Papers – Page 2 – Frank Tramper

A machine-learning photometric classifier for massive stars in nearby galaxies. I. The method

Maravelias, Grigoris; Bonanos, Alceste Z.; Tramper, Frank ; de Wit, Stephan; Yang, Ming; Bonfini, Paolo

Astronomy & Astrophysics, 666, 122 (2022)

Abstract

Context. Mass loss is a key parameter in the evolution of massive stars. Despite the recent progress in the theoretical understanding of how stars lose mass, discrepancies between theory and observations still hold. Moreover, episodic mass loss in evolved massive stars is not included in models, and the importance of its role in the evolution of massive stars is currently undetermined.
Aims: A major hindrance to determining the role of episodic mass loss is the lack of large samples of classified stars. Given the recent availability of extensive photometric catalogs from various surveys spanning a range of metallicity environments, we aim to remedy the situation by applying machine-learning techniques to these catalogs.
Methods: We compiled a large catalog of known massive stars in M 31 and M 33 using IR (Spitzer) and optical (Pan-STARRS) photometry, as well as Gaia astrometric information, which helps with foreground source detection. We grouped them into seven classes (Blue, Red, Yellow, B[e] supergiants, luminous blue variables, Wolf-Rayet stars, and outliers, e.g., quasi-stellar objects and background galaxies). As this training set is highly imbalanced, we implemented synthetic data generation to populate the underrepresented classes and improve separation by undersampling the majority class. We built an ensemble classifier utilizing color indices as features. The probabilities from three machine-learning algorithms (Support Vector Classification, Random Forest, and Multilayer Perceptron) were combined to obtain the final classification.
Results: The overall weighted balanced accuracy of the classifier is ∼83%. Red supergiants are always recovered at ∼94%. Blue and Yellow supergiants, B[e] supergiants, and background galaxies achieve ∼50 − 80%. Wolf-Rayet sources are detected at ∼45%, while luminous blue variables are recovered at ∼30% from one method mainly. This is primarily due to the small sample sizes of these classes. In addition, the mixing of spectral types, as there are no strict boundaries in the features space (color indices) between those classes, complicates the classification. In an independent application of the classifier to other galaxies (IC 1613, WLM, and Sextans A), we obtained an overall accuracy of ∼70%. This discrepancy is attributed to the different metallicity and extinction effects of the host galaxies. Motivated by the presence of missing values, we investigated the impact of missing data imputation using a simple replacement with mean values and an iterative imputer, which proved to be more capable. We also investigated the feature importance to find that r − i and y − [3.6] are the most important, although different classes are sensitive to different features (with potential improvement with additional features).
Conclusions: The prediction capability of the classifier is limited by the available number of sources per class (which corresponds to the sampling of their feature space), reflecting the rarity of these objects and the possible physical links between these massive star phases. Our methodology is also efficient in correctly classifying sources with missing data as well as at lower metallicities (with some accuracy loss), making it an excellent tool for accentuating interesting objects and prioritizing targets for observations.

Keywords: stars: massive; stars: mass-loss; stars: evolution; galaxies: individual: WLM; M 31; IC 1613; M 33; Sextans A; methods: statistical; Astrophysics – Solar and Stellar Astrophysics; Astrophysics – Astrophysics of Galaxies; Astrophysics – Instrumentation and Methods for Astrophysics

The young massive SMC cluster NGC 330 seen by MUSE. II. Multiplicity properties of the massive-star population

Bodensteiner, J.; Sana, H.; Wang, C.; Langer, N. ; Mahy, L.; Banyard, G. ; de Koter, A. ; de Mink, S. E. ; Evans, C. J.; Götberg, Y.; Patrick, L. R.; Schneider, F. R. N.; Tramper, F.

