We propose 35 star systems within similar to 70 pc of Earth as newly identified members of nearby young stellar kinematic groups; these identifications include the first A- and late-B-type members of ...the AB Doradus moving group and field Argus Association. All but one of the 35 systems contain a bright solar- or earlier-type star that should make an excellent target for the next generation of adaptive optics (AO) imaging systems on large telescopes. AO imaging has revealed four massive planets in orbit around the lambda Boo star HR 8799. Initially, the planets were of uncertain mass due in large part to the uncertain age of the star. We find that HR 8799 is a likely member of the similar to 30 Myr old Columba Association, implying planet masses similar to 6 times that of Jupiter. We consider Spitzer Space Telescope MIPS photometry of stars in the similar to 30 Myr old Tucana/Horologium and Columba Associations, the similar to 40 Myr old field Argus Association, and the similar to 70 Myr old AB Doradus moving group. The percentage of stars in these young stellar groups that display excess emission above the stellar photosphere at 24 and 70 mu m wavelengths--indicative of the presence of a dusty debris disk--is compared with corresponding percentages for members of 11 open clusters and stellar associations with ages between 8 and 750 Myr, thus elucidating the decay of debris disks with time.
ABSTRACT We report the discovery of SMSS J160540.18−144323.1, a new ultra metal-poor halo star discovered with the SkyMapper telescope. We measure $\left\rm {Fe}/\rm {H}\right= -6.2 \pm 0.2$ (1D ...LTE), the lowest ever detected abundance of iron in a star. The star is strongly carbon-enhanced, $\left\rm {C}/\rm {Fe}\right = 3.9 \pm 0.2$, while other abundances are compatible with an α-enhanced solar-like pattern with $\left\rm {Ca}/\rm {Fe}\right = 0.4 \pm 0.2$, $\left\rm {Mg}/\rm {Fe}\right = 0.6 \pm 0.2$, $\left\rm {Ti}/\rm {Fe}\right = 0.8 \pm 0.2$, and no significant s- or r-process enrichment, $\left\rm {Sr}/\rm {Fe}\right \lt 0.2$ and $\left\rm {Ba}/\rm {Fe}\right \lt 1.0$ (3σ limits). Population III stars exploding as fallback supernovae may explain both the strong carbon enhancement and the apparent lack of enhancement of odd-Z and neutron-capture element abundances. Grids of supernova models computed for metal-free progenitor stars yield good matches for stars of about $10\, \rm M_\odot$ imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly $20\, \rm M_\odot$ are incompatible with the observed abundance pattern.
Context.
It is more and more suspected that R Coronae Borealis (RCB) stars – rare hydrogen-deficient and carbon-rich supergiant stars – are the products of mergers of CO/He white-dwarf binary systems ...in the intermediate mass regime (0.6 <
M
Tot
< 1.2
M
⊙
). Following the merger, a short-lived cool supergiant phase starts. RCB stars are extremely rare as only 77 have hitherto been known in the Galaxy, while up to 1000 have been predicted from population synthesis models.
Aims.
The goal is to significantly increase the number of known RCB stars in order to better understand their evolutionary paths, their spatial distribution, and their formation rate in the context of population synthesis results. A list of 2356 RCB star candidates was selected using infrared colours from the all-sky 2MASS and WISE surveys. The objective is to follow them up spectroscopically to classify the candidates and, thus, to distinguish RCB stars from other dust-producing stars.
Methods.
A series of brightness and colour-colour cuts that were used as selection criteria were then tested using the sample of known Galactic and Magellanic RCB stars. RCB spectral energy distribution models were also used to understand the effects of each selection criterion in terms of circumstellar shell temperature. Optical, low-resolution spectra were obtained for nearly 500 of the candidate stars. These spectra were compared to synthetic spectra from a new grid of MARCs hydrogen-deficient atmospheric models. This allowed us to define a spectroscopic classification system for RCB stars depending on their effective temperature and photometric status.
Results.
This programme has found 45 new RCB stars, including 30 Cold (4000 <
T
eff
< 6800 K), 14 Warm (6800 <
T
eff
< 8500 K), and one Hot (
T
eff
> 15 000 K). Forty of these belong to the Milky Way and five are located in the Magellanic Clouds. We also confirmed that the candidate KDM 5651 is indeed a new RCB star, increasing the total number of Magellanic RCB stars to 30.
