12 research outputs found
The Stellar Composition of the Star Formation Region CMa R1. II. Spectroscopic and Photometric Observations of 9 Young Stars
We present new high and low resolution spectroscopic and photometric data of
nine members of the young association CMa R1. All the stars have circumstellar
dust at some distance as could be expected from their association with
reflection nebulosity. Four stars (HD 52721, HD 53367, LkHalpha 220 and
LkHalpha 218) show Halpha emission and we argue that they are Herbig Be stars
with discs. Our photometric and spectroscopic observations on these stars
reveal new characteristics of their variability. We present first
interpretations of the variability of HD 52721, HD 53367 and the two LkHalpha
stars in terms of a partially eclipsing binary, a magnetic activity cycle and
circumstellar dust variations, respectively. The remaining five stars show no
clear indications of Halpha emission in their spectra, although their spectral
types and ages are comparable with those of HD 52721 and HD 53367. This
indicates that the presence of a disc around a star in CMa R1 may depend on the
environment of the star. In particular we find that all Halpha emission stars
are located at or outside the arc-shaped border of the H II region, which
suggests that the stars inside the arc have lost their discs through
evaporation by UV photons from nearby O stars, or from the nearby (< 25 pc)
supernova, about 1 Myr ago.Comment: 17 pages, 13 figures, accepted by MNRA
The Stellar Composition of the Star Formation Region CMa R1 -- III. A new outburst of the Be star component in Z CMa
We report on a recent event in which, after more than a decade of slowly
fading, the visual brightness of the massive young binary Z CMa suddenly
started to rise by about 1 magnitude in December 1999, followed by a rapid
decline to its previous brightness over the next six months. This behaviour is
similar to that exhibited by this system around its eruption in February 1987.
A comparison of the intrinsic luminosities of the system with recent
evolutionary calculations shows that Z CMa may consist of a 16 M_sun B0 IIIe
primary star and a ~ 3 M_sun FUOr secondary with a common age of ~ 3 x 10^5 yr.
We also compare new high-resolution spectra obtained in Jan. and Feb. 2000,
during the recent rise in brightness, with archive data from 1991 and 1996. The
spectra are rich in emission lines, which originate from the envelope of the
early B-type primary star. The strength of these emission lines increased
strongly with the brightness of Z CMa. We interpret the collected spectral data
in terms of an accretion disc with atmosphere around the Herbig B0e component
of Z CMa, which has expanded during the outbursts of 1987 and 2000. A high
resolution profile of the 6300 A [O I] emission line, obtained by us in March
2002 shows an increase in flux and a prominent blue shoulder to the feature
extending to ~ -700 km/s, which was much fainter in the pre-outburst spectra.
We propose that this change in profile is a result of a strong change in the
collimation of a jet, as a result of the outburst at the start of this century.Comment: 22 pages, 12 figures, accepted for publication in MNRA
A Photometric Catalogue of Southern Emission-Line Stars
We present a catalogue of previously unpublished optical and infrared
photometry for a sample of 162 emission-line objects and shell stars visible
from the southern hemisphere. The data were obtained between 1978 and 1997 in
the Walraven (WULBV), Johnson/Cousins (UBV(RI)_c) and ESO and SAAO
near-infrared (JHKLM) photometric systems. Most of the observed objects are
Herbig Ae/Be (HAeBe) stars or HAeBe candidates appearing in the list of HAeBe
candidates of Th\'e et al. (1994), although several B[e] stars, LBVs and T
Tauri are also included in our sample. For many of the stars the data presented
here are the first photo-electric measurements in the literature. The resulting
catalogue consists of 1809 photometric measurements. Optical variability was
detected in 66 out of the 116 sources that were observed more than once. 15 out
of the 50 stars observed multiple times in the infrared showed variability at
2.2 microns (K band).Comment: 42 pages, no figures, to appear in Astronomy & Astrophysic
Modeling of PMS Ae/Fe stars using UV spectra
Context.Spectral classification of AeFe stars, based on visual observations, may
lead to ambiguous conclusions.
Aims.
We aim to reduce these ambiguities by using UV spectra for the classification
of these stars, because the rise of the continuum in the UV is highly sensitive
to the stellar spectral type of A/F-type stars.
Methods.
We analyse the low-resolution UV spectra in terms of a 3-component model,
that consists of spectra of a central star, of an optically-thick accretion disc,
and of a boundary-layer between the disc and star.
The disc-component was calculated as a juxtaposition of Planck spectra,
while the 2 other components were simulated by the low-resolution UV spectra
of well-classified standard stars (taken from the IUE spectral atlases).
The hot boundary-layer shows strong similarities to the spectra of late-B
type supergiants (see Appendix A).
Results.
We modeled the low-resolution UV spectra of 37 AeFe stars.
Each spectral match provides 8 model parameters:
spectral type and luminosity-class of photosphere and boundary-layer,
temperature and width of the boundary-layer, disc-inclination and
circumstellar extinction.
From the results of these analyses, combined with available theoretical
PMS evolutionary tracks, we could estimate their masses and ages
and derive their mass-accretion rates.
For a number of analysed PMS stars we calculated the corresponding
SEDs and compared these with the observed SEDs.
Conclusions.
All stars (except βPic) show indications of accretion, that affect the
resulting spectral type of the stellar photosphere.
Formerly this led to ambiguities in classificaton of PMS stars
as the boundary-layer was not taken into consideration.
We give evidence for an increase of the mass-accretion rate with
stellar mass and for a decreases of this rate with stellar age