ORCID Profile
0000-0002-6502-1406
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Astronomical and Space Sciences | Stellar Astronomy and Planetary Systems | Astrobiology | Astronomy And Astrophysics | Space and Solar Physics | Galactic Astronomy | Astronomical and Space Instrumentation
Expanding Knowledge in the Physical Sciences | Physical sciences |
Publisher: American Astronomical Society
Date: 03-08-2020
Publisher: American Astronomical Society
Date: 30-04-2013
Publisher: EDP Sciences
Date: 06-05-2008
Publisher: Oxford University Press (OUP)
Date: 11-01-2017
DOI: 10.1093/MNRAS/STX047
Publisher: Oxford University Press (OUP)
Date: 05-11-2014
Publisher: Oxford University Press (OUP)
Date: 29-04-2016
Publisher: Oxford University Press (OUP)
Date: 15-11-2019
Abstract: We present the characterization of CRTS J055255.7−004426 (=THOR 42), a young eclipsing binary comprising two pre-main sequence M dwarfs (combined spectral type M3.5). This nearby (103 pc), short-period (0.859 d) system was recently proposed as a member of the ∼24 Myr-old 32 Orionis Moving Group. Using ground- and space-based photometry in combination with medium- and high-resolution spectroscopy, we model the light and radial velocity curves to derive precise system parameters. The resulting component masses and radii are 0.497 ± 0.005 and 0.205 ± 0.002 $\\rm {M}_{\\odot }$, and 0.659 ± 0.003 and 0.424 ± 0.002 $\\rm {R}_{\\odot }$, respectively. With mass and radius uncertainties of ∼1 per cent and ∼0.5 per cent, respectively, THOR 42 is one of the most precisely characterized pre-main sequence eclipsing binaries known. Its systemic velocity, parallax, proper motion, colour–magnitude diagram placement and enlarged radii are all consistent with membership in the 32 Ori Group. The system provides a unique opportunity to test pre-main sequence evolutionary models at an age and mass range not well constrained by observation. From the radius and mass measurements we derive ages of 22–26 Myr using standard (non-magnetic) models, in excellent agreement with the age of the group. However, none of the models can simultaneously reproduce the observed mass, radius, temperature and luminosity of the coeval components. In particular, their H–R diagram ages are 2–4 times younger and we infer masses ∼50 per cent smaller than the dynamical values.
Publisher: American Astronomical Society
Date: 04-08-2009
Publisher: EDP Sciences
Date: 09-2017
Publisher: Oxford University Press (OUP)
Date: 17-11-2011
Publisher: American Astronomical Society
Date: 05-07-2017
Publisher: American Astronomical Society
Date: 20-04-2017
Publisher: Oxford University Press (OUP)
Date: 29-03-2022
Abstract: Disentangling distinct stellar populations along the red-giant branches (RGBs) of globular clusters (GCs) is possible by using the pseudo-two-colour diagram dubbed chromosome map (ChM). One of the most intriguing findings is that the so-called first-generation (1G) stars, characterized by the same chemical composition of their natal cloud, exhibit extended sequences in the ChM. Unresolved binaries and internal variations in helium or metallicity have been suggested to explain this phenomenon. Here, we derive high-precision Hubble Space Telescope photometry of the GCs NGC 6362 and NGC 6838 and build their ChMs. We find that both 1G RGB and main-sequence (MS) stars exhibit wider ChM sequences than those of second-generation (2G). The evidence of this feature even among unevolved 1G MS stars indicates that chemical inhomogeneities are imprinted in the original gas. We introduce a pseudo-two-magnitude diagram to distinguish between helium and metallicity, and demonstrate that star-to-star metallicity variations are responsible for the extended 1G sequence. Conversely, binaries provide a minor contribution to the phenomenon. We estimate that the metallicity variations within 1G stars of 55 GCs range from less than [Fe/H]∼0.05 to ∼0.30 and mildly correlate with cluster mass. We exploit these findings to constrain the formation scenarios of multiple populations showing that they are qualitatively consistent with the occurrence of multiple generations. In contrast, the fact that 2G stars have more homogeneous iron content than the 1G challenges the scenarios based on accretion of material processed in massive 1G stars on to existing protostars.
