We present a methodology for the regularization and combination of sparse sampled and irregularly gridded observations from fibre-optic multiobject integral field spectroscopy. The approach minimizes ...interpolation and retains image resolution on combining subpixel dithered data. We discuss the methodology in the context of the Sydney-AAO multiobject integral field spectrograph (SAMI) Galaxy Survey underway at the Anglo-Australian Telescope. The SAMI instrument uses 13 fibre bundles to perform high-multiplex integral field spectroscopy across a 1° diameter field of view. The SAMI Galaxy Survey is targeting ~3000 galaxies drawn from the full range of galaxy environments. We demonstrate the subcritical sampling of the seeing and incomplete fill factor for the integral field bundles results in only a 10 per cent degradation in the final image resolution recovered. We also implement a new methodology for tracking covariance between elements of the resulting data cubes which retains 90 per cent of the covariance information while incurring only a modest increase in the survey data volume.
Context. The chemical abundances of stars encode information on their place and time of origin. Stars formed together in e.g. a cluster, should present chemical homogeneity. Also disk stars ...influenced by the effects of the bar and the spiral arms might have distinct chemical signatures depending on the type of orbit that they follow, e.g. from the inner versus outer regions of the Milky Way. Aims. We explore the correlations between velocity and metallicity and the possible distinct chemical signatures of the velocity over-densities of the local Galactic neighbourhood. Methods. We use the large spectroscopic survey RAVE and the Geneva Copenhagen Survey. We compare the metallicity distribution of regions in the velocity plane (vR,vφ) with that of their symmetric counterparts (−vR,vφ). We expect similar metallicity distributions if there are no tracers of a sub-population (e.g. a dispersed cluster, accreted stars), if the disk of the Galaxy is axisymmetric, and if the orbital effects of the bar and the spiral arms are weak. Results. We find that the metallicity-velocity space of the solar neighbourhood is highly patterned. A large fraction of the velocity plane shows differences in the metallicity distribution when comparing symmetric vR regions. The typical differences in the median metallicity are of 0.05 dex with statistical significant of at least 95% confidence, and with values up to 0.6 dex. For stars with low azimuthal velocity vφ, the ones moving outwards. These include stars in the Hercules and Hyades moving groups and other velocity branch-like structures. For higher vφ, the stars moving inwards have higher metallicity than those moving outwards. We have also discovered a positive gradient in vφ with respect to metallicity at high metallicities, apart from the two known positive and negative gradients for the thick and thin disks. Conclusions. The most likely interpretation of the metallicity asymmetry is that it is mainly due to the orbital effects of the Galactic bar and the radial metallicity gradient of the disk. We present a simulation that supports this idea.
We investigated the prospects for systematic searches of white dwarfs at low Galactic latitudes, using the VLT Survey Telescope H alpha Photometric Survey of the Galactic plane and Bulge (VPHAS+). We ...targeted 17 white dwarf candidates along sightlines of known open clusters, aiming to identify potential cluster members. We confirmed all the 17 white dwarf candidates from blue/optical spectroscopy, and we suggest five of them to be likely cluster members. We estimated progenitor ages and masses for the candidate cluster members, and compare our findings to those for other cluster white dwarfs. A white dwarf in NGC 3532 is the most massive known cluster member (1.13 ...), likely with an oxygen-neon core, for which we estimate an ... progenitor, close to the mass-divide between white dwarf and neutron star progenitors. A cluster member in Ruprecht 131 is a magnetic white dwarf, whose progenitor mass exceeded 2-3 ... We stress that wider searches, and improved cluster distances and ages derived from data of the ESA Gaia mission, will advance the understanding of the mass-loss processes for low- to intermediate-mass stars. (ProQuest: ... denotes formulae/symbols omitted.)
