454 research outputs found
Identification of red supergiants in nearby galaxies with mid-IR photometry
The role of episodic mass loss in massive star evolution is one of the most
important open questions of current stellar evolution theory. Episodic mass
loss produces dust and therefore causes evolved massive stars to be very
luminous in the mid-infrared and dim at optical wavelengths. We aim to increase
the number of investigated luminous mid-IR sources to shed light on the late
stages of these objects. To achieve this we employed mid-IR selection criteria
to identity dusty evolved massive stars in two nearby galaxies. The method is
based on mid-IR colors, using 3.6 {\mu}m and 4.5 {\mu}m photometry from
archival Spitzer Space Telescope images of nearby galaxies and J-band
photometry from 2MASS. We applied our criteria to two nearby star-forming dwarf
irregular galaxies, Sextans A and IC 1613, selecting eight targets, which we
followed up with spectroscopy. Our spectral classification and analysis yielded
the discovery of two M-type supergiants in IC 1613, three K-type supergiants
and one candidate F-type giant in Sextans A, and two foreground M giants. We
show that the proposed criteria provide an independent way for identifying
dusty evolved massive stars, that can be extended to all nearby galaxies with
available Spitzer/IRAC images at 3.6 {\mu}m and 4.5 {\mu}m.Comment: 8 pages, 4 figures, A&A in pres
Elemental abundances in the remnant of the ancient eruption of CK Vulpeculae
CK Vul or Nova 1670 is an enigmatic eruptive object which underwent a
stellar-merger event recorded by seventeenth-century observers. Its remnant was
recently recovered at submillimeter wavelengths, revealing gas of an
extraordinary isotopic composition indicative of past processing in the CNO
cycles and partial He burning. Here, we present long-slit optical spectra of
the remnant acquired with X-shooter at the Very Large Telescope at an
unprecedented sensitivity and spectral coverage. The spectra cover features of
key elements - including H, He, C, N, and O - at ionization degrees I-III. A
classical analysis of the spectra allows us to identify several
spatio-kinematic components in the probed part of the nebula at electron
temperatures of 10-15 kK and densities of 200-600 cm. We find that the
nebula is most likely excited by shocks rather than by direct radiation of the
stellar remnant. We provide a detailed analysis of the elemental abundances in
the remnant and find that helium is twice more abundant than in the Sun.
Nitrogen is also overabundant with a N/O ratio ten times larger than the solar
one. These anomalous abundances strongly indicate that the observed gas was
processed in CNO cycles of H burning, consistent with the submillimeter
studies. Additionally, sub-solar abundances of heavier elements, such as Ne, S,
and Ar, suggest that the progenitor of CK Vul was formed from material poorer
in metals than the Sun and was therefore an old stellar system before the 1670
eruption.Comment: 14 pages, accepted for publication in A&
The three-dimensional structure of the Eta Carinae Homunculus
We investigate, using the modeling code SHAPE, the three-dimensional
structure of the bipolar Homunculus nebula surrounding Eta Carinae, as mapped
by new ESO VLT/X-Shooter observations of the H2 micron
emission line. Our results reveal for the first time important deviations from
the axisymmetric bipolar morphology: 1) circumpolar trenches in each lobe
positioned point-symmetrically from the center and 2) off-planar protrusions in
the equatorial region from each lobe at longitudinal (~55 degrees) and
latitudinal (10-20 degrees) distances from the projected apastron direction of
the binary orbit. The angular distance between the protrusions (~110 degrees)
is similar to the angular extent of each polar trench (~130 degrees) and nearly
equal to the opening angle of the wind-wind collision cavity (~110 degrees). As
in previous studies, we confirm a hole near the centre of each polar lobe and
no detectable near-IR H2 emission from the thin optical skirt seen prominently
in visible imagery. We conclude that the interaction between the outflows
and/or radiation from the central binary stars and their orientation in space
has had, and possibly still has, a strong influence on the Homunculus. This
implies that prevailing theoretical models of the Homunculus are incomplete as
most assume a single star origin that produces an axisymmetric nebula. We
discuss how the newly found features might be related to the Homunculus
ejection, the central binary and the interacting stellar winds. We also include
a 3D printable version of our Homunculus model.Comment: 14 pages, 7 color figures, 1 interactive 3D figure (Figure 5,
requires Adobe Reader), published in MNRAS. A 3D printable version of our
Homunculus model can be downloaded from
http://svs.gsfc.nasa.gov/vis/a010000/a011500/a011568/Eta_Car_Homunuculus_3D_model.zip
or from the 'Supporting Information' link in the electronic version of the
MNRAS articl
Spectroscopic and photometric oscillatory envelope variability during the S Doradus outburst of the Luminous Blue Variable R71
To better understand the LBV phenomenon, we analyze multi-epoch and
multi-wavelength spectra and photometry of R71. Pre-outburst spectra are
analyzed with the radiative transfer code CMFGEN to determine the star's
fundamental stellar parameters. During quiescence, R71 has an effective
temperature of and a luminosity of
log = 5.78 and is thus a classical LBV, but at the lower
luminosity end of this group. We determine its mass-loss rate to yr. We present R71's spectral energy distribution
from the near-ultraviolet to the mid-infrared during its present outburst.