Publisher: Oxford University Press (OUP)
Date: 19-05-2014
Publisher: American Astronomical Society
Date: 16-03-2017
Publisher: American Astronomical Society
Date: 08-06-2015
Publisher: Oxford University Press (OUP)
Date: 02-09-2021
Abstract: Binary star systems are assumed to be co-natal and coeval, and thus to have identical chemical composition. In this work, we aim to test the hypothesis that there is a connection between observed element abundance patterns and the formation of planets using binary stars. Moreover, we also want to test how atomic diffusion might influence the observed abundance patterns. We conduct a strictly line-by-line differential chemical abundance analysis of seven binary systems. Stellar atmospheric parameters and elemental abundances are obtained with extremely high precision (& .5 per cent) using the high-quality spectra from Very Large Telescope/ultraviolet–visual Echelle spectrograph and Keck/high-resolution Echelle spectrometer. We find that four of seven binary systems show subtle abundance differences (0.01–0.03 dex) without clear correlations with the condensation temperature, including two planet-hosting pairs. The other three binary systems exhibit similar degree of abundance differences correlating with the condensation temperature. We do not find any clear relation between the abundance differences and the occurrence of known planets in our systems. Instead, the overall abundance offsets observed in the binary systems (four of seven) could be due to the effects of atomic diffusion. Although giant planet formation does not necessarily imprint chemical signatures on to the host star, the differences in the observed abundance trends with condensation temperature, on the other hand, are likely associated with erse histories of planet formation (e.g. formation location). Furthermore, we find a weak correlation between abundance differences and binary separation, which may provide a new constraint on the formation of binary systems.
Publisher: Oxford University Press (OUP)
Date: 09-05-2016
Publisher: Oxford University Press (OUP)
Date: 04-2015
DOI: 10.1093/MNRAS/STV420
Publisher: American Astronomical Society
Date: 14-07-2015
Publisher: American Astronomical Society
Date: 23-06-2017
Publisher: Oxford University Press (OUP)
Date: 17-07-2019
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.
Publisher: American Astronomical Society
Date: 23-06-2017
Publisher: Oxford University Press (OUP)
Date: 23-03-2016
DOI: 10.1093/MNRAS/STW676
Publisher: Oxford University Press (OUP)
Date: 21-11-2017
Publisher: Springer Science and Business Media LLC
Date: 10-11-2009
Publisher: Oxford University Press (OUP)
Date: 15-05-2019
DOI: 10.1093/MNRAS/STZ112
Publisher: AIP
Date: 2008
DOI: 10.1063/1.2916968
Publisher: EDP Sciences
Date: 22-06-2012
Publisher: Cambridge University Press (CUP)
Date: 08-2009
DOI: 10.1017/S1743921309991001
Abstract: We present abundance measurements in the tidally disrupted globular cluster NGC 6712. In this cluster, there are large star-to-star variations of the light elements C, N, O, F and Na. While such abundance variations are seen in every well-studied globular cluster, they are not found in field stars and indicate that clusters like NGC 6712 cannot provide many field stars and/or field stars do not form in environments with chemical-enrichment histories like those of NGC 6712. Preliminary analysis of NGC 5466, another tidally disrupted cluster, suggests little (if any) abundance variation for O and Na and the abundance ratios [X/Fe] are comparable to field stars at the same metallicity. Therefore, globular clusters like NGC 5466 may have been Galactic building blocks.
Publisher: Oxford University Press (OUP)
Date: 23-12-2019
Abstract: We present a detailed study of stellar rotation in the massive 1.5 Gyr old cluster NGC 1846 in the Large Magellanic Cloud. Similar to other clusters at this age, NGC 1846 shows an extended main-sequence turn-off (eMSTO), and previous photometric studies have suggested it could be bimodal. In this study, we use MUSE integral-field spectroscopy to measure the projected rotational velocities (vsin i) of around $1400$ stars across the eMSTO and along the upper main sequence of NGC 1846. We measure vsin i values up to $\\sim 250\\, {\\rm km\\, s^{-1}}$ and find a clear relation between the vsin i of a star and its location across the eMSTO. Closer inspection of the distribution of rotation rates reveals evidence for a bimodal distribution, with the fast rotators centred around $v\\sin i=140\\, {\\rm km\\, s^{-1}}$ and the slow rotators centred around $v\\sin i=60\\, {\\rm km\\, s^{-1}}$. We further observe a lack of fast rotating stars along the photometric binary sequence of NGC 1846, confirming results from the field that suggest that tidal interactions in binary systems can spin-down stars. However, we do not detect a significant difference in the binary fractions of the fast and slowly rotating sub-populations. Finally, we report on the serendipitous discovery of a planetary nebula associated with NGC 1846.
