ORCID Profile
0000-0002-9634-3002
Current Organisations
DSM Research
,
Rijksinstituut voor Volksgezondheid en Milieu
,
Ministerie van Infrastructuur en Milieu
,
University of Amsterdam
,
Technische Universiteit Delft
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Publications
Gaia and Hubble Unveil the Kinematics of Stellar Populations in the Type II Globular Clusters ω Centauri and M22
Publisher: American Astronomical Society
Date: 03-08-2020
Exploring the Galaxy’s halo and very metal-weak thick disc with SkyMapper and Gaia DR2
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.
The complex stellar system M 22: confirming abundance variations with high precision differential measurements
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.
The halo+cluster system of the Galactic globular cluster NGC 1851★
Publisher: Oxford University Press (OUP)
Date: 28-06-2014
The chemical compositions of multiple stellar populations in the globular cluster NGC 2808
Publisher: Oxford University Press (OUP)
Date: 06-12-2022
Abstract: Pseudo two-colour diagrams or Chromosome maps (ChM) indicate that NGC 2808 host five different stellar populations. The existing ChMs have been derived by the Hubble Space Telescope photometry, and comprise of stars in a small field of view around the cluster centre. To overcome these limitations, we built a ChM with U, B, I photometry from ground-based facilities that disentangle the multiple stellar populations of NGC 2808 over a wider field of view. We used spectra collected by GIRAFFE@VLT in a s le of 70 red giant branch and seven asymptotic giant branch (AGB) stars to infer the abundances of C, N, O, Al, Fe, and Ni, which combined with literature data for other elements (Li, Na, Mg, Si, Ca, Sc, Ti, Cr, and Mn), and together with both the classical and the new ground-based ChMs, provide the most complete chemical characterization of the stellar populations in NGC 2808 available to date. As typical of the multiple population phenomenon in globular clusters, the light elements vary from one stellar population to another whereas the iron peak elements show negligible variation between the different populations (at a level of ≲0.10 dex). Our AGB stars are also characterized by the chemical variations associated with the presence of multiple populations, confirming that this phase of stellar evolution is affected by the phenomenon as well. Intriguingly, we detected one extreme O-poor AGB star (consistent with a high He abundance), challenging stellar evolution models that suggest that highly He-enriched stars should avoid the AGB phase and evolve as AGB-manqué star.
The Gaia-ESO Survey: the most metal-poor stars in the Galactic bulge
Publisher: Oxford University Press (OUP)
Date: 05-11-2014
The EMBLA survey – metal-poor stars in the Galactic bulge
Publisher: Oxford University Press (OUP)
Date: 29-04-2016
Chromosome maps of globular clusters from wide-field ground-based photometry
Publisher: Oxford University Press (OUP)
Date: 11-2022
Abstract: Hubble Space Telescope (HST) photometry is providing an extensive analysis of globular clusters (GCs). In particular, the pseudo-two-colour diagram dubbed 'chromosome map (ChM)’ allowed to detect and characterize their multiple populations with unprecedented detail. The main limitation of these studies is the small field of view of HST, which makes it challenging to investigate some important aspects of the multiple populations, such as their spatial distributions and the internal kinematics in the outermost cluster regions. To overcome this limitation, we analyse state-of-art wide-field photometry of 43 GCs obtained from ground-based facilities. We derived high-resolution reddening maps and corrected the photometry for differential reddening when needed. We use photometry in the U, B, and I bands to introduce the ΔcU, B, I versus ΔB, I ChM of red-giant branch (RGB) and asymptotic-giant branch stars. We demonstrate that this ChM, which is built with wide-band ground-based photometry, is an efficient tool to identify first- and second-generation stars (1G and 2G) over a wide field of view. To illustrate its potential, we derive the radial distribution of multiple populations in NGC 288 and infer their chemical composition. We present the ChMs of RGB stars in 29 GCs and detect a significant degree of variety. The fraction of 1G and 2G stars, the number of subpopulations, and the extension of the ChMs significantly change from one cluster to another. Moreover, the metal-poor and metal-rich stars of Type II GCs define distinct sequences in the ChM. We confirm the presence of extended 1G sequences.
