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Aims. Our aim is to provide an observational view of the old disc structure of the Milky Way galaxy using the distribution of red clump (RC) stars. The spiral arms, warp structure, and other asymmetries present in the disc are revisited using a systematic study of RC star counts over the disc of the Galaxy. Methods. We developed a method for systematically extracting the RC stars from a 2MASS (J − Ks, J) colour-magnitude diagram of 1° ×1° bins in ℓ × b, covering the range 40° ≤ ℓ ≤ 320° and −10° ≤ b ≤ 10°. 2MASS data continue to be important because we are able to identify and trace the RC stars to much farther distances than in any optical survey of the Galactic disc. The foreground star contamination in the selected sample was removed using the accurate astrometric data from Gaia EDR3. We analysed the spiral arms and asymmetry in the Galaxy above and below the Galactic plane in galactocentric coordinates. Results. We have generated a face-on-view (XY-plane) of the Galaxy depicting the density distribution and count ratio above and below the Galactic plane. The resulting over-density of RC stars traces the continuous morphology of the Outer arm from the second to the third Galactic quadrant. This is the first study to map the Outer arms across the disc using RC stars. Through this study, we are able to trace the Outer arm well into the third Galactic quadrant for the first time. In addition to the spiral structures, we also see a wave-like asymmetry above and below the Galactic plane with respect to longitudes, indicating the warp structure. The warp structure is studied systematically by tracing the ratio of RC stars above and below the Galactic plane. We provide the first direct observational evidence of the asymmetry in the outer spiral arms, confirming that the spiral arms traced by the older population are also warped, similar to the disc.

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. Open Access funding provided by Max Planck Society., Context. The determination of absolute and relative distances of molecular clouds along the line-of-sight towards the central molecular zone (CMZ) is crucial for inferring its orbital structure and dynamics and for understanding star formation in the clouds. Aims. Recent work has suggested that the G0.253+0.016 cloud (the Brick) does not belong to the CMZ. This motivated us to crosscheck those results, computing the absolute and relative distances to the Brick as well as to another two molecular clouds (the 50 km s-1 and the 20 km s-1 clouds), and discuss their CMZ membership. Methods. We used the colour magnitude diagrams Ks versus H-Ks to compare stars detected towards the target clouds with stars detected towards three reference regions in the nuclear stellar disc (NSD) and the Galactic bulge. We used red clump (RC) stars to estimate the distance to each region. Results. We found that all the clouds present a double RC feature. Such a double RC has been reported in previous work for the NSD, but not for the bulge adjacent to it. We exclude the possibility that the different RC features are located at significantly different distances. The obtained absolute and relative distances are compatible with the Galactic centre distance (∼8 kpc). © 2021 F. Nogueras-Lara et al., This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 195.B-0283. We thank the staff of ESO for their great efforts and helpfulness. We thank Steven Longmore for useful discussion and comments on the manuscript. F.N.-L. and N.N. gratefully acknowledge support by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project-ID 138713538 - SFB 881 ("The Milky Way System", subproject B8). R.S. acknowledges financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). R.S. acknowledges financial support from national project PGC2018-095049-B-C21 (MCIU/AEI/FEDER, UE).

We developed a set of procedures to automatically detect and measure the DIB around 8620 {\AA} (the Gaia DIB) for a wide range of temperatures. The DIB profile is fit with a Gaussian function. Specifically, the DIB feature is extracted from the spectra of late-type stars by subtracting the corresponding synthetic spectra. For early-type stars we applied a specific model based on the Gaussian process that needs no prior knowledge of the stellar parameters. The method was tested on $\sim$5000 spectra from the Giraffe Inner Bulge Survey (GIBS). After validation, we obtained 4194 reasonable fitting results from the GIBS database. An EW versus $E(J\,{-}\,K_{\rm S})$ relation is derived as $E(J\,{-}\,K_{\rm S})\,{=}\,1.875\,({\pm}\,0.152)\,{\times}\,{\rm EW}\,{-}\,0.011\,({\pm}\,0.048)$, according to $E(B\,{-}\,V)/{\rm EW}\,{=}\,2.721$, which is highly consistent with previous results toward similar sightlines. After a correction based on the VVV database for both EW and reddening, the coefficient derived from individual GIBS fields, $E(J\,{-}\,K_{\rm S})/{\rm EW}\,{=}\,1.884\,{\pm}\,0.225$, is also in perfect agreement with literature values. Based on a subsample of 1015 stars toward the Galactic center within $-3^{\circ}\,{, Comment: 17 pages, 16 figures