Publisher: American Astronomical Society
Date: 14-12-2013
Publisher: American Astronomical Society
Date: 09-08-2012
Publisher: American Astronomical Society
Date: 20-12-2008
DOI: 10.1086/592229
Publisher: Oxford University Press (OUP)
Date: 12-11-2022
Abstract: Star formation models predict that the metal-poor initial mass function (IMF) can be substantially different from that observed in the metal-rich Milky Way. This changeover occurs because metal-poor gas clouds cool inefficiently due to their lower abundance of metals and dust. However, predictions for the metal-poor IMF to date rely on assuming solar-scaled abundances, i.e. [X/O] = 0 at all [O/H]. There is now growing evidence that elements such as C and O that dominate metal line cooling in the ISM do not follow solar scaling at low metallicities. In this work, we extend models that predict the variation in the characteristic (or the peak) IMF mass as a function of metallicity using [C/O] ratios derived from observations of metal-poor Galactic stars and of H ii regions in dwarf galaxies. These data show [C/O] & 0 at subsolar [O/H], which leads to a substantially different metal-poor IMF in the metallicity range where C i and C ii cooling dominate ISM thermodynamics, resulting in an increase in the characteristic mass by a factor as large as 7. An important consequence of this difference is a shift in the location of the transition from a top- to a bottom-heavy IMF upwards by 0.5–1 dex in metallicity. Our findings indicate that the IMF is very sensitive to the assumptions around solar-scaled ISM compositions in metal-poor systems (e.g. dwarf galaxies, the Galactic halo, and metal-poor stars) that are a key focus of JWST.
Publisher: American Astronomical Society
Date: 13-12-2012
Publisher: Oxford University Press (OUP)
Date: 22-05-2020
Abstract: We present a line-by-line differential analysis of a s le of 16 planet-hosting stars and 68 comparison stars using high-resolution, high signal-to-noise ratio spectra gathered using Keck. We obtained accurate stellar parameters and high-precision relative chemical abundances with average uncertainties in Teff, log g, [Fe/H], and [X/H] of 15 K, 0.034 cm s−2 , 0.012 dex, and 0.025 dex, respectively. For each planet host, we identify a set of comparison stars and examine the abundance differences (corrected for Galactic chemical evolution effect) as a function of the dust condensation temperature, Tcond, of the in idual elements. While we confirm that the Sun exhibits a negative trend between abundance and Tcond, we also confirm that the remaining planet hosts exhibit a variety of abundance–Tcond trends with no clear dependence upon age, metallicity, or Teff. The ersity in the chemical compositions of planet-hosting stars relative to their comparison stars could reflect the range of possible planet-induced effects present in these planet hosts, from the sequestration of rocky material (refractory poor) to the possible ingestion of planets (refractory rich). Other possible explanations include differences in the time-scale, efficiency and degree of planet formation, or inhomogeneous chemical evolution. Although we do not find an unambiguous chemical signature of planet formation among our s le, the high-precision chemical abundances of the host stars are essential for constraining the composition and structure of their exoplanets.
Publisher: American Astronomical Society
Date: 02-07-2019
Publisher: American Astronomical Society
Date: 05-2021
Abstract: We combine asteroseismology, optical high-resolution spectroscopy, and kinematic analysis for 26 halo red giant branch stars in the Kepler field in the range of −2.5 [Fe/H] −0.6. After applying theoretically motivated corrections to the seismic scaling relations, we obtain an average mass of 0.97 ± 0.03 M ⊙ for our s le of halo stars. Although this maps into an age of ∼7 Gyr, significantly younger than independent age estimates of the Milky Way stellar halo, we considered this apparently young age to be due to the overestimation of stellar mass in the scaling relations. There is no significant mass dispersion among lower red giant branch stars (log g 2), which constrains the relative age dispersion to %, corresponding to Gyr. The precise chemical abundances allow us to separate the stars with [Fe/H] −1.7 into two [Mg/Fe] groups. While the [ α /Fe] and [Eu/Mg] ratios are different between the two subs les, [ s /Eu], where s stands for Ba, La, Ce, and Nd, does not show a significant difference. These abundance ratios suggest that the chemical evolution of the low-Mg population is contributed by Type Ia supernovae, but not by low- to intermediate-mass asymptotic giant branch stars, providing a constraint on its star formation timescale as 100 Myr τ 300 Myr. We also do not detect any significant mass difference between the two [Mg/Fe] groups, thus suggesting that their formation epochs are not separated by more than 1.5 Gyr.