Multiple Stellar Populations in Asymptotic Giant Branch Stars of Galactic Globular Clusters
Publisher: American Astronomical Society
Date: 03-2021
Abstract: Multiple stellar populations (MPs) are a distinct characteristic of globular clusters (GCs). Their general properties have been widely studied among main-sequence, red giant branch (RGB), and horizontal branch (HB) stars, but a common framework is still missing at later evolutionary stages. We studied the MP phenomenon along the asymptotic giant branch (AGB) sequences in 58 GCs, observed with the Hubble Space Telescope in UV and optical filters. Using UV–optical color–magnitude diagrams, we selected the AGB members of each cluster and identified the AGB candidates of the metal-enhanced population in type II GCs. We studied the photometric properties of the AGB stars and compared them to theoretical models derived from synthetic spectral analysis. We observed the following features: (i) the spread of AGB stars in photometric indices sensitive to variations of light elements and helium is typically larger than that expected from photometric errors (ii) the fraction of metal-enhanced stars in the AGB is lower than that in the RGB in most of the type II GCs (iii) the fraction of 1G stars derived from the chromosome map of AGB stars in 15 GCs is larger than that of RGB stars and (v) the AGB/HB frequency correlates with the average mass of the most helium-enriched population. These findings represent clear evidence of the presence of MPs along the AGB of Galactic GCs and indicate that a significant fraction of helium-enriched stars, which have lower mass in the HB, do not evolve to the AGB phase, leaving the HB sequence toward higher effective temperatures, as predicted by the AGB manqué scenario.
Evidence of globular cluster abundance anomalies in the SMC intermediate-age cluster Kron 3
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.
Chemical Abundances along the 1G Sequence of the Chromosome Maps: The Globular Cluster NGC 3201
Publisher: American Astronomical Society
Date: 13-12-2019
Spectroscopy and Photometry of Multiple Populations along the Asymptotic Giant Branch of NGC 2808 and NGC 6121 (M4)
Publisher: American Astronomical Society
Date: 05-07-2017
Different Stellar Rotations in the Two Main Sequences of the Young Globular Cluster NGC 1818: The First Direct Spectroscopic Evidence
Publisher: American Astronomical Society
Date: 22-08-2018
Multiple Stellar Populations in Metal-poor Globular Clusters with JWST: A NIRCam View of M92
Publisher: American Astronomical Society
Date: 08-2023
Abstract: Recent work on metal-intermediate globular clusters (GCs) with [Fe/H] = −1.5 and −0.75 has illustrated the theoretical behavior of multiple populations in photometric diagrams obtained with the JWST. These results are confirmed by observations of multiple populations among the M dwarfs of 47 Tucanae. Here we explore multiple populations in metal-poor GCs with [Fe/H] = −2.3. We take advantage of synthetic spectra and isochrones that account for the chemical composition of multiple populations to identify photometric diagrams that separate the distinct stellar populations of GCs. We derive high-precision photometry and proper motion for main-sequence (MS) stars in the metal-poor GC M92 from JWST and Hubble Space Telescope images. We identify a first-generation (1G) and two main groups of second-generation (2G A and 2G B ) stars and investigate their kinematics and chemical composition. We find isotropic motions with no differences among the distinct populations. The comparison between the observed colors of the M92 stars and the colors derived by synthetic spectra reveals that the helium abundances of 2G A and 2G B stars are higher than those of the 1G by Δ Y ∼ 0.01 and 0.04, respectively. The m F090W versus m F090W − m F277W color–magnitude diagram shows that below the knee MS stars exhibit a wide color broadening due to multiple populations. We constrain the amount of oxygen variation needed to reproduce the observed MS width, which is consistent with results on red giant branch stars. We conclude that multiple populations with masses of ∼0.1–0.8 M ⊙ share similar chemical compositions.