The Galactic Center region, including the nuclear disk, has until recently been largely avoided in chemical census studies because of extreme extinction and stellar crowding. Making use of the latest APOGEE data release (DR16), we are able for the first time to study cool AGB stars and supergiants in this region. The stellar parameters of five known AGB stars and one supergiant star (VR 5-7) show that their location is well above the tip of the RGB.We study metallicities of 157 M giants situated within 150 pc of the Galactic center from observations obtained by the APOGEE survey with reliable stellar parameters from the APOGEE/ASPCAP pipeline making use of the cool star grid down to 3200 K. Distances, interstellar extinction values, and radial velocities were checked to confirm that these stars are indeed situated in the Galactic Center region. We detect a clear bimodal structure in the metallicity distribution function, with a dominant metal-rich peak of [Fe/H] ~ +0.3 dex and a metal-poor peak around [Fe/H]= -0.5 dex, which is 0.2 dex poorer than Baade's Window. The alpha-elements Mg, Si, Ca, and O show a similar trend to the Galactic Bulge. The metal-poor component is enhanced in the alpha-elements, suggesting that this population could be associated with the classical bulge and a fast formation scenario. We find a clear signature of a rotating nuclear stellar disk and a significant fraction of high velocity stars with $\rm v_{gal} > 300\,km/s$; the metal-rich stars show a much higher rotation velocity ($\rm \sim 200\,km/s$) with respect to the metal-poor stars ($\rm \sim 140\,km/s$). The chemical abundances as well as the metallicity distribution function suggest that the nuclear stellar disc and the nuclear star cluster show distinct chemical signatures and might be formed differently., 12 pages, accepted for Astronomy&Astrophysics

Although there have been numerous studies of chemical abundances in the Galactic bulge, the central two degrees have been relatively unexplored due to the heavy and variable interstellar extinction, extreme stellar crowding, and the presence of complex foreground disk stellar populations. In this paper we discuss the metallicity distribution function, vertical and radial gradients and chemical abundances of $\alpha$-elements in the inner two degrees of the Milky Way, as obtained by recent IR spectroscopic surveys. We use a compilation of recent measurements of metallicities and $\alpha$-element abundances derived from medium-high resolution spectroscopy. We compare these metallicities with low-resolution studies. Defining "metal-rich" as stars with $ \rm [Fe/H]>0$, and "metal-poor" as stars with $\rm [Fe/H], Comment: 9 pages, 8 figures, accepted for Astronomy&Astrophysics

The Galactic bulge has a bimodal metallicity distribution function: different kinematic, spatial, and, potentially, age distributions characterize the metal-poor and metal-rich components. Despite this observed dichotomy, which argues for different formation channels for those stars, the distribution of bulge stars in the $\alpha$-abundance versus metallicity plane has been found so far to be a rather smooth single sequence. We use data from the fourteenth data release of the APOGEE spectroscopic survey (DR14) to investigate the distribution in the Mg abundance (as tracer of the $\alpha$-elements)-versus-metallicity plane of a sample of stars selected to be in the inner region of the bulge. A clean sample has been selected from the DR14 using a set of data- and pipeline-flags to ensure the quality of their fundamental parameters and elemental abundances. An additional selection made use of computed spectro-photometric distances to select a sample of likely bulge stars as those with ${\rm R_{GC}\leq 3.5~kpc}$. We adopt magnesium abundance as an $\alpha$-abundance proxy for our clean sample as it has been proven to be the most accurate $\alpha$-element as determined by ASPCAP, the pipeline for data products from APOGEE spectra. From the distribution of our bulge sample in the [Mg/Fe]-versus-[Fe/H] plane, we found that the sequence is bimodal. This bimodality is given by the presence of a low-Mg sequence of stars parallel to the main high-Mg sequence over a range of $\sim 0.5$~dex around solar metallicity. The two sequences merge above ${\rm [Fe/H]\sim0.15~ dex}$ into a single sequence whose dispersion in [Mg/Fe] is larger than either of the two sequences visible at lower metallicity. This result is confirmed when we consider stars in our sample that are inside the bulge region according to trustworthy Gaia DR2 distances., Comment: accepted for publication in A&A. 11 pages, 8 figures

Context. Thanks to recent large scale surveys in the near infrared such as 2MASS, the galactic plane that most suffers from extinction is revealed and its overall structure can be studied. Aims. This work aims at constraining the structure of the Milky Way external disc as seen in 2MASS data, and in particular the warp. Methods. We use the Two Micron All Sky Survey (hereafter 2MASS) along with the Stellar Population Synthesis Model of the Galaxy, developed in Besancon, to constrain the external disc parameters such as its scale length, its cutoff radius, and the slope of the warp. In order to properly interpret the observations, the simulated stars are reddened using a three dimensional extinction map. The shape of the stellar warp is then compared with previous results and with similar structures in gas and dust. Results. We find new constraints on the stellar disc, which is shown to be asymmetrical, similar to observations of HI. The positive longitude side is found to be easily modelled with a S shape warp but with a slope significantly smaller than the slope seen in the HI warp. At negative longitudes, the disc presents peculiarities which are not well reproduced by any simple model. Finally, comparing with the warp seen in the dust, it seems to follow a slope intermediate between the gas and the stars., 9 pages. Accepted for publication in Astronomy and Astrophysics