Publisher: American Astronomical Society
Date: 25-05-2018
Publisher: Oxford University Press (OUP)
Date: 11-10-2019
Publisher: American Astronomical Society
Date: 23-03-2009
Publisher: Oxford University Press (OUP)
Date: 08-2013
Publisher: Springer Science and Business Media LLC
Date: 07-07-2021
DOI: 10.1038/S41586-021-03611-2
Abstract: Neutron-star mergers were recently confirmed as sites of rapid-neutron-capture (r-process) nucleosynthesis
Publisher: EDP Sciences
Date: 12-2011
Publisher: American Astronomical Society
Date: 21-08-2013
Publisher: EDP Sciences
Date: 12-2015
Publisher: Oxford University Press (OUP)
Date: 16-07-2014
Publisher: Oxford University Press (OUP)
Date: 14-05-2013
DOI: 10.1093/MNRAS/STT533
Publisher: Oxford University Press (OUP)
Date: 29-01-2016
DOI: 10.1093/MNRAS/STW247
Publisher: Oxford University Press (OUP)
Date: 09-2023
Publisher: American Astronomical Society
Date: 09-10-2014
Publisher: EDP Sciences
Date: 13-03-2012
Publisher: Oxford University Press (OUP)
Date: 04-11-2020
Abstract: In this work, we combine spectroscopic information from the SkyMapper survey for Extremely Metal-Poor stars and astrometry from Gaia DR2 to investigate the kinematics of a s le of 475 stars with a metallicity range of $-6.5 \le \rm [Fe/H] \le -2.05$ dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the Gaia Sausage and Gaia Sequoia accretion events, respectively. The most metal poor of these candidates have metallicities of $\rm [Fe/H]=-3.31\, \mathrm{ and }\, -3.74$, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that ∼21 per cent of the s le have orbits that remain confined to within 3 kpc of the Galactic plane, that is, |Zmax| ≤ 3 kpc. Of particular interest is a subs le (∼11 per cent of the total) of low |Zmax| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = –4.30 and the subs le is best interpreted as the very low-metallicity tail of the metal-weak thick disc population. The low |Zmax|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Zmax|, high eccentricity pro- and retrograde stars are plausibly associated with the Gaia Sausage system. We find that a small fraction of our s le (∼4 per cent of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted.
Publisher: EDP Sciences
Date: 22-03-2012
Publisher: Oxford University Press (OUP)
Date: 28-06-2014
Publisher: Oxford University Press (OUP)
Date: 13-08-2022
Abstract: M 22 (NGC 6656) is a chemically complex globular cluster-like system reported to harbour heavy element abundance variations. However, the extent of these variations and the origin of this cluster is still debated. In this work, we investigate the chemical in-homogeneity of M 22 using differential line-by-line analysis of high-quality (R = 110 000, S/N = 300 per pixel at 514 nm) VLT/UVES spectra of six carefully chosen red giant branch stars. By achieving abundance uncertainties as low as ∼0.01 dex (∼2 per cent), this high precision data validates the results of previous studies and reveals variations in Fe, Na, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, Zn, Y, Zr, La, Ce, Nd, Sm, and Eu. Additionally, we can confirm that the cluster hosts two stellar populations with a spread of at least 0.24 dex in [Fe/H] and an average s-process abundance spread of 0.65 dex. In addition to global variations across the cluster, we also find non-negligible variations within each of the two populations, with the more metal-poor population hosting larger spreads in elements heavier than Fe than the metal-rich. We address previous works that do not identify anomalous abundances and relate our findings to our current dynamical understanding of the cluster. Given our results, we suggest that M 22 is either a nuclear star cluster, the product of two merged clusters, or an original building block of the Milky Way.
Publisher: Oxford University Press (OUP)
Date: 08-03-2019
DOI: 10.1093/MNRAS/STZ681
Publisher: American Astronomical Society
Date: 24-11-2014
Publisher: Oxford University Press (OUP)
Date: 11-08-2016
Publisher: American Association for the Advancement of Science (AAAS)
Date: 12-08-2011
Abstract: Most of the progenitors of type Ia supernovae in nearby spiral galaxies may be white dwarf−normal star binary systems.
Publisher: Oxford University Press (OUP)
Date: 21-06-2022
Abstract: Using spectra obtained with the VLT/FORS2 and Gemini-S/GMOS-S instruments, we have investigated carbon, nitrogen, and sodium abundances in a s le of red giant members of the Small Magellanic Cloud star cluster Kron 3. The metallicity and luminosity of the cluster are comparable to those of Galactic globular clusters but it is notably younger (age ≈ 6.5 Gyr). We have measured the strengths of the CN and CH molecular bands, finding a bimodal CN band-strength distribution and a CH/CN anticorrelation. Application of spectrum synthesis techniques reveals that the difference in the mean [N/Fe] and [C/Fe] values for the CN-strong and CN-weak stars are Δ & [N/Fe]& = 0.63 ± 0.16 dex and Δ & [C/Fe]& = −0.01 ± 0.07 dex after applying corrections for evolutionary mixing. We have also measured sodium abundances from the Na D lines finding an observed range in [Na/Fe] of ∼0.6 dex that correlates positively with the [N/Fe] values and a Δ & [Na/Fe]& = 0.12 ± 0.12 dex. While the statistical significance of the sodium abundance difference is not high, the observed correlation between the Na and N abundances supports its existence. The outcome represents the first star-by-star demonstration of correlated abundance variations involving sodium in an intermediate-age star cluster. The results add to existing photometric and spectroscopic indications of the presence of multiple populations in intermediate-age clusters with masses in excess of ∼105 M⊙. It confirms that the mechanism(s) responsible for the multiple populations in ancient globular clusters cannot solely be an early cosmological effect applying only in old clusters.
