1,423 research outputs found
The Instability Strip for Pre--Main-Sequence Stars
We investigate the pulsational properties of Pre--Main-Sequence (PMS) stars
by means of linear and nonlinear calculations. The equilibrium models were
taken from models evolved from the protostellar birthline to the ZAMS for
masses in the range 1 to 4 solar masses. The nonlinear analysis allows us to
define the instability strip of PMS stars in the HR diagram. These models are
used to constrain the internal structure of young stars and to test
evolutionary models. We compare our results with observations of the best case
of a pulsating young star, HR~5999, and we also identify possible candidates
for pulsational variability among known Herbig Ae/Be stars which are located
within or close to the instability strip boundaries.Comment: 14 pages, three postscript figures, accepted for publication on the
Astrophysical Journal Letter
The Mid-Infrared Emitting Dust Around AB Aur
Using the Keck I telescope, we have obtained 11.7 micron and 18.7 micron
images of the circumstellar dust emission from AB Aur, a Herbig Ae star. We
find that AB Aur is probably resolved at 18.7 micron with an angular diameter
of 1.2" at a surface brightness of 3.5 Jy/arcsec^2. Most of the dust mass
detected at millimeter wavelengths does not contribute to the 18.7 micron
emission, which is plausibly explained if the system possesses a relatively
cold, massive disk. We find that models with an optically thick, geometrically
thin disk, surrounded by an optically thin spherical envelope fit the data
somewhat better than flared disk models.Comment: ApJ in press, 4 color figure
Transitional YSOs: Candidates from Flat-Spectrum IRAS Sources
We are searching for Young Stellar Objects (YSOs) near the boundary between
protostars and pre-main sequence objects, what we have termed transitional
YSOs. We have identified a sample of 125 objects as candidate transitional YSOs
on the basis of IRAS colors and optical appearance on DSS images. We find that
the majority of our objects are associated with star-forming regions,
confirming our expectation that the bulk of these are YSOs.
We present optical, near-IR and high-resolution IRAS images of 92 objects
accessible from the northern and 62 from the southern hemisphere. The objects
have been classified on the basis of their morphology and spectral index. Of
the 125 objects, 28 have a variety of characteristics very similar to other
transitional YSOs, while another 22 show some of these characteristics,
suggesting that these transitional YSOs are not as rare as predicted by theory.Comment: 4 pages, 3 figures, to appear in proc. 33rd ESLAB Symposium ``Star
Formation from the Small to the Large Scale'', eds. F. Favata et al., ESA
SP-44
Examining the T Tauri system with SPHERE
Context. The prototypical low-mass young stellar object, T Tauri, is a
well-studied multiple system with at least three components. Aims. We aim to
explore the T Tau system with the highest spatial resolution, study the time
evolution of the known components, and re-determine the orbital parameters of
the stars. Methods. Near-infrared classical imaging and integral field
spectrograph observations were obtained during the Science Verification of
SPHERE, the new high-contrast imaging facility at the VLT. The obtained FWHM of
the primary star varies between 0.050" and 0.059", making these the highest
spatial resolution near-infrared images of the T Tauri system obtained to date.
Results. Our near-infrared images confirm the presence of extended emission
south of T Tau Sa, reported in the literature. New narrow-band images show, for
the first time, that this feature shows strong emission in both the Br-{\gamma}
and H2 1-0 S(1) lines. Broadband imaging at 2.27 {\mu}m shows that T Tau Sa is
0.92 mag brighter than T Tau Sb, which is in contrast to observations from Jan.
2014 (when T Tau Sa was fainter than Sb), and demonstrates that T Tau Sa has
entered a new period of high variability. The newly obtained astrometric
positions of T Tau Sa and Sb agree with orbital fits from previous works. The
orbit of T Tau S (the center of gravity of Sa and Sb) around T Tau N is poorly
constrained by the available observations and can be fit with a range of orbits
ranging from a nearly circular orbit with a period of 475 years to highly
eccentric orbits with periods up to 2.7*10^4 years. We also detected a feature
south of T Tau N, at a distance of mas, which shows the properties
of a new companion.Comment: Accepted by A&A Letter
ISO Spectroscopy of the Young Bipolar Nebulae S106 IR and Cep A East
We present the results of ISO SWS and LWS grating scans towards the embedded
Young Stellar Objects (YSOs) S106 IR and Cep A East. Emission from the pure
rotational lines of H2 and the infrared fine structure lines of [C II], [O I],
[S I], [Si II] and [Fe II], as well as absorption bands due to H2O, CO and CO2
ice were detected toward Cep A. In S106 we detected emission lines of H2, CO, H
I, and a large number of ionized species including Fe, O, N, C, Si, S, Ne and
Ar. S106 also shows many of the infrared PAH bands in emission. Excitation
temperatures and molecular hydrogen masses were derived from the low-lying pure
rotational levels of H2 and are 500 and 730 K and 8 and 3 x 10^{-3} solar
masses for S106 and Cep A, respectively. Since both objects are expected to
have several solar masses of H2 in their environment, we conclude that in both
cases the bulk of the H2 is cooler than a few hundred Kelvins. Excitation
temperatures and line ratios were compared with those predicted by theoretical
models for PDRs and dissociative and non-dissociative shocks. The [S I] 25.2
micron/[Si II] 34.8 micron ratio is a particularly useful shock versus PDR
discriminant and we conclude that S106 IR is dominated by PDR emission while
Cep A East has a large shock component. From an analysis of the ionic lines in
S106 we conclude that the central star must have a temperature around 37,000 K,
corresponding to a spectral type of O8. From its luminosity it is concluded
that the driving source of Cep A must also be a massive early-type star. The
absence of strong high-ionization ionic lines in its ISO spectrum shows that
Cep A has not yet created a significant H II region and must be younger than
S106, illustrating the process of the clearing of the surroundings of a massive
young star.Comment: 15 pages (including 10 figures), to appear in Astronomy &
Astrophysic
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