Conclusions.
We increased the total number of RCB stars known by ∼50%, bringing it up to 147. In addition, we compiled a list of 14 strong RCB candidates, most certainly observed during a dust obscuration phase. From the detection efficiency and success rate so far, we estimate that there should be no more than 500 RCB stars existing in the Milky Way, all HdC stars included.
We present the results of analysis of "snapshot" spectra of 253 metal-poor halo stars-3.8 , Fe/H , -1.5 obtained in the HERES survey. The snapshot spectra have been obtained with VLT/UVES and have ...typically S/N 6 54 per pixel (ranging from 17 to 308), R 6 20 000, l = 3760-4980 AA. This sample represents the major part of the complete HERES sample of 373 stars; however, the CH strong content of the sample is not dealt with here. The spectra are analysed using an automated line profile analysis method based on the Spectroscopy Made Easy (SME) codes of Valenti & Piskunov. Elemental abundances of moderate precision (absolute rms errors of order 0.25 dex, relative rms errors of order 0.15 dex) have been obtained for 22 elements, C, Mg, Al, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, La, Ce, Nd, Sm, and Eu, where detectable. Of these elements, 14 are usually detectable at the 3s confidence level for our typical spectra. The remainder can be detected in the least metal-poor stars of the sample, spectra with higher than average S/N, or when the abundance is enhanced. Among the sample of 253 stars, disregarding four previously known comparison stars, we find 8 r-II stars and 35 r-I stars. The r-II stars, including the two previously known examples CS 22892-052 and CS 31082-001, are centred on a metallicity of Fe/H = -2.81, with a very small scatter, on the order of 0.16 dex. The r-I stars are found across practically the entire metallicity range of our sample. We also find three stars with strong enhancements of Eu which are s-process rich. A significant number of new very metal-poor stars are confirmed: 49 stars with Fe/H < -3 and 181 stars with -3 < Fe/H < -2. We find one star with Fe/H < -3.5. We find the scatter in the abundance ratios of Mg, Ca, Sc, Ti, Cr, Fe, Co, and Ni, with respect to Fe and Mg, to be similar to the estimated relative errors and thus the cosmic scatter to be small, perhaps even non-existent. The elements C, Sr, Y, Ba and Eu, and perhaps Zr, show scatter at Fe/H QQQ ? -2.5 significantly larger than can be explained from the errors in the analysis, implying scatter which is cosmic in origin. Significant scatter is observed in abundance ratios between light and heavy neutron-capture elements at low metallicity and low levels of r-process enrichment.
The first stars are predicted to have formed within 200 million years after the Big Bang, initiating the cosmic dawn. A true first star has not yet been discovered, although stars with tiny amounts ...of elements heavier than helium ('metals') have been found in the outer regions ('halo') of the Milky Way. The first stars and their immediate successors should, however, preferentially be found today in the central regions ('bulges') of galaxies, because they formed in the largest over-densities that grew gravitationally with time. The Milky Way bulge underwent a rapid chemical enrichment during the first 1-2 billion years, leading to a dearth of early, metal-poor stars. Here we report observations of extremely metal-poor stars in the Milky Way bulge, including one star with an iron abundance about 10,000 times lower than the solar value without noticeable carbon enhancement. We confirm that most of the metal-poor bulge stars are on tight orbits around the Galactic Centre, rather than being halo stars passing through the bulge, as expected for stars formed at redshifts greater than 15. Their chemical compositions are in general similar to typical halo stars of the same metallicity although intriguing differences exist, including lower abundances of carbon.
We report deep imaging observations of the young, nearby star AB Pic, a member of the large Tucana-Horologium association. We have detected a faint, red source $5.5''$ South of the star with ...JHK colors compatible with that of a young substellar L dwarf. Follow-up observations at two additional epochs confirm, with a confidence level of 4.7σ, that the faint red object is a companion to AB Pic rather than it being a stationary background object. A low resolution K-band spectrum indicates an early-L spectral type for the companion. Finally, evolutionary model predictions based on the JHK photometry of AB Pic b indicate a mass of 13 to 14 $M_{{\rm Jup}}$ if its age is ~30 Myr. Is AB Pic b a massive planet or a minimum mass brown dwarf?
