arXiv subjects

C. Nitschelm

Publications and source records attributed to C. Nitschelm.

At least 19 recordsLinked to original sources

arXiv2025

Abundances of P, S, and K in 58 bulge spheroid stars from APOGEE

We have previously studied several elements in 58 selected bulge spheroid stars, based on spectral lines in the H-band. We now derive the abundances of the less-studied elements phosphorus (P; Z=15), sulphur (S; Z=16), and potassium (K; Z=19). The abundances of P, S, and K in 58 bulge spheroid stars are compared both with the results of a previous analysis of the data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE), and with a few available studies of these elements. We derive the individual abundances through spectral synthesis, using the stellar physical parameters available for our sample from the DR17 release of the APOGEE project. We provide recommendations for the best lines to be used for the studied elements among those in the H-band. We also compare the present results, together with literature data, with chemical-evolution models. Finally, the neutrino-process was taken into account for the suitable fit to the odd-Z elements P and K. We confirm that the H-band has useful lines for the derivation of the elements P, S, and K in moderately metal-poor stars. The abundances, plotted together with literature results from high-resolution spectroscopy, indicate that: moderately enhanced phosphorus stars are found, reminiscent results obtained for thick disk and halo stars of metallicity [Fe/H]~-1.0. Therefore, for the first time, we identify this effect to occur in the old stars from the bulge spheroid. Sulphur is an alpha-element and behaves as such. Potassium and sulphur both exhibit some star-to-star scatter, but fit within the expectations from chemical evolution models.

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arXiv2024

Abundances of iron-peak elements in 58 bulge spheroid stars from APOGEE

Stars presently identified in the bulge spheroid are probably very old, and their abundances can be interpreted as due to the fast chemical enrichment of the early Galactic bulge. The abundances of the iron-peak elements are important tracers of nucleosynthesis processes, in particular oxygen burning, silicon burning, the weak s-process, and alpha-rich freeze-out. Aims. The aim of this work is to derive the abundances of V, Cr, Mn, Co, Ni, and Cu in 58 bulge spheroid stars and to compare them with the results of a previous analysis of data from APOGEE. We selected the best lines for V, Cr, Mn, Co, Ni, and Cu located within the H-band of the spectrum, identifying the most suitable ones for abundance determination, and discarding severe blends. Using the stellar physical parameters available for our sample from the DR17 release of the APOGEE project, we derived the individual abundances through spectrum synthesis. We then complemented these measurements with similar results from different bulge field and globular cluster stars, in order to define the trends of the individual elements and compare with the results of chemical-evolution models. We verify that the H-band has useful lines for the derivation of the elements V, Cr, Mn, Co, Ni, and Cu in moderately metal-poor stars. The resulting abundances indicate that: V, Cr, and Ni vary in lockstep with Fe; Co tends to vary in lockstep with Fe, but could be showing a slight decrease with decreasing metallicity; and Mn and Cu decrease with decreasing metallicity. These behaviours are well reproduced by chemical-evolution models except for Cu, which appears to drop faster than the models predict for moderate metallicities. Finally, abundance indicators combined with kinematical and dynamical criteria appear to show that our 58 sample stars are likely to have originated in situ.

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arXiv2022

Application limit of the photocentre displacement to fundamental stellar parameters of fast rotators -- Illustration on the edge-on fast rotator Regulus

Differential Interferometry allows to obtain the differential visibility and phase, in addition to the spectrum. The differential phase contains important information about the structure and motion of stellar photosphere such as stellar spots and non-radial pulsations, and particularly the rotation. Thus, this interferometric observable strongly helps to constrain the stellar fundamental parameters of fast rotators. The spectro-astrometry mainly uses the photocentre displacements, which is a first approximation of the differential phase, and is applicable only for unresolved or marginally objects. We study here the sensitivity of relevant stellar parameters to the simulated photocentres using the SCIROCCO code: a semi-analytical algorithm dedicated to fast rotators, applied to two theoretical modeling stars based on Achernar and Regulus, in order to classify the importance of these parameters and their impact on the modeling. We compare our simulations with published VLTI/AMBER data. This current work sets the limits of application of photocentre displacements to fast rotators, and under which conditions we can use the photocentres and/or the differential phase, through a pre-established physical criterion. To validate our theoretical study, we apply our method of analysis on observed data of the edge-on fast rotator Regulus. For unresolved targets, with a visibility $V\sim 1$, the photocentre can constrain the main stellar fundamental parameters of fast rotators, whereas from marginally resolved objects ($0.8 \leq V < 1$), mainly the rotation axis position angle ($\rm PA_{\rm rot}$) can be directly deduced from the vectorial photocentre displacement, which is very important for young cluster studies.