Peeking beneath the precision floor − II. Probing the chemo-dynamical histories of the potential globular cluster siblings, NGC 288 and NGC 362
Publisher: Oxford University Press (OUP)
Date: 25-04-2023
Abstract: The assembly history of the Milky Way (MW) is a rapidly evolving subject, with numerous small accretion events and at least one major merger proposed in the MW’s history. Accreted alongside these dwarf galaxies are globular clusters (GCs), which act as spatially coherent remnants of these past events. Using high precision differential abundance measurements from our recently published study, we investigate the likelihood that the MW clusters NGC 362 and NGC 288 are galactic siblings, accreted as part of the Gaia-Sausage-Enceladus (GSE) merger. To do this, we compare the two GCs at the 0.01 dex level for 20 + elements for the first time. Strong similarities are found, with the two showing chemical similarity on the same order as those seen between the three LMC GCs, NGC 1786, NGC 2210-, and NGC 2257. However, when comparing GC abundances directly to GSE stars, marked differences are observed. NGC 362 shows good agreement with GSE stars in the ratio of Eu to Mg and Si, as well as a clear dominance in the r- compared to the s-process, while NGC 288 exhibits only a slight r-process dominance. When fitting the two GC abundances with a GSE-like galactic chemical evolution model, NGC 362 shows agreement with both the model predictions and GSE abundance ratios (considering Si, Ni, Ba, and Eu) at the same metallicity. This is not the case for NGC 288. We propose that the two are either not galactic siblings, or GSE was chemically inhomogeneous enough to birth two similar, but not identical clusters with distinct chemistry relative to constituent stars.
Discovery of Extended Main Sequence Turnoffs in Galactic Open Clusters
Publisher: American Astronomical Society
Date: 17-08-2018
Nucleosynthesis in a Primordial Supernova: Carbon and Oxygen Abundances in SMSS J031300.36-670839.3
Publisher: American Astronomical Society
Date: 08-06-2015
Peeking beneath the precision floor – I. Metallicity spreads and multiple elemental dispersions in the globular clusters NGC 288 and NGC 362
Publisher: Oxford University Press (OUP)
Date: 22-10-2022
Abstract: The view of globular clusters (GCs) as simple systems continues to unravel, revealing complex objects hosting multiple chemical peculiarities. Using differential abundance analysis, we probe the chemistry of the Type I GC, NGC 288 and the Type II GC, NGC 362 at the 2 per cent level for the first time. We measure 20 elements and find differential measurement uncertainties of the order of 0.01–0.02 dex in both clusters. The smallest uncertainties are measured for Fe i in both clusters, with an average uncertainty of ∼0.013 dex. Dispersion in the abundances of Na, Al, Ti i, Ni, Fe i, Y, Zr, Ba, and Nd are recovered in NGC 288, none of which can be explained by a spread in He. This is the first time, to our knowledge, a statistically significant spread in s-process elements and a potential spread in metallicity has been detected in NGC 288. In NGC 362, we find significant dispersion in the same elements as NGC 288, with the addition of Co, Cu, Zn, Sr, La, Ce, and Eu. Two distinct groups are recovered in NGC 362, separated by 0.3 dex in average differential s-process abundances. Given strong correlations between Al and several s-process elements, and a significant correlation between Mg and Si, we propose that the s-process rich group is younger. This agrees with asymptotic giant branch star (AGB) enrichment between generations, if there is overlap between low- and intermediate-mass AGBs. In our scenario, the older population is dominated by the r-process with a ΔLa–ΔEu ratio of −0.16 ± 0.06. We propose that the r-process dominance and dispersion found in NGC 362 are primordial.
Iron and s-elements abundance variations in NGC 5286: comparison with ‘anomalous' globular clusters and Milky Way satellites
Publisher: Oxford University Press (OUP)
Date: 04-2015
DOI: 10.1093/MNRAS/STV420
Keck HIRES spectroscopy of SkyMapper commissioning survey candidate extremely metal-poor stars
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.