Publisher: American Astronomical Society
Date: 13-12-2019
Publisher: Oxford University Press (OUP)
Date: 25-10-2019
Publisher: American Astronomical Society
Date: 09-09-2011
Publisher: EDP Sciences
Date: 12-2014
Publisher: Oxford University Press (OUP)
Date: 31-12-2015
Publisher: American Astronomical Society
Date: 13-08-2010
Publisher: American Astronomical Society
Date: 09-07-2015
Publisher: EDP Sciences
Date: 18-03-2016
Publisher: Oxford University Press (OUP)
Date: 15-02-2014
DOI: 10.1093/MNRAS/STU118
Publisher: Oxford University Press (OUP)
Date: 07-03-2019
DOI: 10.1093/MNRAS/STZ645
Abstract: We present results from the analysis of high-resolution spectra obtained with the Keck HIRES spectrograph for a s le of 17 candidate extremely metal-poor (EMP) stars originally selected from commissioning data obtained with the SkyMapper telescope. Fourteen of the stars have not been observed previously at high dispersion. Three have [Fe/H] ≤ −3.0, while the remainder, with two more metal-rich exceptions, have −3.0 ≤ [Fe/H] ≤ −2.0 dex. Apart from Fe, we also derive abundances for the elements C, N, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Co, Ni, and Zn, and for n-capture elements Sr, Ba, and Eu. None of the current s le of stars is found to be carbon-rich. In general, our chemical abundances follow previous trends found in the literature, although we note that two of the most metal-poor stars show very low [Ba/Fe] (∼−1.7) coupled with low [Sr/Ba] (∼−0.3). Such stars are relatively rare in the Galactic halo. One further star, and possibly two others, meet the criteria for classification as a r-I star. This study, together with that of Jacobson et al. (2015), completes the outcomes of the SkyMapper commissioning data survey for EMP stars.
Publisher: Oxford University Press (OUP)
Date: 17-10-2018
Publisher: EDP Sciences
Date: 07-2019
DOI: 10.1051/0004-6361/201935306
Abstract: Context. The benchmark open cluster M 67 is known to have solar metallicity and an age similar to that of the Sun. It thus provides us with a great opportunity to study the properties of solar twins, as well as the evolution of Sun-like stars. Aims. Previous spectroscopic studies of M 67 reported possible subtle changes in stellar surface abundances throughout the stellar evolutionary phase, namely the effect of atomic diffusion. In this study we attempt to confirm and quantify more precisely the effect of atomic diffusion, and to explore the level of chemical (in)homogeneity in M 67. Methods. We presented a strictly line-by-line differential chemical abundance analysis of two groups of stars in M 67: three turn-off stars and three subgiants. Stellar atmospheric parameters and elemental abundances were obtained with very high precision using the Keck/HIRES spectra. Results. The subgiants in our s le show negligible abundance variations (≤0.02 dex), which implies that M 67 was born chemically homogeneous. We note that there is a significant abundance difference (~0.1–0.2 dex) between subgiants and turn-off stars, which can be interpreted as the signature of atomic diffusion. Qualitatively stellar models with diffusion agree with the observed abundance results. Some turn-off stars do not follow the general pattern, which suggests that in some cases diffusion can be inhibited, or they might have undergone some sort of mixing event related to planets. Conclusions. Our results pose additional challenges for chemical tagging when using turn-off stars. In particular, the effects of atomic diffusion, which could be as large as 0.1–0.2 dex, must be taken into account in order for chemical tagging to be successfully applied.
Publisher: EDP Sciences
Date: 06-04-2016
Publisher: Oxford University Press (OUP)
Date: 15-08-2016
Publisher: Oxford University Press (OUP)
Date: 16-05-2018
Publisher: Springer Science and Business Media LLC
Date: 11-2015
DOI: 10.1038/NATURE15747
Abstract: 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.