Context. The formation and evolution of the Milky Way bulge is still largely an unanswered question. One of the most essential observables needed in its modelling are the metallicity distribution and the trends of the alpha elements as measured in stars. While Bulge regions beyond R > 50 pc of the centre has been targeted in several surveys, the central part has escaped detailed study due to the extreme extinction and crowding. The abundance gradients from the center are, however, of large diagnostic value. Aims. We aim at investigating the Galactic Centre environment by probing M giants in the field, avoiding supergiants and cluster members. Methods. For 9 field M-giants in the Galactic Centre region, we have obtained high- and low-resolution spectra observed simultaneously with CRIRES and ISAAC on UT1 and UT3 of the VLT. The low-resolution spectra provide a means of determining the effective temperatures, and the high-resolution spectra provide detailed abundances of Fe, Mg, Si, and Ca. Results. We find a metal-rich population at [Fe/H]=+0.11+-0.15 and a lack of the metal-poor population, found further out in the Bulge, corroborating earlier studies. Our [alpha/Fe] element trends, however, show low values, following the outer Bulge trends. A possible exception of the [Ca/Fe] trend is found and needs further investigation. Conclusions. The results of the analysed field M-giants in the Galactic Centre region, excludes a scenario with rapid formation, in which SNIIe played a dominated role in the chemical enrichment of the gas. The metal-rich metallicities together with low alpha-enhancement seems to indicate a bar-like population perhaps related to the nuclear bar., Accepted for publication in A&A

Context: The stellar populations in the central region of the Galaxy are poorly known because of the high visual extinction and very great source density in this direction. Aims: To use recent infrared surveys for studying the dusty stellar objects in this region. Methods: We analyse the content of a 20x20 arcmin^2 field centred at (l,b)=(-0.27,-0.06) observed at 7 and 15 microns as part of the ISOGAL survey. These ISO observations are more than an order of magnitude better in sensitivity and spatial resolution than the IRAS observations. The sources are cross-associated with other catalogues to identify various types of objects. We then derive criteria to distinguish young objects from post-main sequence stars. Results: We find that a sample of about 50 young stellar objects and ultra-compact HII regions emerges, out of a population of evolved AGB stars. We demonstrate that the sources colours and spatial extents, as they appear in the ISOGAL catalogue, possibly complemented with MSX photometry at 21 microns, can be used to determine whether the ISOGAL sources brighter than 300 mJy at 15 microns (or [15] < 4.5 mag) are young objects or late-type evolved stars., 15 pages, 12 figures. Accepted for publication in Astronomy and Astrophysics

We present a stellar populations analysis of the first release of the CFHTLS (Canada-France-Hawai Telescope Legacy Survey) data. A detailed comparison between the Besan\c{c}on model of the Galaxy and the first data release of the CFHTLS-Deep survey is performed by implementing the MEGACAM photometric system in this model using stellar atmosphere model libraries. The reliability of the theoretical libraries to reproduce the observed colours in the MEGACAM system is investigated. The locations of various stellar species like subdwarfs, white dwarfs, late-type and brown dwarfs, binary systems are identified. The contamination of the stellar sample by quasars and compact galaxies is quantified using spectroscopic data from the VIMOS-VLT Deep Survey (VVDS) as a function of $i'$ magnitude and $r'-i'$ colour. A comparison between simulated counts using the standard IMF at low masses show that the number of very low mass dwarfs may have been underestimated in previous studies. These observations favour a power law IMF following $d(n)/dm \propto m^{-\alpha}$ with $\alpha=2.5$ for $m < $ 0.25 \Msun or $\alpha=3.0$ for $m < $ 0.2 \Msun for single stars. The resulting LF is in agreement with the local LF as measured from the 5 or 25 pc samples. It is in strong disagreement with the Zheng et al (2001) LF measured from deep HST data. We show that this discrepancy can be understood as an indication of a different IMF at low masses at early epochs of the Galaxy compared to the local thin disc IMF., Comment: 15 pages The original paper with the figures at high resolution can be found under the anonymous ftp (ftp.iap.fr: pub/from_users/schulthe/cfhtls.pdf)