We have conducted a detailed multi-wavelength study of the peculiar nebula Abell 48 and its central star. We classify the nucleus as a helium-rich, hydrogen-deficient star of type WN4-5. The evidence ...for either a massive WN or a low-mass WN interpretation is critically examined, and we firmly conclude that Abell 48 is a planetary nebula (PN) around an evolved low-mass star, rather than a Population I ejecta nebula. Importantly, the surrounding nebula has a morphology typical of PNe, and is not enriched in nitrogen, and thus not the 'peeled atmosphere' of a massive star. We estimate a distance of 1.6 kpc and a reddening, E(B − V) = 1.90 mag, the latter value clearly showing the nebula lies on the near side of the Galactic bar, and cannot be a massive WN star. The ionized mass (∼0.3 M) and electron density (700 cm−3) are typical of middle-aged PNe. The observed stellar spectrum was compared to a grid of models from the Potsdam Wolf-Rayet (PoWR) grid. The best-fitting temperature is 71 kK, and the atmospheric composition is dominated by helium with an upper limit on the hydrogen abundance of 10 per cent. Our results are in very good agreement with the recent study of Todt et al., who determined a hydrogen fraction of 10 per cent and an unusually large nitrogen fraction of ∼5 per cent. This fraction is higher than any other low-mass H-deficient star, and is not readily explained by current post-AGB models. We give a discussion of the implications of this discovery for the late-stage evolution of intermediate-mass stars. There is now tentative evidence for two distinct helium-dominated post-AGB lineages, separate to the helium- and carbon-dominated surface compositions produced by a late thermal pulse. Further theoretical work is needed to explain these recent discoveries.
Abstract
We apply the twin method to determine parallaxes to 232 545 stars of the RAVE survey using the parallaxes of Gaia DR1 as a reference. To search for twins in this large data set, we apply the ...t-student stochastic neighbour embedding projection that distributes the data according to their spectral morphology on a two-dimensional map. From this map, we choose the twin candidates for which we calculate a χ2 to select the best sets of twins. Our results show a competitive performance when compared to other model-dependent methods relying on stellar parameters and isochrones. The power of the method is shown by finding that the accuracy of our results is not significantly affected if the stars are normal or peculiar since the method is model free. We find twins for 60 per cent of the RAVE sample that are not contained in Tycho–Gaia Astrometric Solution (TGAS) or that have TGAS uncertainties that are larger than 20 per cent. We could determine parallaxes with typical errors of 28 per cent. We provide a complementary data set for the RAVE stars not covered by TGAS, or that have TGAS uncertainties which are larger than 20 per cent, with model-free parallaxes scaled to the Gaia measurements.
We present a supplement to the Macquarie/AAO/Strasbourg Hα planetary nebulae (PNe) catalogue (MASH), which we denote MASH-II. The supplement consists of over 300 true, likely and possible new ...Galactic PNe found after re-examination of the entire AAO/UKST Hα survey of the Southern Galactic Plane in digital form. We have spectroscopically confirmed over 240 of these new candidates as bona fide PNe, and we include other high-quality candidates awaiting spectroscopic confirmation as possible PNe. These latest discoveries largely comprise two distinct groups: small, star like or moderately resolved PNe at one end and mostly large, extremely low surface brightness PNe at the other. Neither group were easy to discover from simple visual scrutiny of the original survey exposures as for MASH but were relatively straightforward to uncover from the digital images via application of semi-automated discovery techniques. We suspect the few PNe still hidden in the Hα survey will lie outside our search criteria or be difficult to find.
Context.
Data-driven methods play an increasingly important role in the field of astrophysics. In the context of large spectroscopic surveys of stars, data-driven methods are key in deducing physical ...parameters for millions of spectra in a short time. Convolutional neural networks (CNNs) enable us to connect observables (e.g. spectra, stellar magnitudes) to physical properties (atmospheric parameters, chemical abundances, or labels in general).
Aims.
We test whether it is possible to transfer the labels derived from a high-resolution stellar survey to intermediate-resolution spectra of another survey by using a CNN.
Methods.
We trained a CNN, adopting stellar atmospheric parameters and chemical abundances from APOGEE DR16 (resolution
R
= 22 500) data as training set labels. As input, we used parts of the intermediate-resolution RAVE DR6 spectra (
R
∼ 7500) overlapping with the APOGEE DR16 data as well as broad-band ALL_WISE and 2MASS photometry, together with
Gaia
DR2 photometry and parallaxes.
Results.
We derived precise atmospheric parameters
T
eff
, log(
g
), and M/H, along with the chemical abundances of Fe/H,
α
/M, Mg/Fe, Si/Fe, Al/Fe, and Ni/Fe for 420 165 RAVE spectra. The precision typically amounts to 60 K in
T
eff
, 0.06 in log(
g
) and 0.02−0.04 dex for individual chemical abundances. Incorporating photometry and astrometry as additional constraints substantially improves the results in terms of the accuracy and precision of the derived labels, as long as we operate in those parts of the parameter space that are well-covered by the training sample. Scientific validation confirms the robustness of the CNN results. We provide a catalogue of CNN-trained atmospheric parameters and abundances along with their uncertainties for 420 165 stars in the RAVE survey.