Publisher: Oxford University Press (OUP)
Date: 25-07-2014
Publisher: Oxford University Press (OUP)
Date: 26-10-2020
Abstract: The ‘chromosome map’ diagram (ChM) proved a successful tool to identify and characterize multiple populations (MPs) in 59 Galactic globular clusters (GCs). Here, we construct ChMs for 11 GCs of both Magellanic Clouds (MCs) and with different ages to compare MPs in Galactic and extragalactic environments, and explore whether this phenomenon is universal through ‘place’ and ‘time’. MPs are detected in five clusters. The fractions of 1G stars, ranging from ∼50 per cent to & per cent, are significantly higher than those observed in Galactic GCs with similar present-day masses. By considering both Galactic and MC clusters, the fraction of 1G stars exhibits: (i) a strong anticorrelation with the present-day mass, and (ii) with the present-day mass of 2G stars (iii) a mild anticorrelation with 1G present-day mass. All Galactic clusters without MPs have initial masses smaller than ∼1.5 · 105 M⊙ but a mass threshold governing the occurrence of MPs seems challenged by massive simple-population MC GCs (iv) Milky Way clusters with large perigalactic distances typically host larger fractions of 1G stars, but the difference disappears when we use initial cluster masses. These facts are consistent with a scenario where the stars lost by GCs mostly belong to the 1G. By exploiting recent work based on Gaia, half of the known Type II GCs appear clustered in a distinct region of the integral of motions space, thus suggesting a common progenitor galaxy. Except for these Type II GCs, we do not find any significant difference in the MPs between clusters associated with different progenitors.
Publisher: EDP Sciences
Date: 06-2018
DOI: 10.1051/0004-6361/201832701
Abstract: Differences in the elemental abundances of planet-hosting stars in binary systems can give important clues and constraints about planet formation and evolution. In this study we performed a high-precision, differential elemental abundance analysis of a wide binary system, HD 80606 / 80607 , based on high-resolution spectra with high signal-to-noise ratio obtained with Keck/HIRES. HD 80606 is known to host a giant planet with the mass of four Jupiters, but no planet has been detected around HD 80607 so far. We determined stellar parameters as well as abundances for 23 elements for these two stars with extremely high precision. Our main results are that (i) we confirmed that the two components share very similar chemical compositions, but HD 80606 is marginally more metal-rich than HD 80607 , with an average difference of +0.013 ± 0.002 dex ( σ = 0.009 dex) and (ii) there is no obvious trend between abundance differences and condensation temperature. Assuming that this binary formed from material with the same chemical composition, it is difficult to understand how giant planet formation could produce the present-day photospheric abundances of the elements we measure. We cannot exclude the possibility that HD 80606 might have accreted about 2.5 to 5 M Earth material onto its surface, possibly from a planet destabilised by the known highly eccentric giant.
Publisher: Oxford University Press (OUP)
Date: 09-09-2020
Abstract: We present new fluorine abundance estimations in two carbon enhanced metal-poor (CEMP) stars, HE 1429−0551 and HE 1305+0007. HE 1429−0551 is also enriched in slow neutron-capture process (s-process) elements, a CEMP-s, and HE 1305+0007 is enhanced in both, slow and rapid neutron-capture process elements, a CEMP-s/r. The F abundances estimates are derived from the vibration–rotation transition of the HF molecule at 23358.6 Å using high-resolution infrared spectra obtained with the Immersion Grating Infrared Spectrometer (IGRINS) at the 4-m class Lowell Discovery Telescope. Our results include an F abundance measurement in HE 1429−0551 of A(F) = +3.93 ([F/Fe] = +1.90) at [Fe/H] = −2.53, and an F upper limit in HE 1305+0007 of A(F) & +3.28 ([F/Fe] & +1.00) at [Fe/H] = −2.28. Our new derived F abundance in HE 1429−0551 makes this object the most metal-poor star where F has been detected. We carefully compare these results with literature values and state-of-the-art CEMP-s model predictions including detailed asymptotic giant branch (AGB) nucleosynthesis and binary evolution. The modelled fluorine abundance for HE 1429−0551 is within reasonable agreement with our observed abundance, although is slightly higher than our observed value. For HE 1429−0551, our findings support the scenario via mass transfer by a primary companion during its thermally pulsing phase. Our estimated upper limit in HE 1305+0007, along with data from the literature, shows large discrepancies compared with AGB models. The discrepancy is principally due to the simultaneous s- and r-process element enhancements which the model struggles to reproduce.