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arXiv2022

The eROSITA Final Equatorial-Depth Survey (eFEDS): Identification and characterization of the counterparts to the point-like sources

In November 2019, eROSITA on board of SRG observatory started to map the entire sky in X-rays. After the 4-year survey program, it will reach flux limits about 25 times deeper than ROSAT. During the SRG Performance Verification phase, eROSITA observed a contiguous 140 deg$^2$ area of the sky down to the final depth of the eROSITA all-sky survey ("eROSITA Final Equatorial-Depth Survey": eFEDS), with the goal of obtaining a census of the X-ray emitting populations (stars, compact objects, galaxies, clusters of galaxies, AGN) that will be discovered over the entire sky. This paper presents the identification of the counterparts to the point-sources detected in eFEDS in the Main and Hard samples described in Brunner et al 2021, and their multi-wavelength properties, including redshift. For the identification of the counterparts we combined the results from two independent methods (NWAY and ASTROMATCH), trained on the multi-wavelength properties of a sample of 23k XMM-Newton sources detected in the DESI Legacy Imaging Survey DR8. Then spectroscopic redshifts and photometry from ancillary surveys are collated for the computation of photometric redshifts. The eFEDS sources with a reliable counterparts are 24774/27369 (90.5\%) in the Main sample and 231/246 (93.9\%) in the Hard sample, including 2514 (3) sources for which a second counterpart is equally likely. [abridged] This paper is accompanying the eROSITA early data release of all the observations performed during the performance and verification phase. Together with the catalogs of primary and secondary counterparts to the Main and Hard samples of the eFEDS survey this paper releases their multi-wavelength properties and redshifts.

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arXiv2021

Exploring the s-process history in the Galactic disk: Cerium abundances and gradients in Open Clusters from the OCCAM/APOGEE sample

The APOGEE Open Cluster Chemical Abundances and Mapping (OCCAM) survey is used to probe the chemical evolution of the s-process element cerium in the Galactic disk. Cerium abundances were derived from measurements of Ce II lines in the APOGEE spectra using the Brussels Automatic Code for Characterizing High Accuracy Spectra (BACCHUS) in 218 stars belonging to 42 open clusters. Our results indicate that, in general, for Ages $<$ 4 Gyr, younger open clusters have higher [Ce/Fe] and [Ce/$α$-element] ratios than older clusters. In addition, metallicity segregates open clusters in the [Ce/X]-Age plane (where X can be H, Fe, and the $α$-elements O, Mg, Si, or Ca). These metallicity-dependant relations result in [Ce/Fe] and [Ce/$α$] ratios with age that are not universal clocks. Radial gradients of [Ce/H] and [Ce/Fe] ratios in open clusters, binned by age, were derived for the first time, with d[Ce/H]dR$_{GC}$ being negative, while d[Ce/Fe]/dR$_{GC}$ is positive. [Ce/H] and [Ce/Fe] gradients are approximately constant over time, with the [Ce/Fe] gradient becoming slightly steeper, changing by $\sim$+0.009 dex-kpc$^{-1}$-Gyr$^{-1}$. Both the [Ce/H] and [Ce/Fe] gradients are shifted to lower values of [Ce/H] and [Ce/Fe] for older open clusters. The chemical pattern of Ce in open clusters across the Galactic disk is discussed within the context of s-process yields from AGB stars, $\sim$Gyr time delays in Ce enrichment of the interstellar medium, and the strong dependence of Ce nucleosynthesis on the metallicity of its AGB stellar sources.

