44 research outputs found
Measuring helium abundance difference in giants of NGC 2808
Multiple populations have been detected in several globular clusters (GC)
that do not display a spread in metallicity. Unusual features of their CMD can
be interpreted in terms of differences in the Helium content of the stars
belonging to the sub-populations. Differences in He abundance have never been
directly observed. We attempt to measure these differences in two giant stars
of NGC 2808 with very similar parameters but different Na and O abundances,
hence that presumably belong to different sub-populations, by directly
comparing their He I 10830 {\AA} lines. The He 10830 {\AA} line forms in the
upper chromosphere. Our detailed models derive the chromospheric structure
using the Ca II and H, and simulate the corresponding He I 10830 line
profiles. We show that, at a given value of He abundance, the He I 10830
equivalent width cannot significantly change without a corresponding much
larger change in the Ca II lines. We have used the VLT-CRIRES to obtain
high-resolution spectra in the 10830 {\AA} region, and the VLT-UVES to obtain
spectra of the Ca II and H lines of our target stars. The two target
stars have very similar Ca II and H lines, but different appearances in
the He region. One line, blueshifted by 17 km s with respect to the He
10830 rest wavelength, is detected in the spectrum of the Na-rich star, whereas
the Na-poor star spectrum is consistent with a non-detection. The difference in
the spectra is consistent and most closely explained by an He abundance
difference between the two stars of 0.17.We provide direct
evidence of a significant He line strength difference in giant stars of NGC
2808 belonging to different sub-populations, which had been previously detected
by other photometric and spectroscopic means.Comment: Accepted for publication in A&
The core flux of the brightest 10 micron galaxies in the southern sky
Aims. Near diffraction-limited images have been taken at 8.9, 11.9, and 12.9
micron for the brightest extragalactic sources in the southern sky, in order to
optimally plan N-band observations with MIDI (MID-infrared Interferometric
instrument) at the VLTI. Methods. We have assembled a sample of 21 objects
consisting of all the AGNs observable from Paranal observatory, Chile, plus
three non-AGN objects, with an estimated N-band flux greater than 400mJy. We
used the TIMMI2 Mid Infrared instrument mounted on the ESO's 3.6m telescope to
obtain near diffraction-limited images in order to establish the unresolved
core flux within < 0.5 arscsec. Results. Positions and core total fluxes were
obtained for all sources in our sample and compared with similar investigations
in the literature. We find that 15 AGN and the nuclear starburst in NGC 253
exhibit an unresolved core flux < 300mJy at 11.9 micron, making them promising
targets for MIDI at the VLTI. For extended sources, near diffraction-limited
images are presented and discussed.Comment: Accepted to A&
Probabilistic point source inversion of strong-motion data in 3-D media using pattern recognition: A case study for the 2008 M w 5.4 Chino Hills earthquake
Despite the ever increasing availability of computational power, real-time source inversions based on physical modeling of wave propagation in realistic media remain challenging. We investigate how a nonlinear Bayesian approach based on pattern recognition and synthetic 3-D Green's functions can be used to rapidly invert strong-motion data for point source parameters by means of a case study for a fault system in the Los Angeles Basin. The probabilistic inverse mapping is represented in compact form by a neural network which yields probability distributions over source parameters. It can therefore be evaluated rapidly and with very moderate CPU and memory requirements. We present a simulated real-time inversion of data for the 2008 Mw 5.4 Chino Hills event. Initial estimates of epicentral location and magnitude are available ∼14 s after origin time. The estimate can be refined as more data arrive: by ∼40 s, fault strike and source depth can also be determined with relatively high certainty
Chemical abundances for 11 bulge stars from high-resolution, near-IR spectra
It is debated whether the Milky Way bulge has the characteristics of a
classical bulge sooner than those of a pseudobulge. Detailed abundance studies
of bulge stars is a key to investigate the origin, history, and classification
of the bulge. The aim is to add to the discussion on the origin of the bulge
and to study detailed abundances determined from near-IR spectra for bulge