Publisher: Oxford University Press (OUP)
Date: 12-09-2019
Abstract: We present and discuss the results of a search for extremely metal-poor stars based on photometry from data release DR1.1 of the SkyMapper imaging survey of the southern sky. In particular, we outline our photometric selection procedures and describe the low-resolution (R ≈ 3000) spectroscopic follow-up observations that are used to provide estimates of effective temperature, surface gravity, and metallicity ([Fe/H]) for the candidates. The selection process is very efficient: of the 2618 candidates with low-resolution spectra that have photometric metallicity estimates less than or equal to −2.0, 41 per cent have [Fe/H] ≤ −2.75 and only approximately seven per cent have [Fe/H] −2.0 dex. The most metal-poor candidate in the s le has [Fe/H] −4.75 and is notably carbon rich. Except at the lowest metallicities ([Fe/H] −4), the stars observed spectroscopically are dominated by a ‘carbon-normal’ population with [C/Fe]1D, LTE ≤ +1 dex. Consideration of the A(C)1D, LTE versus [Fe/H]1D, LTE diagram suggests that the current selection process is strongly biased against stars with A(C)1D, LTE 7.3 (predominantly CEMP-s) while any bias against stars with A(C)1D, LTE 7.3 and [C/Fe]1D,LTE +1 (predominantly CEMP-no) is not readily quantifiable given the uncertainty in the SkyMapper v-band DR1.1 photometry. We find that the metallicity distribution function of the observed s le has a power-law slope of Δ(Log N)/Δ[Fe/H] = 1.5 ± 0.1 dex per dex for −4.0 ≤ [Fe/H] ≤ −2.75, but appears to drop abruptly at [Fe/H] ≈ −4.2, in line with previous studies.
Publisher: Oxford University Press (OUP)
Date: 04-12-2012
DOI: 10.1093/MNRAS/STS305
Abstract: We describe the motivation, field locations and stellar selection for the Abundances and Radial velocity Galactic Origins Survey (ARGOS) spectroscopic survey of 28 000 stars in the bulge and inner disc of the Milky Way galaxy across latitudes of b = −5° to −10°. The primary goal of this survey is to constrain the formation processes of the bulge and establish whether it is predominantly a merger or instability remnant. From the spectra (R = 11 000), we have measured radial velocities and determined stellar parameters, including metallicities and [α/Fe] ratios. Distances were estimated from the derived stellar parameters and about 14 000 stars are red giants within 3.5 kpc of the Galactic Centre. In this paper, we present the observations and analysis methods. Subsequent papers (III and IV) will discuss the stellar metallicity distribution and kinematics of the Galactic bulge and inner disc, and the implications for the formation of the bulge.
Publisher: Springer Science and Business Media LLC
Date: 29-05-2013
DOI: 10.1038/NATURE12191
Abstract: The asymptotic giant branch (AGB) phase is the final stage of nuclear burning for low-mass stars. Although Milky Way globular clusters are now known to harbour (at least) two generations of stars, they still provide relatively homogeneous s les of stars that are used to constrain stellar evolution theory. It is predicted by stellar models that the majority of cluster stars with masses around the current turn-off mass (that is, the mass of the stars that are currently leaving the main sequence phase) will evolve through the AGB phase. Here we report that all of the second-generation stars in the globular cluster NGC 6752--70 per cent of the cluster population--fail to reach the AGB phase. Through spectroscopic abundance measurements, we found that every AGB star in our s le has a low sodium abundance, indicating that they are exclusively first-generation stars. This implies that many clusters cannot reliably be used for star counts to test stellar evolution timescales if the AGB population is included. We have no clear explanation for this observation.
Publisher: Oxford University Press (OUP)
Date: 31-01-2013
DOI: 10.1093/MNRAS/STS629
Publisher: Oxford University Press (OUP)
Date: 31-05-2014
DOI: 10.1093/MNRAS/STU806
Publisher: American Astronomical Society
Date: 15-11-2012
Publisher: Oxford University Press (OUP)
Date: 29-09-2023
Publisher: Springer Science and Business Media LLC
Date: 02-2014
DOI: 10.1038/NATURE12990
Abstract: The element abundance ratios of four low-mass stars with extremely low metallicities (abundances of elements heavier than helium) indicate that the gas out of which the stars formed was enriched in each case by at most a few--and potentially only one--low-energy supernova. Such supernovae yield large quantities of light elements such as carbon but very little iron. The dominance of low-energy supernovae seems surprising, because it had been expected that the first stars were extremely massive, and that they disintegrated in pair-instability explosions that would rapidly enrich galaxies in iron. What has remained unclear is the yield of iron from the first supernovae, because hitherto no star has been unambiguously interpreted as encapsulating the yield of a single supernova. Here we report the optical spectrum of SMSS J031300.36-670839.3, which shows no evidence of iron (with an upper limit of 10(-7.1) times solar abundance). Based on a comparison of its abundance pattern with those of models, we conclude that the star was seeded with material from a single supernova with an original mass about 60 times that of the Sun (and that the supernova left behind a black hole). Taken together with the four previously mentioned low-metallicity stars, we conclude that low-energy supernovae were common in the early Universe, and that such supernovae yielded light-element enrichment with insignificant iron. Reduced stellar feedback both chemically and mechanically from low-energy supernovae would have enabled first-generation stars to form over an extended period. We speculate that such stars may perhaps have had an important role in the epoch of cosmic reionization and the chemical evolution of early galaxies.