HIGH-RESOLUTION SPECTROSCOPIC STUDY of EXTREMELY METAL-POOR STAR CANDIDATES from the SKYMAPPER SURVEY
Publisher: American Astronomical Society
Date: 09-07-2015
The lowest detected stellar Fe abundance: the halo star SMSS J160540.18-144323.1
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.
STRÖMGREN SURVEY FOR ASTEROSEISMOLOGY AND GALACTIC ARCHAEOLOGY: LET THE SAGA BEGIN
Publisher: American Astronomical Society
Date: 09-05-2014
Chemical abundances in the multiple sub-giant branch of 47 Tucanae: insights on its faint sub-giant branch component
Publisher: Oxford University Press (OUP)
Date: 15-03-2016
DOI: 10.1093/MNRAS/STW611
Multiple stellar populations in Magellanic Cloud clusters – VI. A survey of multiple sequences and Be stars in young clusters
Publisher: Oxford University Press (OUP)
Date: 14-03-2018
DOI: 10.1093/MNRAS/STY661
The chemical compositions of solar twins in the open cluster M67
Publisher: Oxford University Press (OUP)
Date: 15-08-2016
Risk-maps informing land-use planning processes
Publisher: Elsevier BV
Date: 06-2007
DOI: 10.1016/J.JHAZMAT.2006.11.032
Abstract: The definition of safety distances as required by Art 12 of the Seveso II Directive on dangerous substances (96/82/EC) is necessary to minimize the consequences of potential major accidents. As they affect the land-use destinations of involved areas, safety distances can be considered as risk tolerability criteria with a territorial reflection. Recent studies explored the suitability of using Geographical Information System technologies to support their elaboration and visual rendering. In particular, the elaboration of GIS "risk-maps" has been recognized as functional to two objectives: connecting spatial planners and safety experts during decision making processes and communicating risk to non-experts audiences. In order to elaborate on these findings and to verify their reflection on European practices, the article presents the result of a comparative study between the United Kingdom and the Netherlands recent developments. Their land-use planning practices for areas falling under Seveso II requirements are explored. The role of GIS risk-maps within decisional processes is analyzed and the reflection on the transparency and accessibility of risk-information is commented. Recommendations for further developments are given.
Multiple populations in globular clusters and their parent galaxies
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.
The closest extremely low-mass white dwarf to the Sun
Publisher: Oxford University Press (OUP)
Date: 24-04-2020
Abstract: We present the orbit and properties of 2MASS J050051.85−093054.9, establishing it as the closest (d ≈ 71 pc) extremely low-mass white dwarf to the Sun. We find that this star is hydrogen rich with $T_\\textrm {eff}\\approx 10\\, 500$ K, log g ≈ 5.9, and, following evolutionary models, has a mass of ≈0.17 M⊙. Independent analysis of radial velocity and Transiting Exoplanet Survey Satellite(TESS) photometric time series reveals an orbital period of ≈9.5 h. Its high velocity litude ($K\\approx 144~\\textrm {km}\\, \\textrm {s}^{-1}$) produces a measurable Doppler beaming effect in the TESSlight curve with an litude of 1 mmag. The unseen companion is most likely a faint white dwarf. J0500−0930 belongs to a class of post-common envelope systems that will most likely merge through unstable mass transfer and in specific circumstances lead to Type Ia supernova explosions.
The SkyMapper DR1.1 search for extremely metal-poor stars
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.
