Near-Infrared Variability Study of the Central 2.3 arcmin x 2.3 arcmin of the Galactic Centre I. Catalog of Variable Sources

We used four-year baseline HST/WFC3 IR observations of the Galactic Centre in the F153M band (1.53 micron) to identify variable stars in the central ~2.3'x2.3' field. We classified 3845 long-term (periods from months to years) and 76 short-term (periods of a few days or less) variables among a total sample of 33070 stars. For 36 of the latter ones, we also derived their periods (<3 days). Our catalog not only confirms bright long period variables and massive eclipsing binaries identified in previous works, but also contains many newly recognized dim variable stars. For example, we found \delta Scuti and RR Lyrae stars towards the Galactic Centre for the first time, as well as one BL Her star (period < 1.3 d). We cross-correlated our catalog with previous spectroscopic studies and found that 319 variables have well-defined stellar types, such as Wolf-Rayet, OB main sequence, supergiants and asymptotic giant branch stars. We used colours and magnitudes to infer the probable variable types for those stars without accurately measured periods or spectroscopic information. We conclude that the majority of unclassified variables could potentially be eclipsing/ellipsoidal binaries and Type II Cepheids. Our source catalog will be valuable for future studies aimed at constraining the distance, star formation history and massive binary fraction of the Milky Way nuclear star cluster.Comment: has been accepted to be published in MNRAS, 64 pages, 26 figures. The complete lists of table 3, 4, 8 and 9 will be published onlin

GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre III. Evidence for wavelength dependence of the extinction curve in the near-infrared

The characterisation of the extinction curve in the near infrared (NIR) is fundamental to analyse the structure and stellar population of the Galactic centre (GC), whose analysis is hampered by the extreme interstellar extinction ($A_V\sim 30$ mag) that varies on arc-second scales. Recent studies indicate that the behaviour of the extinction curve might be more complex than previously assumed, pointing towards a variation of the extinction curve as a function of wavelength. We aim at analysing the variations of the extinction index, $\alpha$, with wavelength, line-of-sight, and absolute extinction, extending previous analysis to a larger area of the innermost regions of the Galaxy. We analysed the whole GALACTICNUCLEUS survey, a high-angular resolution ($\sim 0.2''$) $JHK_s$ NIR survey specially designed to observe the GC in unprecedented detail. It covers a region of $\sim 6000$\,pc$^2$, comprising fields in the nuclear stellar disc, the inner bulge, and the transition region between them. We applied two independent methods based on red clump (RC) stars to constrain the extinction curve and analysed its variation superseding previous studies. We used more than 165,000 RC stars and increased significantly the size of the regions analysed to confirm that the extinction curve varies with the wavelength. We estimated a difference $\Delta\alpha = 0.21\pm0.07$ between the obtained extinction indices, $\alpha_{JH}=2.44\pm0.05$ and $\alpha_{HK_s} = 2.23\pm0.05$. We also concluded that there is no significant variation of the extinction curve with wavelength, with the line-of-sight or the absolute extinction. Finally, we computed the ratios between extinctions, $A_J/A_H = 1.87\pm0.03$ and $A_{H}/A_{K_s} = 1.84\pm0.03$, consistent with all the regions of the GALACTICNUCLEUS catalogue.Comment: 10 pages, 8 figures, accepted for publication in Astronomy & Astrophysic

The formation history of our Galaxy’s nuclear stellar disc constrained from HST observations of the Quintuplet field

This is an Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Context. Until recently it was thought that the nuclear stellar disc at the centre of our Galaxy was formed via quasi-continuous star formation over billions of years. However, an analysis of GALACTICNUCLEUS survey data indicates that > 80% of the mass of the stellar disc formed at least 8 Gyr ago and about 5% roughly 1 Gyr ago. Aims. Our aim is to derive new constraints on the formation history of the nuclear stellar disc. Methods. We analysed a catalogue of HST/WFC3-IR observations of the Quintuplet cluster field. From this catalogue, we selected about 24 000 field stars that probably belong to the nuclear stellar disc. We used red clump giants to deredden the sample and fit the resulting F153M luminosity function with a linear combination of theoretical luminosity functions created from different stellar evolutionary models. Results. We find that ≳70% of the stellar population in the nuclear disc probably formed more than 10 Gyr ago, while ∼15% formed in an event (or series of events) ∼1 Gyr ago. Up to 10% of the stars appear to have formed in the past tens to hundreds of Myr. These results do not change significantly for reasonable variations in the assumed mean metallicity, sample selection, reddening correction, or stellar evolutionary models. Conclusions. We confirm previous work that changed the formation paradigm for stars in the Galactic Centre. The nuclear stellar disc is indeed a very old structure. There seems to have been little star formation activity between its formation and about 1 Gyr ago. © The Authors 2023.RS, AMA, AG, EGC, MCG, and ATGC acknowledge financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709) and financial support from national project PGC2018-095049-B-C21 (MCIU/AEI/FEDER, UE). M.H. is supported by the Brinson Prize Fellowship. FNL gratefully acknowledges the sponsorship provided by the European Southern Observatory through a research fellowship.With funding from the Spanish government through the "Severo Ochoa Centre of Excellence" accreditation (CEX2021-001131-S).Peer reviewe