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arXiv2021

The Milky Way bar and bulge revealed by APOGEE DR16 and Gaia EDR3

We investigate the inner regions of the Milky Way with a sample of unprecedented size and coverage thanks to APOGEE DR16 and Gaia EDR3 data. Our inner Galactic sample has more than 26,000 stars within $|X_{\rm Gal}| <5$ kpc, $|Y_{\rm Gal}| <3.5$ kpc, $|Z_{\rm Gal}| <1$ kpc, and we also make the analysis for a foreground-cleaned sub-sample of 8,000 stars more representative of the bulge-bar populations. The inner Galaxy shows a clear chemical discontinuity in key abundance ratios [$α$/Fe], [C/N], and [Mn/O], probing different enrichment timescales, which suggests a star formation gap (quenching) between the high- and low-$α$ populations. For the first time, we are able to fully characterize the different populations co-existing in the innermost regions of the Galaxy via joint analysis of the distributions of rotational velocities, metallicities, orbital parameters and chemical abundances. The chemo-kinematic analysis reveals the presence of the bar; of an inner thin disk; of a thick disk, and of a broad metallicity population, with a large velocity dispersion, indicative of a pressure supported component. We find and characterize chemically and kinematically a group of counter-rotating stars, which could be the result of a gas-rich merger event or just the result of clumpy star formation during the earliest phases of the early disk, which migrated into the bulge. Finally, based on the 6D information we assign stars a probability value of being on a bar orbit and find that most of the stars with large bar orbit probabilities come from the innermost 3 kpcs. Even stars with a high probability of belonging to the bar show the chemical bimodality in the [$α$/Fe] vs. [Fe/H] diagram. This suggests bar trapping to be an efficient mechanism, explaining why stars on bar orbits do not show a significant distinct chemical abundance ratio signature.

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arXiv2021

APOGEE view of the globular cluster NGC 6544

The second phase of the APOGEE survey is providing near-infrared, high-resolution, high signal-to-noise spectra of stars in the halo, disk, bar and bulge of the Milky Way. The near-infrared spectral window is especially important in the study of the Galactic bulge, where stars are obscured by the dust and gas of the disk in its line-of-sight. We present a chemical characterisation of the globular cluster NGC 6544 with high-resolution spectroscopy. The characterisation of the cluster chemical fingerprint, given its status of "interloper" towards the Galactic bulge and clear signatures of tidal disruption in its core is crucial for future chemical tagging efforts. Cluster members were selected from the DR16 of the APOGEE survey, using chemo-dynamical criteria of individual stars. A sample of 23 members of the cluster was selected. An analysis considering the intra-cluster abundance variations, known anticorrelations is given. According to the RGB content of the cluster, the iron content and $α$-enhancement are [Fe/H] $= -1.44 \pm 0.04$ dex and [$α$/Fe] $= 0.20 \pm 0.04$ dex, respectively. Cluster members show a significant spread in [Fe/H] and [Al/Fe] that is larger than expected based on measurement errors. An [Al/Fe] spread, signal of an Mg-Al anticorrelation is observed and used to constraint the cluster mass budget, along with C, N, Mg, Si, K, Ca, and Ce element variations are discussed. Across all the analysed evolutionary stages (RGB and AGB), about $\sim2/3$ (14 out of 23) show distinct chemical patterns, possibly associated with second-generation stars.

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arXiv2020

How Many Components? Quantifying the Complexity of the Metallicity Distribution in the Milky Way Bulge with APOGEE

We use data of $\sim$13,000 stars from the SDSS/APOGEE survey to study the shape of the bulge MDF within the region $|\ell|\leq11^\circ$ and $|b|\leq13^\circ$, and spatially constrained to ${\rm R_{GC}\leq3.5}$ kpc. We apply Gaussian Mixture Modeling and Non-negative Matrix Factorization decomposition techniques to identify the optimal number and the properties of MDF components. We find the shape and spatial variations of the MDF (at ${\rm [Fe/H]\geq-1}$ dex) are well represented as a smoothly varying contribution of three overlapping components located at [Fe/H]=+$0.32$, $-0.17$ and $-0.66$ dex. The bimodal MDF found in previous studies is in agreement with our trimodal assessment once the limitations in sample size and individual measurement errors are taken into account. The shape of the MDF and its correlations with kinematics reveal different spatial distributions and kinematical structure for the three components co-existing in the bulge region. We confirm the consensus physical interpretation of metal-rich stars as associated with the secularly evolved disk into a boxy/peanut X-shape bar. On the other hand, metal-intermediate stars could be the product of in-situ formation at high redshift in a gas-rich environment characterized by violent and fast star formation. This interpretation would help to link a present-day structure with those observed in formation in the center of high redshift galaxies. Finally, metal-poor stars may correspond to the metal-rich tail of the population sampled at lower metallicity from the study of RR Lyrae stars. Conversely, they could be associated with the metal-poor tail of the early thick disc.