Mid-infrared observations suggest that we are witnessing dust formation and
grain evolution. Semi-regular oscillatory variability in the star's light curve
is observed during the current outburst. Absorption lines develop a second blue
component on a timescale twice that length. The variability may consist of one
(quasi-)periodic component with P ~ 425/850 d with additional variations
superimposed. During its current S Doradus outburst, R71 occupies a region in
the HR diagram at the high-luminosity extension of the Cepheid instability
strip and exhibits similar irregular variations as RV Tau variables. LBVs do
not pass the Cepheid instability strip because of core evolution, but they
develop comparable cool, low-mass, extended atmospheres in which convective
instabilities may occur. As in the case of RV Tau variables, the occurrence of
double absorption lines with an apparent regular cycle may be due to shocks
within the atmosphere and period doubling may explain the factor of two in the
lengths of the photometric and spectroscopic cycles.Comment: 18 pages, 14 figures, submitted to A&
Stellar parameters of Be stars observed with X-shooter
Aims. The X-shooter archive of several thousand telluric star spectra was
skimmed for Be and Be-shell stars to derive the stellar fundamental parameters
and statistical properties, in particular for the less investigated late type
Be stars, and the extension of the Be phenomenon into early A stars. Methods.
An adapted version of the BCD method is used, utilizing the Balmer
discontinuity parameters to determine effective temperature and surface
gravity. This method is optimally suited for late B stars. The projected
rotational velocity was obtained by profile fitting to the Mg ii lines of the
targets, and the spectra were inspected visually for the presence of peculiar
features such as the infrared Ca ii triplet or the presence of a double Balmer
discontinuity. The Balmer line equivalent widths were measured, but due to
uncertainties in determining the photospheric contribution are useful only in a
subsample of Be stars for determining the pure emission contribution. Results.
A total of 78 Be stars, mostly late type ones, were identified in the X-shooter
telluric standard star archive, out of which 48 had not been reported before.
The general trend of late type Be stars having more tenuous disks and being
less variable than early type ones is confirmed. The relatively large number
(48) of relatively bright (V > 8.5) additional Be stars casts some doubt on the
statistics of late type Be stars; they are more common than currently thought:
The Be/B star fraction may not strongly depend on spectral subtype.Comment: Accepted for publication in A&
Short-term variability and mass loss in Be stars III. BRITE and SMEI satellite photometry of 28 Cygni
The BRITE Constellation of nanosatellites obtained mmag photometry of 28
Cygni for 11 months in 2014-2016. Observations with the Solar Mass Ejection
Imager in 2003-2010 and 118 H line profiles were added.
