301 research outputs found
A representative sample of Be stars III: H band spectroscopy
We present H band (1.53 - 1.69 micron) spectra of 57 isolated Be stars of
spectral types O9-B9 and luminosity classes III,IV & V. The HI Brackett (n-4)
series is seen in emission from Br 11-18, and FeII emission is also apparent
for a subset of those stars with HI emission. No emission from species with a
higher excitation temperature, such as He II or CIII is seen, and no forbidden
line emission is present. A subset of 12 stars show no evidence for emission
from any species; these stars appear indistinguishable from normal B stars of a
comparable spectral type. In general the line ratios constructed from the
transitions in the range Br 11-18 do not fit case B recombination theory
particularly well. Strong correlations between the line ratios with Br-gamma
and spectral type are found. These results most likely represent systematic
variations in the temperature and ionization of the circumstellar disc with
spectral type. Weak correlations between the line widths and projected
rotational velocity of the stars are observed; however no systematic trend for
increasing line width through the Brackett series is observed.Comment: accepted for publication in A&A (this version correctly formatted
Physical conditions in the gas phases of the giant HII region LMC-N11 unveiled by Herschel - I. Diffuse [CII] and [OIII] emission in LMC-N11B
(Abridged) The Magellanic Clouds provide a nearby laboratory for metal-poor
dwarf galaxies. The low dust abundance enhances the penetration of UV photons
into the interstellar medium (ISM), resulting in a relatively larger filling
factor of the ionized gas. Furthermore, there is likely a hidden molecular gas
reservoir probed by the [CII]157um line. We present Herschel/PACS maps in
several tracers, [CII], [OI]63um,145um, [NII]122um, [NIII]57um, and [OIII]88um
in the HII region N11B in the Large Magellanic Cloud. Halpha and [OIII]5007A
images were used as complementary data to investigate the effect of dust
extinction. Observations were interpreted with photoionization models to infer
the gas conditions and estimate the ionized gas contribution to the [CII]
emission. Photodissociation regions (PDRs) are probed through polycyclic
aromatic hydrocarbons (PAHs). We first study the distribution and properties of
the ionized gas. We then constrain the origin of [CII]157um by comparing to
tracers of the low-excitation ionized gas and of PDRs. [OIII] is dominated by
extended emission from the high-excitation diffuse ionized gas; it is the
brightest far-infrared line, ~4 times brighter than [CII]. The extent of the
[OIII] emission suggests that the medium is rather fragmented, allowing far-UV
photons to permeate into the ISM to scales of >30pc. Furthermore, by comparing
[CII] with [NII], we find that 95% of [CII] arises in PDRs, except toward the
stellar cluster for which as much as 15% could arise in the ionized gas. We
find a remarkable correlation between [CII]+[OI] and PAH emission, with [CII]
dominating the cooling in diffuse PDRs and [OI] dominating in the densest PDRs.
The combination of [CII] and [OI] provides a proxy for the total gas cooling in
PDRs. Our results suggest that PAH emission describes better the PDR gas
heating as compared to the total infrared emission.Comment: Accepted for publication in Astronomy and Astrophysics. Fixed
inverted line ratio in Sect. 5.
Probing the mass-loss history of AGB and red supergiant stars from CO rotational line profiles. I. Theoretical model - Mass-loss history unravelled in VY CMa
Context: Mass loss plays a dominant role in the evolution of low mass stars
while they are on the Asymptotic Giant Branch (AGB). The gas and dust ejected
during this phase are a major source in the mass budget of the interstellar
medium. Recent studies have pointed towards the importance of variations in the
mass-loss history of such objects. Aims: By modelling the full line profile of
low excitation CO lines emitted in the circumstellar envelope, we can study the
mass-loss history of AGB stars. Methods: We have developed a non-LTE radiative
transfer code, which calculates the velocity structure and gas kinetic
temperature of the envelope in a self-consistent way. The resulting structure
of the envelope provides the input for the molecular line radiative
calculations which are evaluated in the comoving frame. The code allows for the
implementation of modulations in the mass-loss rate. This code has been
benchmarked against other radiative transfer codes and is shown to perform well
and efficiently. Results: We illustrate the effects of varying mass-loss rates
in case of a superwind phase. The model is applied to the well-studied case of
VY CMa. We show that both the observed integrated line strengths as the
spectral structure present in the observed line profiles, unambiguously
demonstrate that this source underwent a phase of high mass loss (~ 3.2E-4
Msun/yr) some 1000 yr ago. This phase took place for some 100 yr, and was
preceded by a low mass-loss phase (~ 1E-6 Msun/yr) taking some 800 yr. The
current mass-loss rate is estimated to be in the order of 8E-5 Msun/yr.
Conclusions: In this paper, we demonstrate that both the relative strength of
the CO rotational line profiles and the (non)-occurrence of spectral structure
in the profile offer strong diagnostics to pinpoint the mass-loss history.Comment: 18 pages, 10 figures, accepted for publication in "Astronomy &
Astrophysics
A milestone toward understanding PDR properties in the extreme environment of LMC-30Dor
More complete knowledge of galaxy evolution requires understanding the
process of star formation and interaction between the interstellar radiation
field and the interstellar medium in galactic environments traversing a wide
range of physical parameter space. Here we focus on the impact of massive star
formation on the surrounding low metallicity ISM in 30 Doradus in the Large
Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the
early universe, results in less ultra-violet shielding for the formation of the
molecular gas necessary for star formation to proceed. The half-solar
metallicity gas in this region is strongly irradiated by the super star cluster
R136, making it an ideal laboratory to study the structure of the ISM in an
extreme environment. Our spatially resolved study investigates the gas heating
and cooling mechanisms, particularly in the photo-dissociation regions where
the chemistry and thermal balance are regulated by far-ultraviolet photons (6
eV< h\nu <13.6 eV).