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arXiv2020

Open Cluster Chemical Homogeneity Throughout the Milky Way

The chemical homogeneity of surviving stellar clusters contains important clues about interstellar medium (ISM) mixing efficiency, star formation, and the enrichment history of the Galaxy. Existing measurements in a handful of open clusters suggest homogeneity in several elements at the 0.03 dex level. Here we present (i) a new cluster member catalog based only on APOGEE radial velocities and Gaia-DR2 proper motions, (ii) improved abundance uncertainties for APOGEE cluster members, and (iii) the dependence of cluster homogeneity on Galactic and cluster properties, using abundances of eight elements from the APOGEE survey for ten high-quality clusters. We find that cluster homogeneity is uncorrelated with Galactocentric distance, |Z|, age, and metallicity. However, velocity dispersion, which is a proxy for cluster mass, is positively correlated with intrinsic scatter at relatively high levels of significance for [Ca/Fe] and [Mg/Fe]. We also see a possible positive correlation at a low level of significance for [Ni/Fe], [Si/Fe], [Al/Fe], and [Fe/H], while [Cr/Fe] and [Mn/Fe] are uncorrelated. The elements that show a correlation with velocity dispersion are those that are predominantly produced by core-collapse supernovae (CCSNe). However, the small sample size and relatively low correlation significance highlight the need for follow-up studies. If borne out by future studies, these findings would suggest a quantitative difference between the correlation lengths of elements produced predominantly by Type~Ia SNe versus CCSNe, which would have implications for Galactic chemical evolution models and the feasibility of chemical tagging.

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arXiv2020

Cool stars in the Galactic Center as seen by APOGEE: M giants, AGB stars and supergiant stars/candidates

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.

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arXiv2020

From the bulge to the outer disc: StarHorse stellar parameters, distances, and extinctions for stars in APOGEE DR16 and other spectroscopic surveys

We combine high-resolution spectroscopic data from APOGEE-2 Survey Data Release 16 (DR16) with broad-band photometric data from several sources, as well as parallaxes from {\it Gaia} Data Release 2 (DR2). Using the Bayesian isochrone-fitting code {\tt StarHorse}, we derive distances, extinctions and astrophysical parameters for around 388,815 APOGEE stars, achieving typical distance uncertainties of $\sim 6\%$ for APOGEE giants, $\sim 2\%$ for APOGEE dwarfs, as well as extinction uncertainties of $\sim 0.07$ mag when all photometric information is available, and $\sim 0.17$ mag if optical photometry is missing. {\tt StarHorse} uncertainties vary with the input spectroscopic catalogue, with the available photometry, and with the parallax uncertainties. To illustrate the impact of our results, we show that, thanks to {\it Gaia} DR2 and the now larger sky coverage of APOGEE-2 (including APOGEE-South), we obtain an extended map of the Galactic plane, providing unprecedented coverage of the disk close to the Galactic mid-plane ($|Z_{Gal}|<1$ kpc) from the Galactic Centre out to $R_{\rm Gal}\sim 20$ kpc. The improvements in statistics as well as distance and extinction uncertainties unveil the presence of the bar in stellar density, as well as the striking chemical duality in the innermost regions of the disk, now clearly extending to the inner bulge. We complement this paper with distances and extinctions for stars in other public released spectroscopic surveys: 324,999 in GALAH DR2, 4,928,715 in LAMOST DR5, 408,894 in RAVE DR6, and 6,095 in GES DR3

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arXiv2019

The K2 Galactic Caps Project -- Going Beyond the \textit{Kepler} Field and Ageing the Galactic Disc