We have spectroscopically identified ~100 G-, K-, and M-type members of the Scorpius-Centaurus complex. To deduce the age of these young stars we compare their Li lambda6708 absorption line strengths ...against those of stars in the TW Hydrae association and beta Pictoris moving group. These line strengths indicate that Sco-Cen stars are younger than beta Pic stars whose ages of ~12 Myr have previously been derived from a kinematic traceback analysis. Our derived age, ~10 Myr, for stars in the Lower Centaurus Crux and Upper Centaurus Lupus subgroups of ScoCen is younger than previously published ages based on the moving cluster method and upper main-sequence fitting. The discrepant ages are likely due to an incorrect (or lack of) cross-calibration between model-dependent and model-independent age-dating methods.
Context. Despite their large number in the Galaxy, M dwarfs remain elusive objects and the modeling of their photosphere has long remained a challenge (molecular opacities, dust cloud formation). ...Aims. Our objectives are to validate the BT-Settl model atmospheres, update the M dwarf Teff-spectral type relation, and find the atmospheric parameters of the stars in our sample. Methods. We compare two samples of optical spectra covering the whole M dwarf sequence with the most recent BT-Settl synthetic spectra and use a χ2 minimization technique to determine Teff. The first sample consists of 97 low-resolution spectra obtained with New Technology Telescope (NTT) at La Silla Observatory. The second sample contains 55 medium-resolution spectra obtained at the Siding Spring Observatory (SSO). The spectral typing is realized by comparison with already classified M dwarfs. Results. We show that the BT-Settl synthetic spectra reproduce the slope of the spectral energy distribution and most of its features. Only the CaOH band at 5570 Å and AlH and NaH hydrides in the blue part of the spectra are still missing in the models. The Teff scale obtained with the higher resolved SSO 2.3 m spectra is consistent with that obtained with the NTT spectra. We compare our Teff scale with those of other authors and with published isochrones using the BT-Settl colors. We also present relations between effective temperature, spectral type, and colors of the M dwarfs.
ABSTRACT
We present chemical abundances for 21 elements (from Li to Eu) in 150 metal-poor Galactic stars spanning −4.1 < Fe/H < −2.1. The targets were selected from the SkyMapper survey and include ...90 objects with Fe/H ≤ −3 of which some 15 have Fe/H ≤ −3.5. When combining the sample with our previous studies, we find that the metallicity distribution function has a power-law slope of Δ(log N)/ΔFe/H = 1.51 ± 0.01 dex per dex over the range −4 ≤ Fe/H ≤ −3. With only seven carbon-enhanced metal-poor stars in the sample, we again find that the selection of metal-poor stars based on SkyMapper filters is biased against highly carbon-rich stars for Fe/H > −3.5. Of the 20 objects for which we could measure nitrogen, 11 are nitrogen-enhanced metal-poor (NEMP) stars. Within our sample, the high NEMP fraction (55 per cent ± 21 per cent) is compatible with the upper range of predicted values (between 12 per cent and 35 per cent). The chemical abundance ratios X/Fe versus Fe/H exhibit similar trends to previous studies of metal-poor stars and Galactic chemical evolution models. We report the discovery of nine new r-I stars, four new r-II stars, one of which is the most metal-poor known, nine low-α stars with α/Fe ≤ 0.15 as well as one unusual star with Zn/Fe = +1.4 and Sr/Fe = +1.2 but with normal Ba/Fe. Finally, we combine our sample with literature data to provide the most extensive view of the early chemical enrichment of the Milky Way Galaxy.
The Carina-Near Moving Group Zuckerman, B; Bessell, M. S; Song, Inseok ...
The Astrophysical journal,
10/2006, Letnik:
649, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We identify a group of 620 comoving, mostly southern hemisphere, 6200 Myr old stars near Earth. Of the stars likely to be members of this Carina-Near moving group, in either its nucleus (630 pc from ...Earth) or its surrounding stream, all but three are plausible members of a multiple star system. The nucleus is (coincidentally) located quite close to the nucleus of the AB Doradus moving group notwithstanding that the two groups have substantially different ages and Galactic space motions, UVW.
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