giants already investigated with optical spectra, the latter also providing the
stellar parameters which are very significant for the results of the present
study. Especially, the important CNO elements are better determined in the
near-IR. High-resolution, near-infrared spectra in the H band are recorded
using the CRIRES spectrometer on the Very Large Telescope. The CNO abundances
can all be determined from the numerous molecular lines in the wavelength range
observed. Abundances of the alpha elements are also determined from the near-IR
spectra. [O/Fe], [Si/Fe] and [S/Fe] are enhanced up to metallicities of at
least [Fe/H]=-0.3, after which they decline. This suggests that the Milky Way
bulge experienced a rapid and early star-formation history like that of a
classical bulge. However, a similarity between the bulge trend and the trend of
the local thick disk seems present. Such a similarity could suggest that the
bulge has a pseudobulge origin. Our [C/Fe] trend does not show any increase
with [Fe/H] which could have been expected if W-R stars have contributed
substantially to the C abundances. No "cosmic scatter" can be traced around our
observed abundance trends; the scatter found is expected, given the
observational uncertainties.Comment: Accepted for publication in A&
The composition and nature of the dust shell surrounding the binary AFGL 4106
We present infrared spectroscopy and imaging of AFGL~4106. The 2.4-5 micron
ISO-SWS spectrum reveals the presence of a cool, luminous star (T_eff ~ 3750 K)
in addition to an almost equally luminous F star (T_eff ~ 7250 K). The 5-195
micron SWS and LWS spectra are dominated by strong emission from circumstellar
dust. We find that the dust consists of amorphous silicates, with a minor but
significant contribution from crystalline silicates. The amorphous silicates
consist of Fe-rich olivines. The presence of amorphous pyroxenes cannot be
excluded but if present they contain much less Fe than the amorphous olivines.
Comparison with laboratory data shows that the pure Mg-end members of the
crystalline olivine and pyroxene solid solution series are present. In
addition, we find strong evidence for simple oxides (FeO and Al2O3) as well as
crystalline H2O ice. Several narrow emission features remain unidentified.
Modelling of the dust emission using a dust radiation transfer code shows that
large grains (~1 micron) must be present and that the abundance of the
crystalline silicates is between 7 and 15% of the total dust mass, depending on
the assumed enstatite to forsterite ratio, which is estimated to be between 1
and 3. The amorphous and crystalline dust components in the shell do not have
the same temperature, implying that the different dust species are not
thermally coupled. We find a dust mass of ~3.9 x 10^-2 M_sol expelled over a
period of 4 x 10^3 years for a distance of 3.3 kpc. The F-star in the AFGL~4106
binary is likely a post-red-supergiant in transition to a blue supergiant or WR
phase.Comment: 22 pages (including 12 figures), accepted by Astronomy and
Astrophysic
Large halloween asteroid at lunar distance
The near-Earth asteroid (NEA) 2015 TB had a very close encounter with Earth at 1.3 lunar distances on October 31, 2015. We obtained 3-band mid-infrared observations of this asteroid with the ESO VLT-VISIR instrument covering approximately four hours in total. We also monitored the visual lightcurve during the close-encounter phase. The NEA has a (most likely) rotation period of 2.939 ± 0.005 h and the visual lightcurve shows a peak-to-peak amplitude of approximately 0.12 ± 0.02 mag. A second rotation period of 4.779 ± 0.012 h, with an amplitude of the Fourier fit of 0.10 ± 0.02 mag, also seems compatible with the available lightcurve measurements. We estimate a V-R colour of 0.56 ± 0.05 mag from different entries in the MPC database. A reliable determination of the object's absolute magnitude was not possible. Applying different phase relations to the available R-/V-band observations produced H = 18.6 mag (standard H-G calculations) or H = 19.2 mag and H = 19.8 mag (via the H-G procedure for sparse and low-quality data), with large uncertainties of approximately 1 mag. We performed a detailed thermophysical model analysis by using spherical and partially also ellipsoidal shape models. The thermal properties are best explained by an equator-on (± 30°) viewing geometry during our measurements with a thermal inertia in the range 250-700 J m s K (retrograde rotation) or above 500 J m s K (prograde rotation). We find that the NEA has a minimum size of approximately 625 m, a maximum size of just below 700 m, and a slightly elongated