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$\alpha$, 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$\alpha$ lines of our target stars. The two target stars have very similar Ca II and H$\alpha$ lines, but different appearances in the He region. One line, blueshifted by 17 km s$^{-1}$ 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 $\Delta Y \ge$ 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&

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

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

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

Son of X--Shooter: a multi--band instrument for a multi--band universe

Son Of X-Shooter (SOXS) will be a new instrument designed to be mounted at the Nasmyth--A focus of the ESO 3.5 m New Technology Telescope in La Silla site (Chile). SOXS is composed of two high-efficiency spectrographs with a resolution slit product 4500, working in the visible (350 -- 850 nm) and NIR (800 -- 2000 nm) range respectively, and a light imager in the visible (the acquisition camera usable also for scientific purposes). The science case is very broad, it ranges from moving minor bodies in the solar system, to bursting young stellar objects, cataclysmic variables and X-ray binary transients in our Galaxy, supernovae and tidal disruption events in the local Universe, up to gamma-ray bursts in the very distant and young Universe, basically encompassing all distance scales and astronomy branches. At the moment, the instrument passed the Preliminary Design Review by ESO (July 2017) and the Final Design (with FDR in July 2018).Comment: 23 pages, 10 Figures, accepted to be published in Frontier Research in Astrophysics 2018 Conference proceedings in Proceeding of Science. arXiv admin note: text overlap with arXiv:1807.0882