Astronomy & Astrophysics, 652, 70 (2021)

ADS – Journal – arXiv

Abstract

Context. Observations of massive stars in open clusters younger than ∼8 Myr have shown that a majority of them are in binary systems, most of which will interact during their life. While these can be used as a proxy of the initial multiplicity properties, studying populations of massive stars older than ∼20 Myr allows us to probe the outcome of these interactions after a significant number of systems have experienced mass and angular momentum transfer and may even have merged.
Aims: Using multi-epoch integral-field spectroscopy, we aim to investigate the multiplicity properties of the massive-star population in the dense core of the ∼40 Myr old cluster NGC 330 in the Small Magellanic Cloud in order to search for possible imprints of stellar evolution on the multiplicity properties.
Methods: We obtained six epochs of VLT/MUSE observations operated in wide-field mode with the extended wavelength setup and supported by adaptive optics. We extracted spectra and measured radial velocities for stars brighter than m_F814W = 19. We identified single-lined spectroscopic binaries through significant RV variability with a peak-to-peak amplitude larger than 20 km s⁻¹. We also identified double-lined spectroscopic binaries, and quantified the observational biases for binary detection. In particular, we took into account that binary systems with similar line strengths are difficult to detect in our data set.
Results: The observed spectroscopic binary fraction among stars brighter than m_F814W = 19 (approximately 5.5 M_⊙ on the main sequence) is f_SB^obs = 13.2 ± 2.0%. Considering period and mass ratio ranges from log(P) = 0.15−3.5 (about 1.4 to 3160 d), q = 0.1−1.0, and a representative set of orbital parameter distributions, we find a bias-corrected close binary fraction of f_cl = 34₋₇⁺⁸%. This fraction seems to decline for the fainter stars, which indicates either that the close binary fraction drops in the B-type domain, or that the period distribution becomes more heavily weighted toward longer orbital periods. We further find that both fractions vary strongly in different regions of the color-magnitude diagram, which corresponds to different evolutionary stages. This probably reveals the imprint of the binary history of different groups of stars. In particular, we find that the observed spectroscopic binary fraction of Be stars (f_SB^obs = 2 ± 2%) is significantly lower than that of B-type stars (f_SB^obs = 9 ± 2%).
Conclusions: We provide the first homogeneous radial velocity study of a large sample of B-type stars at a low metallicity ([Fe/H] ≲ −1.0). The overall bias-corrected close binary fraction (log(P) < 3.5 d) of the B-star population in NGC 330 is lower than the fraction reported for younger Galactic and Large Magellanic Cloud clusters in previous works. More data are needed, however, to establish whether the observed differences are caused by an age or a metallicity effect.

Keywords: stars: massive; stars: emission-line; Be; binaries: spectroscopic; blue stragglers; open clusters and associations: individual: NGC 330; Magellanic Clouds; Astrophysics – Solar and Stellar Astrophysics; Astrophysics – Astrophysics of Galaxies

Massive stars in extremely metal-poor galaxies: a window into the past

Garcia, M. ; Evans, C. J. ; Bestenlehner, J. M. ; Bouret, J. C. ; Castro, N. ; Cerviño, M.; Fullerton, A. W. ; Gieles, M. ; Herrero, A. ; de Koter, A. ; Lennon, D. J. ; van Loon, J. Th. ; Martins, F. ; de Mink, S. E. ; Najarro, F. ; Negueruela, I. ; Sana, H. ; Simón-Díaz, S. ; Szécsi, D. ; Tramper, F..; Vink, J. ; Wofford, A.

Experimental Astronomy, 114 (2021)

ADS – Journal – arXiv

Abstract

Cosmic History has witnessed the lives and deaths of multiple generations of massive stars, all of them invigorating their host galaxies with ionizing photons, kinetic energy, fresh material and stellar-mass black holes. Ubiquitous engines as they are, Astrophysics needs a good understanding of their formation, evolution, properties and yields throughout the history of the Universe, and with decreasing metal content mimicking the environment at the earliest epochs. Ultimately, a physical model that could be extrapolated to zero metallicity would enable tackling long-standing questions such as “What did the First, very massive stars of the Universe look like?” or “What was their role in the re-ionization of the Universe?”. Yet, most our knowledge of metal-poor massive stars is drawn from one single point in metallicity. Massive stars in the Small Magellanic Cloud (SMC, ∼1/5Z⊙∼1/5Z⊙) currently serve as templates for low-metallicity objects in the early Universe, even though significant differences with respect to massive stars with poorer metal content have been reported. This White Paper summarizes the current knowledge on extremely (sub-SMC) metal poor massive stars, highlighting the most outstanding open questions and the need to supersede the SMC as standard. A new paradigm can be built from nearby extremely metal-poor galaxies that make a new metallicity ladder, but massive stars in these galaxies are out of reach to current observational facilities. Such task would require an L-size mission, consisting of a 10m-class space telescope operating in the optical and the ultraviolet ranges. Alternatively, we propose that ESA unites efforts with NASA to make the LUVOIR mission concept a reality, thus continuing the successful partnership that made Hubble Space Telescope one of the greatest observatories of all time.

