113 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
HD 34700: A new T Tauri double-lined spectroscopic binary
We find the star HD 34700 to be a double-lined spectroscopic binary. We also
identify it as a weak-line T Tauri object The spectra of both components are
very similar and both show the Li I feature at 6708 \AA. Strong arguments in
favour of the binary nature of the star as opposed to other possibilities are
offered. It is very likely that the companion is also a T Tauri star of similar
mass. A projected rotational velocity, sin, of 25 and 23 km/s has
been estimated for the blue and red components. We present a list of the lines
identified and the radial velocities of the two components on three spectra
obtained on consecutive nights.Comment: 4 pages, 5 figures, 2 tables (one electronic) Accepted for
publication in A&A Letter
Analysis of the circumstellar environment of the B[e] star HD 45677 (FS CMa)
We studied the circumstellar environment of the B[e] star HD 45677 through
the analysis of the emission lines from ionized metals. We used the statistical
approach of the self absorption curve method (SAC) to derive physical
parameters of the line emitting region. The Fe II and Cr II double-peaked
emission line structure is explained by the presence of a thin absorption
component red shifted by ~3 km/s. This absorption component can be interpreted
geometricaly as being due to infalling material perpendicularly to the disk
seen nearly pole-on, as indicated by the emission line structure. The Cr II and
Fe II emission lines have a complex structure with two (narrow and broad)
components, of 45 and 180 km/s FWHM for the permitted lines and 25 and 100 km/s
FWHM for the forbidden ones, respectively. We argue that the narrow components
are principaly emitted by an optically thin disk seen nearly pole-on, in a
region whose minimum radius is estimated to be 4 10^12 cm, while the broad ones
are formed in a disk-linked wind.Comment: 14 pages, submitted to Astronomy and Astrophysic
EXPORT: optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of the EXPORT
sample obtained at the 2.5m Nordic Optical Telescope. The database consists of
multi-epoch photo-polarimetry of 68 pre-main-sequence and main-sequence stars.
An investigation of the polarization variability indicates that 22 objects are
variable at the 3sigma level in our data. All these objects are pre-main
sequence stars, consisting of both T Tauri and Herbig Ae/Be objects while the
main sequence, Vega type and post-T Tauri type objects are not variable. The
polarization properties of the variable sources are mostly indicative of the
UXOR-type behaviour; the objects show highest polarization when the brightness
is at minimum. We add seven new objects to the class of UXOR variables (BH Cep,
VX Cas, DK Tau, HK Ori, LkHa 234, KK Oph and RY Ori). The main reason for their
discovery is the fact that our data-set is the largest in its kind, indicating
that many more young UXOR-type pre-main sequence stars remain to be discovered.
The set of Vega-like systems has been investigated for the presence of
intrinsic polarization. As they lack variability, this was done using indirect
methods, and apart from the known case of BD +31.643, the following stars were
found to be strong candidates to exhibit polarization due to the presence of
circumstellar disks: 51 Oph, BD +31.643C, HD 58647 and HD 233517.Comment: A&A accepte
Mobster: Accurate detection of mobile element insertions in next generation sequencing data
Mobile elements are major drivers in changing genomic architecture and can cause disease. The detection of mobile elements is hindered due to the low mappability of their highly repetitive sequences. We have developed an algorithm, called Mobster, to detect non-reference mobile element insertions in next generation sequencing data from both whole genome and whole exome studies. Mobster uses discordant read pairs and clipped reads in combination with consensus sequences of known active mobile elements. Mobster has a low false discovery rate and high recall rate for both L1 and Alu elements. Mobster is available at http://sourceforge.net/projects/mobster. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s13059-014-0488-x) contains supplementary material, which is available to authorized users
Accretion Rates in Herbig Ae stars
Accretion rates from disks around pre-main sequence stars are of importance
for our understanding of planetary formation and disk evolution. We provide in
this paper estimates of the mass accretion rates in the disks around a large
sample of Herbig Ae stars. We obtained medium resolution 2 micron spectra and
used the results to compute values of Macc from the measured luminosity of the
Br_gamma emission line, using a well established correlation between
L(Br_gamma) and the accretion luminosity Lacc. We find that 80% of the stars,
all of which have evidence of an associated circumstellar disk, are accreting
matter, with rates 3x10^{-9} < Macc} < 10^{-6} Msun/yr; for 7 objects, 6 of
which are located on the ZAMS in the HR diagram, we do not detect any line
emission. Few HAe stars (25%) have Macc>10^{-7} Msun/yr. In most HAe stars the
accretion rate is sufficiently low that the gas in the inner disk, inside the
dust evaporation radius, is optically thin and does not prevent the formation
of a puffed-up rim, where dust is directly exposed to the stellar radiation.