The COMBS Survey - II. Distinguishing the metal-poor bulge from the halo interlopers
Publisher: Oxford University Press (OUP)
Date: 06-01-2021
Abstract: The metal-poor stars in the bulge are important relics of the Milky Way’s formation history, as simulations predict that they are some of the oldest stars in the Galaxy. In order to determine if they are truly ancient stars, we must understand their origins. Currently, it is unclear if the metal-poor stars in the bulge ([Fe/H] & −1 dex) are merely halo interlopers, a unique accreted population, part of the boxy eanut-shaped bulge, or a classical bulge population. In this work, we use spectra from the VLT/FLAMES spectrograph to obtain metallicity estimates using the Ca-II triplet of 473 bulge stars (187 of which have [Fe/H] & −1 dex), targeted using SkyMapper photometry. We also use Gaia DR2 data to infer the Galactic positions and velocities along with orbital properties for 523 stars. We employ a probabilistic orbit analysis and find that about half of our s le has a & per cent probability of being bound to the bulge, and half are halo interlopers. We also see that the occurrence rate of halo interlopers increases steadily with decreasing metallicity across the full range of our s le (−3 & [Fe/H] & 0.5). Our examination of the kinematics of the confined compared to the unbound stars indicates the metal-poor bulge comprises at least two populations those confined to the boxy eanut bulge and halo stars passing through the inner galaxy. We conclude that an orbital analysis approach, as we have employed, is important to understand the composite nature of the metal-poor stars in the inner region.
Metallicity Variations in the Type II Globular Cluster NGC 6934
Publisher: American Astronomical Society
Date: 25-05-2018
High precision differential abundance measurements in globular clusters: chemical inhomogeneities in NGC 6752★
Publisher: Oxford University Press (OUP)
Date: 08-2013
Iron and neutron-capture element abundance variations in the globular cluster M2 (NGC 7089)★
Publisher: Oxford University Press (OUP)
Date: 31-05-2014
DOI: 10.1093/MNRAS/STU806
TheGaia-ESO Survey: A globular cluster escapee in the Galactic halo
Publisher: EDP Sciences
Date: 03-2015
Multiple stellar populations in Magellanic Cloud clusters - IV. The double main sequence of the young cluster NGC 1755
Publisher: Oxford University Press (OUP)
Date: 15-03-2016
DOI: 10.1093/MNRAS/STW608
Helium enhanced stars and multiple populations along the horizontal branch of NGC 2808: direct spectroscopic measurements★
Publisher: Oxford University Press (OUP)
Date: 20-11-2013
The COMBS survey – I. Chemical origins of metal-poor stars in the Galactic bulge
Publisher: Oxford University Press (OUP)
Date: 08-07-2019
Abstract: Chemistry and kinematic studies can determine the origins of stellar population across the Milky Way. The metallicity distribution function of the bulge indicates that it comprises multiple populations, the more metal-poor end of which is particularly poorly understood. It is currently unknown if metal-poor bulge stars ([Fe/H] −1 dex) are part of the stellar halo in the inner most region, or a distinct bulge population or a combination of these. Cosmological simulations also indicate that the metal-poor bulge stars may be the oldest stars in the Galaxy. In this study, we successfully target metal-poor bulge stars selected using SkyMapper photometry. We determine the stellar parameters of 26 stars and their elemental abundances for 22 elements using R∼ 47 000 VLT/UVES spectra and contrast their elemental properties with that of other Galactic stellar populations. We find that the elemental abundances we derive for our metal-poor bulge stars have lower overall scatter than typically found in the halo. This indicates that these stars may be a distinct population confined to the bulge. If these stars are, alternatively, part of the innermost distribution of the halo, this indicates that the halo is more chemically homogeneous at small Galactic radii than at large radii. We also find two stars whose chemistry is consistent with second-generation globular cluster stars. This paper is the first part of the Chemical Origins of Metal-poor Bulge Stars (COMBS) survey that will chemodynamically characterize the metal-poor bulge population.
Spectroscopy and Photometry of the Least Massive Type II Globular Clusters: NGC 1261 and NGC 6934*
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.
High-resolution spectroscopic follow-up of the most metal-poor candidates from SkyMapper DR1.1
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.
The two metallicity groups of the globular cluster M 22: a chemical perspective
Publisher: EDP Sciences
Date: 12-07-2011
Related Organisations
Rijksinstituut Voor Volksgezondheid En Milieu
Location: No location found
Related Funding Activities
No related grants have been discovered for Ben Ale.