Keywords: Astrophysics – Instrumentation and Methods for Astrophysics;Astrophysics – Solar and Stellar Astrophysics

The young massive SMC cluster NGC 330 seen by MUSE. II. Multiplicity properties of the massive-star population

Bodensteiner, J.; Sana, H.; Wang, C.; Langer, N. ; Mahy, L.; Banyard, G.; de Koter, A. ; de Mink, S. E. ; Evans, C. J. ; Götberg, Y.; Patrick, L. R.; Schneider, F. R. N.; Tramper, F.

Astronomy & Astrophysics, 652, A70, 18 (2021)

ADS – Journal – arXiv

Abstract

Keywords: stars: massive;stars: emission-line;Be;binaries: spectroscopic;blue stragglers;open clusters and associations: individual: NGC 330;Magellanic Clouds;Astrophysics – Solar and Stellar Astrophysics;Astrophysics – Astrophysics of Galaxies

Evolved massive stars at low-metallicity. IV. Using the 1.6 μm H-bump to identify red supergiant stars: Case study of NGC 6822

Yang, Ming; Bonanos, Alceste Z.; Jiang, Biwei; Lam, Man I.; Gao, Jian; Gavras, Panagiotis; Maravelias, Grigoris; Wang, Shu; Chen, Xiao-Dian; Tramper, Frank; Ren, Yi; Spetsieri, Zoi T.

Astronomy & Astrophysics, 647, 167 (2021)

ADS – Journal – arXiv

Abstract

We present a case study in which we used a novel method to identify red supergiant (RSG) candidates in NGC 6822 based on their 1.6 μm H-bump. We collected 32 bands of photometric data for NGC 6822 ranging from the optical to the mid-infrared, derived from Gaia, PS1, LGGS, VHS, UKIRT, IRSF, HAWK-I, Spitzer, and WISE. Using the theoretical spectra from MARCS, we demonstrate that there is a prominent difference around 1.6 μm (H-bump) between targets with high and low surface gravity (HSG and LSG). Taking advantage of this feature, we identify efficient color-color diagrams of rzH (r ‒ z vs. z ‒ H) and rzK (r ‒ z vs. z ‒ K) to separate HSG (mostly foreground dwarfs) and LSG targets (mainly background red giant stars, asymptotic giant branch stars, and RSGs) from crossmatching of optical and near-infrared (NIR) data. Moreover, synthetic photometry from ATLAS9 gives similar results. We further separated RSG candidates from the remaining LSG candidates as determined by the H-bump method by using semi-empirical criteria on NIR color-magnitude diagrams, where both the theoretic cuts and morphology of the RSG population are considered. This separation produced 323 RSG candidates. The simulation of foreground stars with Besançon models also indicates that our selection criteria are largely free from the contamination of Galactic giants. In addition to the H-bump method, we used the traditional BVR method (B ‒ V vs. V ‒ R) as a comparison and/or supplement by applying a slightly aggressive cut to select as many RSG candidates as possible (358 targets). Furthermore, the Gaia astrometric solution was used to constrain the sample, where 181 and 193 targets were selected with the H-bump and BVR method, respectively. The percentages of selected targets in the two methods are similar at ∼60%, indicating a comparable accuracy of the two methods. In total, there are 234 RSG candidates after combining targets from the two methods, and 140 (∼60%) of them are in common. The final RSG candidates are in the expected locations on the mid-infrared color-magnitude diagram with [3.6]‒[4.5] ≲ 0 and J ‒ [8.0] ≈ 1.0. The spatial distribution is also coincident with the far-ultraviolet-selected star formation regions, suggesting that the selection is reasonable and reliable. We indicate that our method can also be used to identify other LSG targets, such as red giants and asymptotic giant branch stars, and it can also be applied to most of the nearby galaxies by using recent large-scale ground-based surveys. Future ground- and space-based facilities may promote its application beyond the Local Group.

Keywords: infrared: stars;galaxies: dwarf;stars: late-type;stars: massive;stars: mass-loss;stars: variables: general;Astrophysics – Solar and Stellar Astrophysics;Astrophysics – Astrophysics of Galaxies