Publisher: Oxford University Press (OUP)
Date: 03-10-2023
Publisher: Oxford University Press (OUP)
Date: 04-11-2022
Abstract: Young star clusters enable us to study the effects of stellar rotation on an ensemble of stars of the same age and across a wide range in stellar mass and are therefore ideal targets for understanding the consequences of rotation on stellar evolution. We combine MUSE spectroscopy with HST photometry to measure the projected rotational velocities (Vsin i) of 2184 stars along the split main sequence and on the main sequence turn-off (MSTO) of the 100 Myr-old massive ($10^5\\, {\\rm M_{\\odot }}$) star cluster NGC 1850 in the Large Magellanic Cloud. At fixed magnitude, we observe a clear correlation between Vsin i and colour, in the sense that fast rotators appear redder. The average Vsin i values for stars on the blue and red branches of the split main sequence are $\\sim \\! 100\\, {\\rm km\\, s^{-1}}$ and $\\sim \\! 200\\, {\\rm km\\, s^{-1}}$, respectively. The values correspond to about $25-30{{\\ \\rm per\\ cent}}$ and $50-60{{\\ \\rm per\\ cent}}$ of the critical rotation velocity and imply that rotation rates comparable to those observed in field stars of similar masses can explain the split main sequence. Our spectroscopic s le contains a rich population of ∼200 fast rotating Be stars. The presence of shell features suggests that 23 per cent of them are observed through their decretion discs, corresponding to a disc opening angle of 15 degrees. These shell stars can significantly alter the shape of the MSTO, hence care should be taken when interpreting this photometric feature. Overall, our findings impact our understanding of the evolution of young massive clusters and provide new observational constraints for testing stellar evolutionary models.
Publisher: American Astronomical Society
Date: 12-2021
Abstract: Recent work has revealed two classes of globular clusters (GCs), dubbed Type I and Type II. Type II GCs are characterized by both a blue and a red red giant branch composed of stars with different metallicities, often coupled with distinct abundances in the slow neutron-capture elements (s-elements). Here we continue the chemical tagging of Type II GCs by adding the two least massive clusters of this class, NGC 1261 and NGC 6934. Based on both spectroscopy and photometry, we find red stars in NGC 1261 to be slightly enhanced in [Fe/H] by ∼0.1 dex and confirm that red stars of NGC 6934 are enhanced in iron by ∼0.2 dex. Neither NGC 1261 nor NGC 6934 show internal variations in the s-elements, which suggests a GC mass threshold for the occurrence of s -process enrichment. We found a significant correlation between the additional Fe locked in the red stars of Type II GCs and the present-day mass of the cluster. Nevertheless, most Type II GCs retained a small fraction of Fe produced by SNe II, lower than the 2% NGC 6273, M54, and ω Centauri are remarkable exceptions. In the Appendix, we infer for the first time chemical abundances of lanthanum, assumed as representative of the s-elements, in M54, the GC located in the nucleus of the Sagittarius dwarf galaxy. Red-sequence stars are marginally enhanced in [La/Fe] by 0.10 ± 0.06 dex, in contrast with the large [La/Fe] spread of most Type II GCs. We suggest that different processes are responsible for the enrichment in iron and s -elements in Type II GCs.
Publisher: Oxford University Press (OUP)
Date: 15-07-2021
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 s le with our previous studies, we find that the metallicity distribution function has a power-law slope of Δ(log N)/Δ[Fe/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 s le, 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 s le, 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 s le with literature data to provide the most extensive view of the early chemical enrichment of the Milky Way Galaxy.
No related organisations have been discovered for David Yong.
Start Date: 2012
End Date: 12-2015
Amount: $355,000.00
Funder: Australian Research Council
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Amount: $28,500.00
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End Date: 12-2019
Amount: $674,404.00
Funder: Australian Research Council
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End Date: 07-2023
Amount: $792,859.00
Funder: Australian Research Council
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