 Mpc. The mass estimated from the (CO)-C-12(2-1) emission is similar to 1.4x10(6)(d/50.7 Mpc)M-2(circle dot), while the attenuation derived from the optical spectrum implies a dust mass of similar to 2.6x10(4)(d/50.7 Mpc)M-2(circle dot). The derived attenuation ratio A ' (B)/(A ' (B)-A ' (R)) of 1.6 +/- 0.11 is substantially lower than the corresponding value for the mean Milky Way extinction curve for point sources (2.3).Conclusions. We established the extragalactic nature of the patch, but its origin remains elusive. One possibility is that the dust patch is left over from the removal of interstellar matter in NGC 3269 through the interaction with its neighbour, NGC 3268.Peer reviewe
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