Conclusions.
CNN-based methods provide a powerful way to combine spectroscopic, photometric, and astrometric data without the need to apply any priors in the form of stellar evolutionary models. The developed procedure can extend the scientific output of RAVE spectra beyond DR6 to ongoing and planned surveys such as
Gaia
RVS, 4MOST, and WEAVE. We call on the community to place a particular collective emphasis and on efforts to create unbiased training samples for such future spectroscopic surveys.
Using the RAdial Velocity Experiment fourth data release (RAVE DR4), and a new metallicity calibration that will be also taken into account in the future RAVE DR5, we investigate the existence and ...the properties of supersolar metallicity stars (M/H ... +0.1 dex) in the sample, and in particular in the solar neighbourhood. We find that RAVE is rich in supersolar metallicity stars, and that the local metallicity distribution function declines remarkably slowly up to +0.4 dex. Our results show that the kinematics and height distributions of the supersolar metallicity stars are identical to those of the M/H ... 0 thin-disc giants that we presume were locally manufactured. The eccentricities of the supersolar metallicity stars indicate that half of them are on a roughly circular orbit (e = 0.15), so under the assumption that the metallicity of the interstellar medium at a given radius never decreases with time, they must have increased their angular momenta by scattering at corotation resonances of spiral arms from regions far inside the solar annulus. The likelihood that a star will migrate radially does not seem to decrease significantly with increasing amplitude of vertical oscillations within range of oscillation amplitudes encountered in the disc. (ProQuest: ... denotes formulae/symbols omitted.)
We present optical, radio and X-ray data that confirm a new supernova remnant (SNR) in the Large Magellanic Cloud (LMC) discovered using our deep Hα imagery. Optically, the new SNR has a somewhat ...filamentary morphology and a diameter of 56 × 64 arcsec (13.5 × 15.5 pc at the 49.9 kpc distance of the LMC). Spectroscopic follow-up of multiple regions show high S ii/Hα emission-line ratios ranging from 0.66 ± 0.02 to 0.93 ± 0.01, all of which are typical of an SNR. We found radio counterparts for this object using our new Australia Telescope Compact Array (ATCA) 6 cm pointed observations as well as a number of available radio surveys at 8640 MHz, 4850 MHz, 1377 MHz and 843 MHz. With these combined data we provide a spectral index α ≈ −0.5 between 843 and 8640 MHz. Both spectral line analysis and the magnetic field strength, ranging from 124 to 184 μG, suggest a dynamical age between ∼2200 and ∼4700 yr. The SNR has a previously catalogued X-ray counterpart listed as HP 483 in the ROSAT Position Sensitive Proportional Counter (PSPC) catalogue.
Context. Radial velocity surveys such as the RAdial Velocity Experiment (RAVE) provide us with measurements of hundreds of thousands of nearby stars most of which belong to the Galactic thin, thick ...disk or halo. Ideally, to study the Galactic disks (both thin and thick) one should make use of the multi-dimensional phase-space and the whole pattern of chemical abundances of their stellar populations. Aims. In this paper, with the aid of the RAVE survey, we study the thin and thick disks of the Milky Way, focusing on the latter. We present a technique to disentangle the stellar content of the two disks based on the kinematics and other stellar parameters such as the surface gravity of the stars. Using the Padova Galaxy model, we checked the ability of our method to correctly isolate the thick disk component from the Galaxy mixture of stellar populations. Methods. We introduce selection criteria in order to clean the observed radial velocities from the Galactic differential rotation and to take into account the partial sky coverage of RAVE. We developed a numerical technique to statistically disentangle thin and thick disks from their mixture. Results. We deduce the components of the solar motion relative to the local standard of rest (LSR) in the radial and vertical direction, the rotational lag of the thick disk component relative to the LSR, and the square root of the absolute value of the velocity dispersion tensor for the thick disk alone. The analysis of the thin disk is presented in another paper. We find good agreement with previous independent parameter determinations. In our analysis we used photometrically determined distances. In the Appendix we show that similar values can be found for the thick disk alone as derived in the main sections of our paper even without the knowledge of photometric distances.
PDF