We examine the metallicity trends in the Milky Way (MW) bulge - using APOGEE DR13 data - and explore their origin by comparing two N-body models of isolated galaxies which develop a bar and a boxy/peanut (b/p) bulge. Both models have been proposed as scenarios for reconciling a disc origin of the MW bulge with a negative vertical metallicity gradient. The first model is a superposition of co-spatial, i.e. overlapping, disc populations with different scaleheights, kinematics and metallicities. In this model the thick, metal-poor, and centrally concentrated disc populations contribute significantly to the stellar mass budget in the inner galaxy. The second model is a single disc with an initial steep radial metallicity gradient, which is mapped by the bar into the b/p bulge in such a way that the vertical metallicity gradient of the MW bulge is reproduced -- as shown already in previous works in the literature. However, as we show here, the latter model does not reproduce the positive longitudinal metallicity gradient of the inner disc, nor the metal-poor innermost regions of the Bulge seen in the data. On the other hand, the model with co-spatial thin and thick disc populations reproduces all the aforementioned trends. We therefore see that it is possible to reconcile a (primarily) disc origin for the MW bulge with the observed trends in metallicity by mapping the inner thin and thick discs of the MW into a b/p. For this scenario to reproduce the observations, the $\alpha$-enhanced, metal-poor, thick disc populations must have a significant mass contribution in the inner regions -- as has been suggested for the Milky Way., Comment: 6 pages, accepted for publication in A&A Letters

International audience; We investigate the characteristics of the thick disk in the Canada – France – Hawaii – Telescope Legacy Survey (CFHTLS) fields, complemented at bright magnitudes with Sloan Digital Sky Survey (SDSS) data. The ([Fe/H], Z) distributions are derived in the W1 and W3 fields, and compared with simulated maps produced using the Besançon model. It is shown that the thick disk, represented in star-count models by a distinct component, is not an adequate description of the observed ([Fe/H], Z) distributions in these fields.

Three dimensional interstellar extinction maps provide a powerful tool for stellar population analysis. We use data from the VISTA Variables in the Via Lactea survey together with the Besan��on stellar population synthesis model of the Galaxy to determine interstellar extinction as a function of distance in the Galactic bulge covering $ -10 < l < 10$ and $-10 < b, 11 pages, 18 figures, accepted for publication in Astronomy&Astrophysics

LHS1070 is a nearby multiple system of low mass stars. It is an important source of information for probing the low mass end of the main sequence, down to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and two components are late-M to early L dwarfs, at the star-brown dwarf transition. Hence LHS1070 is a valuable object to understand the onset of dust formation in cool stellar atmospheres.This work aims at determining the fundamental stellar parameters of LHS1070 and to test recent model atmospheres: BT-Dusty,BT-Settl, DRIFT, and MARCS models.Unlike in previous studies, we have performed a chi^2-minimization comparing well calibrated optical and infrared spectra with recent cool star synthetic spectra leading to the determination of the physical stellar parameters Teff, radius, and log g for each of the three components of LHS1070., Comment: Accepted in Astronomy and Astrophysics, 15 pages, 11 figures

We re-analyse photometric near-infrared data in order to investigate why it is so hard to get a consensus for the shape and density law of the bulge, as seen from the literature. To solve the problem we use the Besancon Galaxy Model to provide a scheme for parameter fitting of the structural characteristics of the bulge region. The fitting process allows the determination of the global shape of the bulge main structure. We explore various parameters and shape for the bulge/bar structure based on Ferrer's ellipsoids and fit the shape of the inner disc in the same process. The results show that the main structure is a quite standard triaxial boxy bar/bulge with an orientation of about 13 degree with respect to the Sun-centre direction. But the fit is greatly improved when we add a second structure, which is a longer and thicker ellipsoid. We emphasize that our first ellipsoid represent the main boxy bar of the Galaxy, and that the thick bulge could be either a classical bulge slightly flattened by the effect of the bar potential, or a inner thick disc counterpart. We show that the double clump seen at intermediate latitudes can be reproduced by adding a slight flare to the bar. In order to better characterize the populations, we further simulate several fields which have been surveyed in spectroscopy and for which metallicity distribution function (MDF) are available. The model is in good agreement with these MDF along the minor axis if we assume that the main bar has a mean solar metallicity and the second thicker population has a lower metallicity. It then creates naturally a vertical metallicity gradient by the mixing of the two poulations. (abridged), 16 pages, 17 figures, Astronomy and Astrophysics in press, new submission after langage editing and minor corrections (including the title)

We obtained high-resolution near-IR spectra of 45 AGB stars located in the Galactic bulge. The aim of the project is to determine key elemental abundances in these stars to help constrain the formation history of the bulge. A further aim is to link the photospheric abundances to the dust species found in the winds of the stars. Here we present a progress report of the analysis of the spectra., Comment: 2 pages, 1 figure. To appear in the proceedings of the conference "Assembling the Puzzle of the Milky Way", Le Grand-Bornand, France, 17-22 April 2011, European Physical Journal, editors C. Reyl\'e, A. Robin and M. Schultheis