For decades, 28 Cyg has exhibited four large-amplitude frequencies: two
closely spaced frequencies of spectroscopically confirmed modes near 1.5
c/d, one slightly lower exophotospheric (Stefl) frequency, and at 0.05 c/d the
difference frequency between the two g modes. This top-level framework is
indistinguishable from eta Cen (Paper I), which is also very similar in
spectral type, rotation rate, and viewing angle. The Stefl frequency is the
only one that does not seem to be affected by the difference frequency. The
amplitude of the latter undergoes large variations; around maximum the amount
of near-circumstellar matter is increased, and the amplitude of the Stefl
frequency grows by some factor. During such brightenings dozens of transient
spikes appear in the frequency spectrum, concentrated in three groups. Only
eleven frequencies were common to all years of BRITE observations.
Be stars seem to be controlled by several coupled clocks, most of which are
not very regular on timescales of weeks to months but function for decades. The
combination of g modes to the low difference frequency and/or the atmospheric
response to it appears significantly nonlinear. Like in eta Cen, the
difference-frequency variability seems the main responsible for the modulation
of the star-to-disc mass transfer in 28 Cyg. A hierarchical set of difference
frequencies may reach the longest timescales known of the Be phenomenon.Comment: 17 pages, 21 figures, submitted to Astronomy & Astrophysic
The cataclysmic variable QZ Lib : a period bouncer
While highly evolved cataclysmic variables (CVs) with brown dwarf donors, often called āperiod bouncersā, are predicted to make up ā 40 ā 70% of the Galactic CV population, only a handful of such systems are currently known. The identification and characterization of additional period bouncers is therefore important to probe this poorly understood phase of CV evolution. We investigate the evolution of the CV QZ Lib following its 2004 superāoutburst using multiāepoch spectroscopy. From timeā resolved spectroscopic observations we measure the orbital period of the system, Porb = 0.06436(20) d, which, combined with the superhump period PSH = 0.064602(24) d, yields the system mass ratio, q = 0.040(9). From the analysis of the spectral energy distribution we determine the structure of the accretion disc and the white dwarf effective temperature, Teff = 10 500 Ā± 1500 K. We also derive an upper limit on the effective temperature of the secondary, Teff < 1700 K, corresponding to a brown dwarf of T spectral type. The low temperature of the white dwarf, the small mass ratio and the fact that the donor is not dominating the nearāinfrared emission are all clues of a post bounce system. Although it is possible that QZ Lib could have formed as a white dwarf plus a brown dwarf binary, binary population synthesis studies clearly suggest this scenario to be less likely than a period bouncer detection and we conclude that QZ Lib is a CV that has already evolved through the period minimum
Molecular remnant of Nova 1670 (CK Vulpeculae): I. Properties and enigmatic origin of the gas
CK Vul erupted in 1670 and is considered a stellar-merger candidate. Its
remnant contains a molecular component of surprisingly rich composition. We
present interferometric line surveys with subarcsec resolution with ALMA and
SMA. The observations provide interferometric maps of molecular line emission
at frequencies between 88 and 243 GHz that allow imaging spectroscopy of more
than 180 transitions of 26 species. We present, classify, and analyze the
different morphologies of the emission regions displayed by the molecules. We
also perform a non-LTE radiative-transfer analysis of emission of most of the
observed species, deriving temperatures and column densities in five parts of
the nebula. Non-LTE effects are clearly seen in complex species including
methanol absorption against the CMB. The temperatures are 17 K in the inner
remnant and 14 K in the extended lobes. We find total (hydrogen plus helium)
densities in the range of cm. The column densities provide
relative abundance patterns in the remnant which currently are not understood.
Attempts to derive elemental abundances within the assumption of a chemical
equilibrium give only loose constraints on the CNO elements. That the formation
of many of the observed molecules requires a major involvement of circumstellar
shocks remains the preferred possibility. The molecular gas could have formed
350 yr ago or more recently. The molecules are well shielded from the
interstellar radiation field by the circumstellar dust. Their presence alone
indicates that the unobservable central star cannot be a hot object such as a
white dwarf. This excludes some of the proposed scenarios on the nature of CK
Vul. The general characteristics of the molecular environment of CK Vul derived
in this study resemble quite well those of some pre-planetary nebulae and AGB
stars, most notably that of OH231.8.Comment: submitted to A&A; comments welcom
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