Analyses of data from spectroscopic and astrometric surveys have led to conflicting results concerning the vertical characteristics of the Milky Way. Ages are often used to provide clarity, but typical uncertainties of $>$ 40\,\% restrict the validity of the inferences made. Using the \textit{Kepler} APOKASC sample for context, we explore the global population trends of two K2 campaign fields (3 and 6), which extend further vertically out of the Galactic plane than APOKASC. We analyse the properties of red giant stars utilising three asteroseismic data analysis methods to cross-check and validate detections. The Bayesian inference tool PARAM is used to determine the stellar masses, radii and ages. Evidence of a pronounced red giant branch bump and an [$α$/Fe] dependence on the position of the red clump is observed from the radii distribution of the K2 fields. Two peaks in the age distribution centred at $\sim$5 and and $\sim$12 Gyr are found using a sample with $σ_{\rm{age}}$ $<$ 35\,\%. In a comparison with \textit{Kepler}, we find the older peak to be more prominent for K2. This age bimodality is also observed based on a chemical selection of low- ($\leq$ 0.1) and high- ($>$ 0.1) [$α$/Fe] stars. As a function of vertical distance from the Galactic mid-plane ($|Z|$), the age distribution shows a transition from a young to old stellar population with increasing $|Z|$ for the K2 fields. Further coverage of campaign targets with high resolution spectroscopy is required to increase the yield of precise ages achievable with asteroseismology.

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arXiv2019

A CO-multilayer outer atmosphere for eight evolved stars revealed with VLTI/AMBER

We determine the physical parameters of the outer atmosphere of a sample of eight evolved stars, including the red supergiant α Scorpii, the red giant branch stars α Bootis and γ Crucis, the K giant λ Velorum, the normal M giants BK Virginis and SW Virginis, and the Mira star W Hydrae (in two different luminosity phases) by spatially resolving the stars in the individual carbon monoxide (CO) first overtone lines. We used the Astronomical Multi-BEam combineR (AMBER) instrument at the Very Large Telescope Interferometer (VLTI), in high-resolution mode ($λ/Δλ {\approx} 12000$) between 2.28 and 2.31 $μm$ in the K band.The maximal angular resolution is 10 mas, obtained using a triplet telescope configuration, with baselines from 7 to 48 m. By using a numerical model of a molecular atmosphere in a spherical shells (MOLsphere), called PAMPERO (an acronym for the "Physical Approach of Molecular Photospheric Ejection at high-angular-Resolution for evOlved stars"), we add multiple extended CO layers above the photospheric MARCS model at an adequate spatial resolution. We use the differential visibilities and the spectrum to estimate the size ($R$) of the CO MOLsphere, its column density (${N_{CO}}$) and temperature (${T_{mol}}$) distributions along the stellar radius. The combining of the ${χ^2}$ minimization and a fine grid approach for uncertainty analysis leads to reasonable ${N_{CO}}$ and ${T_{mol}}$ distributions along the stellar radius of the MOLsphere.

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arXiv2019

Study of Eclipsing Binary and Multiple Systems in OB Associations V: MQ Cen in Crux OB1

The early-type massive binary MQ Cen (P$_{orb}$=3.7 d) has been investigated by means of high-resolution ($R\sim48\,000$) spectral analysis and multi-band (Johnson $BVRI$ and Strömgren $vby$) light curve modeling. The physical parameters of the components have been found to be $M_1= 4.26\pm0.10$ M$_{\odot}$, $R_1= 3.72\pm0.05 $R$_{\odot}$, $T_{\rm eff1}=16\,600\pm520$ K, and $M_2= 5.14\pm0.09 $M$_{\odot}$, $R_2= 7.32\pm0.03 $R$_{\odot}$, $T_{\rm eff2}=15\,000\pm500$ K for the primary and secondary, respectively. The orbital inclination is $i=87.0\pm0.2$ deg. The distance to MQ Cen has been derived to be $d=2\,460\pm310$ pc which locates it in the Crux OB1 association. However, the age of MQ~Cen ($\sim70$ Myr) is higher than the one reported for the Crux OB1 association ($\sim$6 Myr). The derived masses are implying a spectral type of B5 for the primary and B4 for the secondary component. Nevertheless, the secondary component, which is more massive, appears to be cooler than the primary component: It has completed its lifetime on the main-sequence and it is now positioned at the turn-off point of the giant branch, meanwhile the less massive primary component is still staying on the main-sequence.