Early formation and recent starburst activity in the nuclear disk of the Milky Way

The nuclear disk is a dense stellar structure at the centre of the Milky Way, with a radius of ~150 pc (ref. 1). It has been a place of intense star formation in the past several tens of millions of years1-3, but its overall formation history has remained unknown2. Here, we report that the bulk of its stars formed at least 8 Gyr ago. After a long period of quiescence, a starburst event followed about 1 Gyr ago that formed roughly 5% of its mass within ~100 Myr, in what may arguably have been one of the most energetic events in the history of the Milky Way. Star formation continued subsequently on a lower level, creating a few per cent of the stellar mass in the past ~500 Myr, with an increased rate up to ~30 Myr ago. Our findings contradict the previously accepted paradigm of quasi-continuous star formation at the Galactic Centre4. The long quiescent phase agrees with the overall quiescent history of the Milky Way2,5 and suggests that our Galaxy's bar may not have existed until recently, or that gas transport through the bar was extremely inefficient during a long stretch of the Milky Way's life. Consequently, the central black hole may have acquired most of its mass already in the early days of the Milky Way

The JWST Galactic Center Survey -- A White Paper

The inner hundred parsecs of the Milky Way hosts the nearest supermassive black hole, largest reservoir of dense gas, greatest stellar density, hundreds of massive main and post main sequence stars, and the highest volume density of supernovae in the Galaxy. As the nearest environment in which it is possible to simultaneously observe many of the extreme processes shaping the Universe, it is one of the most well-studied regions in astrophysics. Due to its proximity, we can study the center of our Galaxy on scales down to a few hundred AU, a hundred times better than in similar Local Group galaxies and thousands of times better than in the nearest active galaxies. The Galactic Center (GC) is therefore of outstanding astrophysical interest. However, in spite of intense observational work over the past decades, there are still fundamental things unknown about the GC. JWST has the unique capability to provide us with the necessary, game-changing data. In this White Paper, we advocate for a JWST NIRCam survey that aims at solving central questions, that we have identified as a community: i) the 3D structure and kinematics of gas and stars; ii) ancient star formation and its relation with the overall history of the Milky Way, as well as recent star formation and its implications for the overall energetics of our galaxy's nucleus; and iii) the (non-)universality of star formation and the stellar initial mass function. We advocate for a large-area, multi-epoch, multi-wavelength NIRCam survey of the inner 100\,pc of the Galaxy in the form of a Treasury GO JWST Large Program that is open to the community. We describe how this survey will derive the physical and kinematic properties of ~10,000,000 stars, how this will solve the key unknowns and provide a valuable resource for the community with long-lasting legacy value.Comment: This White Paper will be updated when required (e.g. new authors joining, editing of content). Most recent update: 24 Oct 202

New windows onto the stellar population at the Galactic Centre: multi–wavelength and time-domain studies