shape with a/b 1.1. The best match to all thermal measurements is found for: (i) thermal inertia Γ = 900 J m s K; D = 644 m, p = 5.5% (prograde rotation with 2.939 h); regolith grain sizes of 50-100 mm; (ii) thermal inertia Γ = 400 J m s K; D = 667 m, p = 5.1% (retrograde rotation with 2.939 h); regolith grain sizes of 10-20 mm. A near-Earth asteroid model (NEATM) confirms an object size well above 600 m (best NEATM solution at 690 m, beaming parameter η = 1.95), significantly larger than early estimates based on radar measurements. In general, a high-quality physical and thermal characterisation of a close-encounter object from two-week apparition data is not easily possible. We give recommendations for improved observing strategies for similar events in the future. © ESO, 2017.The research leading to these results has received funding from the European Union's Horizon 2020 Research and Innovation Programme, under Grant Agreement No. 687378. Funding from Spanish grant AYA-2014-56637-C2-1-P is acknowledged. Hungarian funding from the NKFIH grant GINOP-2.3.2-15-2016-00003 is also acknowledged. R.D. acknowledges the support of MINECO for his Ramon y Cajal Contract.Peer Reviewe
Discovery of a double ring in the core of η Carinae
We report the discovery of a double ring structure in the waist of the nebula surrounding η Carinae. The rings are detected in the mid-IR dust continuum at wavelengths of 7.9, 11.9, 12.9 and 20 μm. The dust in the rings has a temperature of about 300 K. The orientation of the rings is inclined with respect to the axis of the homunculus by either 37 or 58 degrees. The central star is not in the projected centre of the structure defined by the two rings. This geometry is reminiscent of that seen in SN1987A and some planetary nebulae. We discuss several possible origins for this remarkable geometry and its orientation
PAHs and crystalline silicates in the post-AGB star IRAS 16279-4757
IRAS 16279-4757 belongs to a group of post-AGB stars showing both PAH bands
and crystalline silicates. We present mid-infrared images, that resolve the
object for the first time. The morphology is similar to that of the `Red
Rectangle' (HD 44179), the prototype object with PAHs and crystalline
silicates. A two-component model and images suggest a dense oxygen-rich torus,
an inner, low-density carbon-rich region and a carbon-rich bipolar outflow. The
PAH bands are enhanced at the outflow, while the continuum emission is
concentrated towards the center. Our findings support the suggestion that mixed
chemistry and morphology are closely related. We discuss the ISO/SWS spectra of
IRAS 16279-4757. Several bands in the ISO/SWS spectrum show a match with
anorthite: this would be the first detection of this mineral outside the solar
system. Compared to HD 44179, the shapes of PAH bands are closer to those of
planetary nebulae, possibly related to a population of small PAHs present HD
44179, but absent around IRAS 16279-4757. Detailed examination of the spectra
shows the individual character of these two objects. The comparison suggests
that the torus found in IRAS 16279-4757 may have formed more recently than that
in HD 44179.Comment: accepted to Ap
Towards a physical understanding of the thermal background in large ground-based telescopes
Ground-based thermal-infrared observations have a unique scientific potential, but are also extremely challenging due to the need to accurately subtract the high thermal background. Since the established techniques of chopping and nodding need to be modified for observations with the future mid-infrared ELT imager and spectrograph (METIS), we investigate the sources of thermal background subtraction residuals. Our aim is to either remove or at least minimise the need for nodding in order to increase the observing efficiency for METIS. To this end we need to improve our knowledge about the origin of chop residuals and devise observing methods to remove them most efficiently, i.e. with the slowest possible nodding frequency. Thanks to dedicated observations with VLT/VISIR and GranTeCan/CanariCam, we have successfully traced the origin of three kinds of chopping residuals to (1) the entrance window, (2) the spiders and (3) other warm emitters in the pupil, in particular the VLT M3 mirror cell in its parking position. We conclude that, in order to keep chopping residuals stable over a long time (and therefore allow for slower nodding cycles), the pupil illumination needs to be kept constant, i.e. (imaging) observations should be performed in pupil-stabilised, rather than field-stabilised mode, with image de-rotation in the pInstrumentatio