When compared to the Macc values found for lower-mass stars in the star forming
regions Taurus and Ophiuchus, HAe stars have on average higher accretion rates
than solar-mass stars; however, there is a lack of very strong accretors among
them, probably due to the fact that they are on average older.Comment: To be published in Astronomy & Astrophysic
A spectroscopic survey of Herbig Ae/Be stars with X-shooter – I. Stellar parameters and accretion rates
Herbig Ae/Be stars (HAeBes) span a key mass range that links low- and high-mass stars, and thus provide an ideal window from which to explore their formation. This paper presents Very Large Telescope/X-shooter spectra of 91 HAeBes, the largest spectroscopic study of HAeBe accretion to date. A homogeneous approach to determining stellar parameters is undertaken for the majority of the sample. Measurements of the ultraviolet are modelled within the context of magnetospheric accretion, allowing a direct determination of mass accretion rates. Multiple correlations are observed across the sample between accretion and stellar properties: the youngest and often most massive stars are the strongest accretors, and there is an almost 1:1 relationship between the accretion luminosity and stellar luminosity. Despite these overall trends of increased accretion rates in HAeBes when compared to classical T Tauri stars, we also find noticeable differences in correlations when considering the Herbig Ae and Herbig Be subsets. This, combined with the difficulty in applying a magnetospheric accretion model to some of the Herbig Be stars, could suggest that another form of accretion may be occurring within Herbig Be mass range
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
ISO spectroscopy of disks around Herbig Ae/Be stars
We have investigated the infrared spectra of all 46 Herbig Ae/Be stars for
which spectroscopic data is available in the ISO data archive. Our quantitative
analysis of these spectra focusses on the emission bands linked to polycyclic
aromatic hydrocarbons (PAHs), the amorphous 10 micron silicate band and the
crystalline silicate band at 11.3 micron. We have detected PAH emission in 57%
of the Herbig stars in our sample. Clear examples of differences in the PAH
spectra are present within our sample, indicating differences in PAH size,
chemistry and/or ionization. Amorphous silicate emission was detected in the
spectra of 52% of the sample stars, amorphous silicate absorption in 13%. We
have detected crystalline silicate emission in 11 stars (24% of our sample), of
which four (9%) also display strong PAH emission. We have classified the sample
sources according to the strength of their mid-IR energy distribution. The
systems with stronger mid-infared (20-100 um) excesses relative to their
near-infrared (1-5 um) excess display significantly more PAH emission than
those with weaker mid-infrared excesses. This provides strong observational
support for the disk models by Dullemond (2002), in which systems with a
flaring disk geometry display a strong mid-infrared excess, whereas those with
disks that are strongly shadowed by the puffed-up inner rim of the disk only
display modest amounts of mid-infrared emission. The PAH emission is expected
to be produced mainly in the part of the disk atmosphere that is directly
exposed to radiation from the central star. In this model, self-shadowed disks
should display weaker PAH emission than flared disks, consistent with our
observations.Comment: 27 pages, 26 figures, A&A accepted (22/06/2004
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