El objetivo de esta tesis es el estudio de las estrellas masivas pertenecientes a dos de los cúmulos más emblemáticos del centro Galáctico, los cúmulos de Arches y Quintuplet, ambos situados a unos 30 parsecs en proyección de Sagittarius A*, el agujero negro central de la Vía Láctea, utilizando técnicas de muy alta resolución angular radio interferométricas. Estas técnicas nos permiten la observación de objetos lejanos separados por una distancia angular muy pequeña, sintetizando una apertura equivalente a la distancia de separación de pares de antenas, apertura que no sería posible conseguir con un único radio telescopio. Debido a que la extinción interestelar en esta región es muy elevada, los estudios observacionales de estos dos cúmulos se han visto limitados fundamentalmente a observaciones en el infrarrojo cercano, con espectroscopía y fotometría, por ser el rango óptico prácticamente inaccesible y estar el infrarrojo medio limitado por la resolución angular y/o la sensibilidad de los instrumentos en un enclave muy poblado de estrellas. Dada la baja estadística de observaciones en radio de los vientos ionizados de las estrellas masivas y la existencia de ellas en los cúmulos de Arches y Quintuplet, este trabajo es un nuevo paso hacia la comprensión de este tipo de estrellas y de los cúmulos que las contienen. Todos los estudios que aquí se presentan se han realizado gracias a la utilización del radio interferómetro Karl G. Janksy Very Large Array (JVLA) operado por el National Radio Astronomy Observatory (NRAO)1. En este trabajo se ha obtenido: a) un nuevo censo, en ambos cúmulos, del número de estrellas masivas observadas en longitudes de onda de radio centimétricas, b) los flujos y posiciones de las mismas, c) las tasas de pérdida de masa estimadas, d) una valoración de su posible multiplicidad en base al índice espectral calculado, e) una evaluación de la variabilidad, y finalmente, f) una estimación, entre unos ciertos límites, de la edad y/o masa de los cúmulos, así como de la función inicial de masa en base, precisamente, al número de radio fuentes detectadas. Esta tesis esta compuesta de seis capítulos. En el primero de ellos se da una breve introducción sobre la ciencia que hay en la misma. En el segundo capítulo se exponen los principios de la radio interferometría, haciendo hincapié en los aspectos fundamentales que han dado lugar a este trabajo. En los capítulos tres y cuatro se presentan los principales tópicos de investigación de esta tesis. Éstos incluyen los datos, el análisis y los resultados descritos en una publicación arbitrada y en una que será enviada de forma inminente. Finalmente, los capítulos cinco y seis presentan las conclusiones generales y las futuras líneas de investigación de esta tesis.The objective of this thesis is the study of the massive stars belonging to two of the most emblematic clusters of the Galactic centre, the Arches and Quintuplet clusters, both located about 30 parsecs in projection of Sagittarius A*, the central black hole of the Milky Way, using very high angular resolution radio interferometric techniques. These techniques allow us to observe distant objects separated by a very small angular distance, synthesising an aperture equivalent to the distance between pairs of antennas, an aperture that would not be possible to achieve with a single dish radio telescope. Due to the very high extinction in this region, the observational studies of these two clusters have been mainly limited to observations in the near infrared, with spectroscopy and photometry, as the optical range is practically inaccesible and the mid-infrared is limited due to the angular resolution and/or the sensitivity of the instruments in an enclave heavily populated with stars. Given the low statistics of radio observations of the ionised winds of massive stars and the existence of them in the Arches and Quintuplet clusters, this work is a new step towards understanding this type of stars and the clusters that contain them. All the studies presented here have been carried out using the Karl G. Janksy Very Large Array (JVLA) radio interferometer operated by the National Radio Astronomy Observatory (NRAO)2. In this work, we have obtained: a) a new census, in both clusters, of the number of massive stars observed in centimetre radio wavelengths, b) their fluxes and positions, c) estimated mass-loss rates, d) an assessment of their possible multiplicity based on the calculated spectral index, e) an assessment of variability, and finally, f) an estimate, within certain limits, of the age and/or mass of the clusters, as well as the IMF based, precisely, on the number of radio sources detected. This thesis is composed of six chapters. In the first one, a brief introduction is given about the science that is in it. In the second chapter, the principles of radio interferometry are exposed, emphasising the fundamental aspects that have given rise to this work. In chapters three and four, the main research topics of this thesis are presented. These include the data, analysis, and results described in a peerreviewed publication and in one to be submitted imminently. Finally, chapters five and six present the general conclusions and future lines of research of this thesis.Tesis Univ. Granada

Near-infrared variability study of the central 2.3 × 2.3 arcmin(2) of the Galactic Centre - II. Identification of RR Lyrae stars in the Milky Way nuclear star cluster

Dong, Hui et. al.Because of strong and spatially highly variable interstellar extinction and extreme source crowding, the faint (K ≥ 15) stellar population in the Milky Way's nuclear star cluster is still poorly studied. RR Lyrae stars provide us with a tool to estimate the mass of the oldest, relative dim stellar population. Recently, we analysed HST/WFC3/IR observations of the central 2.3 × 2.3 arcmin2 of the Milky Way and found 21 variable stars with periods between 0.2 and 1 d. Here, we present a further comprehensive analysis of these stars. The period- luminosity relationship of RR Lyrae is used to derive their extinctions and distances. Using multiple approaches, we classify our sample as 4 RRc stars, 4 RRab stars, 3 RRab candidates and 10 binaries. Especially, the four RRab stars show sawtooth light curves and fall exactly on to the Oosterhoff I division in the Bailey diagram. Compared to the RRab stars reported by Minniti et al., our new RRab stars have higher extinction (AK > 1.8) and should be closer to the Galactic Centre. The extinction and distance of one RRab stars match those for the Milky Way's nuclear star cluster given in previous works. We perform simulations and find that after correcting for incompleteness, there could be not more than 40 RRab stars within the Milky Way's nuclear star cluster and in our field of view. Through comparing with the known globular clusters of the Milky Way, we estimate that if there exists an old, metal-poor (-1.5 < [Fe/H] < -1) stellar population in the Milky Way nuclear star cluster on a scale of 5 × 5 pc, then it contributes at most 4.7 × 105 M⊙, i.e. ~18 per cent of the stellar mass. © 2017 The Authors.The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no [614922]. The work is also supported partly by NASA via the grant GO-14589, provided by the Space Telescope Science Institute. FN-L acknowledges financial support from an MECD predoctoral contract, code FPU14/01700. This work uses observations made with the NASA/ESA HST and the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.Peer reviewe