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arXiv2018

APOGEE DR14/DR15 Abundances in the Inner Milky Way

We present an overview of the distributions of 11 elemental abundances in the Milky Way's inner regions, as traced by APOGEE stars released as part of SDSS Data Release 14/15 (DR14/DR15), including O, Mg, Si, Ca, Cr, Mn, Co, Ni, Na, Al, and K. This sample spans ~4000 stars with R_GC<4 kpc, enabling the most comprehensive study to date of these abundances and their variations within the innermost few kiloparsecs of the Milky Way. We describe the observed abundance patterns ([X/Fe]-[Fe/H]), compare to previous literature results and to patterns in stars at the solar Galactic radius, and discuss possible trends with DR14/DR15 effective temperatures. We find that the position of the [Mg/Fe]-[Fe/H] "knee" is nearly constant with R_GC, indicating a well-mixed star-forming medium or high levels of radial migration in the early inner Galaxy. We quantify the linear correlation between pairs of elements in different subsamples of stars and find that these relationships vary; some abundance correlations are very similar between the alpha-rich and alpha-poor stars, but others differ significantly, suggesting variations in the metallicity dependencies of certain supernova yields. These empirical trends will form the basis for more detailed future explorations and for the refinement of model comparison metrics. That the inner Milky Way abundances appear dominated by a single chemical evolutionary track and that they extend to such high metallicities underscore the unique importance of this part of the Galaxy for constraining the ingredients of chemical evolution modeling and for improving our understanding of the evolution of the Galaxy as a whole.

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arXiv2018

A chemical and kinematical analysis of the intermediate-age open cluster IC 166 from APOGEE and Gaia DR2

IC 166 is an intermediate-age open cluster ($\sim 1$ Gyr) which lies in the transition zone of the metallicity gradient in the outer disc. Its location, combined with our very limited knowledge of its salient features, make it an interesting object of study. We present the first high-resolution spectroscopic and precise kinematical analysis of IC 166, which lies in the outer disc with $R_{GC} \sim 12.7$ kpc. High resolution \textit{H}-band spectra were analyzed using observations from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. We made use of the Brussels Automatic Stellar Parameter (BACCHUS) code to provide chemical abundances based on a line-by-line approach for up to eight chemical elements (Mg, Si, Ca, Ti, Al, K, Mn and Fe). The $α-$element (Mg, Si, Ca and whenever available Ti) abundances, and their trends with Fe abundances have been analysed for a total of 13 high-likelihood cluster members. No significant abundance scatter was found in any of the chemical species studied. Combining the positional, heliocentric distance, and kinematic information we derive, for the first time, the probable orbit of IC 166 within a Galactic model including a rotating boxy bar, and found that it is likely that IC 166 formed in the Galactic disc, supporting its nature as an unremarkable Galactic open cluster with an orbit bound to the Galactic plane.

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arXiv2017

Metallicity effect on stellar granulation detected from oscillating red giants in open clusters

The effect of metallicity on the granulation activity in stars is still poorly understood. Available spectroscopic parameters from the updated APOGEE-\textit{Kepler} catalog, coupled with high-precision photometric observations from NASA's \textit{Kepler} mission spanning more than four years of observation, make oscillating red giant stars in open clusters crucial testbeds. We determine the role of metallicity on the stellar granulation activity by discriminating its effect from that of different stellar properties such as surface gravity, mass, and temperature. We analyze 60 known red giant stars belonging to the open clusters NGC 6791, NGC 6819, and NGC 6811, spanning a metallicity range from [Fe/H] $\simeq -0.09$ to $0.32$. The parameters describing the granulation activity of these stars and their $ν_\mathrm{max}$, are studied by considering the different masses, metallicities, and stellar evolutionary stages. We derive new scaling relations for the granulation activity, re-calibrate existing ones, and identify the best scaling relations from the available set of observations. We adopted the Bayesian code DIAMONDS for the analysis of the background signal in the Fourier spectra of the stars. We performed a Bayesian parameter estimation and model comparison to test the different model hypotheses proposed in this work and in the literature. Metallicity causes a statistically significant change in the amplitude of the granulation activity, with a dependency stronger than that induced by both stellar mass and surface gravity. We also find that the metallicity has a significant impact on the corresponding time scales of the phenomenon. The effect of metallicity on the time scale is stronger than that of mass. A higher metallicity increases the amplitude of granulation and meso-granulation signals and slows down their characteristic time scales toward longer periods.

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arXiv2017

Atypical Mg-poor Milky Way field stars with globular cluster second-generation like chemical patterns

We report the peculiar chemical abundance patterns of eleven atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low-Mg abundances ([Mg/Fe]$<$0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H]$\gtrsim - 1.0$) sample stars, which is at odds with actual observations of SG stars in Galactic CGs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy; a fundamental step forward to understand the Galactic formation and evolution.

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