We present Herschel observations of far-infrared fine-structure lines
obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines
from Herschel and Spitzer observations with ground-based CO data to provide
diagnostics on the properties and the structure of the gas by modeling it with
the Meudon PDR code. We derive the spatial distribution of the radiation field,
the pressure, the size, and the filling factor of the photodissociated gas and
molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K
and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR
modeling. Assuming a plane-parallel geometry and a uniform medium, we find a
total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to
3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three
dimensional structure of the gas. (Abridged)Comment: 20 pages, 23 figures, accepted in A&
The effects of star formation on the low-metallicity ISM: NGC4214 mapped with Herschel/PACS spectroscopy
We present Herschel/PACS spectroscopic maps of the dwarf galaxy NC4214
observed in 6 far infrared fine-structure lines: [C II] 158mu, [O III] 88mu, [O
I] 63mu, [O I] 146mu, [N II] 122mu, and [N II] 205mu. The maps are sampled to
the full telescope spatial resolution and reveal unprecedented detail on ~ 150
pc size scales. We detect [C II] emission over the whole mapped area, [O III]
being the most luminous FIR line. The ratio of [O III]/[C II] peaks at about 2
toward the sites of massive star formation, higher than ratios seen in dusty
starburst galaxies. The [C II]/CO ratios are 20 000 to 70 000 toward the 2
massive clusters, which are at least an order of magnitude larger than spiral
or dusty starbursts, and cannot be reconciled with single-slab PDR models.
Toward the 2 massive star-forming regions, we find that L[CII] is 0.5 to 0.8%
of the LTIR . All of the lines together contribute up to 2% of LTIR . These
extreme findings are a consequence of the lower metallicity and young,
massive-star formation commonly found in dwarf galaxies. These conditions
promote large-scale photodissociation into the molecular reservoir, which is
evident in the FIR line ratios. This illustrates the necessity to move to
multiphase models applicable to star-forming clusters or galaxies as a whole.Comment: Accepted for publication in the A&A Herschel Special Issu
High Resolution Chandra Spectroscopy of Gamma Cassiopeia (B0.5IVe)
gamma Cas has long been famous for its unique hard X-ray characteristics. We
report herein on a 53 ks Chandra HETGS observation of this target. An
inspection of our spectrum shows that it is quite atypical for a massive star,
with abnormally weak Fe XXV, XXVI lines, Ly-alpha lines of H-like species from
Fe XVII, XXIII, XXIV, S XVI, Si XIV, Mg XII, Ne X, O VII, VIII, and N VII.
Also, line ratios of the rif-triplet of for a few He-like ions XVII are
consistent with the dominance of collisional atomic processes. Yet, the
presence of Fe and Si fluorescence K features indicates that photoionization
also occurs in nearby cold gas. The line profiles indicate a mean velocity at
rest and a broadening of 500 km/s. A global fitting analysis of the line and
continuum spectrum finds that there are 3-4 plasma emission components. The
dominant hot (12 keV) component and has a Fe abundance of 0.22 solar. Some
fraction of this component (10-30%) is heavily absorbed. The other 2-3
components, with temperatures 0.1, 0.4, 3 keV, are "warm," have a nearly solar
composition, a lower column absorption, and are responsible for most other
emission lines. The strength of the fluorescence features and the dual-column
absorption model for the hot plasma component suggest the presence near the hot
sites of a cold gas structure with a column density of 10^23 cm^-2. Since this
value is consistent with theoretical estimates of the vertical disk column of
this star, these attributes suggest that the X-rays originate near the star or
disk. It is possible that the Fe anomaly in the hot component is related to the
First Ionization Potential effect found in coronal structures around active
cool stars. This would be yet another indication that the X-rays -rays are
produced in the immediate vicinity of the Be star.Comment: 32 pages, 4 figures (Fig. 3 colorized.) To be published in 01/10/04
Astrophysical Journal, Main Journal; included figures and updated formattin
The 21 micron and 30 micron circumstellar dust features in evolved C-rich objects
The 21micron and 30micron bands are the strongest dust emission features
detected in evolved low- and intermediate-mass C-rich stars (i.e. asymptotic
giant branch [AGB] stars, proto-planetary nebulae [PPN], and planetary nebulae
[PN]). While the 21micron feature is rare and exists only in the transient PPN
phase, the 30micron feature is more common and seen in the entire late stage of
stellar evolution, from AGB to PPN and PN phases, as well as in the
low-metallicity galaxies: the Large Magellanic Cloud (LMC) and the Small
Magellanic Cloud (SMC). The carriers of these features remain unidentified.
Eleven of the twelve well-identified 21micron sources also emit in the 30micron
band, suggesting that their carriers may be somewhat related.Comment: 7 pages, 1 figure, uses eps.cls. Accepted for publication in "Earth,
Planets and Space" (special issue on Cosmic Dust
- …