arXiv subjects

M. Schultheis

Publications and source records attributed to M. Schultheis.

At least 55 records · Page 3Linked to original sources

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

Optical interferometry and Gaia measurement uncertainties reveal the physics of asymptotic giant branch stars

Context. Asymptotic giant branch stars are cool luminous evolved stars that are well observable across the Galaxy and populating Gaia data. They have complex stellar surface dynamics Aims. On the AGB star CL Lac, it has been shown that the convection-related variability accounts for a substantial part of the Gaia DR2 parallax error. We observed this star with the MIRC-X beam combiner installed at the CHARA interferometer to detect the presence of stellar surface inhomogeneities. Methods. We performed the reconstruction of aperture synthesis images from the interferometric observations at different wavelengths. Then, we used 3D radiative hydrodynamics simulations of stellar convection with CO5BOLD and the post-processing radiative transfer code Optim3D to compute intensity maps in the spectral channels of MIRC-X observations. Then, we determined the stellar radius and compared the 3D synthetic maps to the reconstructed ones focusing on matching the intensity contrast, the morphology of stellar surface structures, and the photocentre position at two different spectral channels, 1.52 and 1.70 micron, simultaneously. Results. We measured the apparent diameter of CL Lac at two wavelengths and recovered the radius using a Gaia parallax. In addition to this, the reconstructed images are characterised by the presence of a brighter area that largely affects the position of the photocentre. The comparison with 3D simulation shows good agreement with the observations both in terms of contrast and surface structure morphology, meaning that our model is adequate for explaining the observed inhomogenities. Conclusions. This work confirms the presence of convection-related surface structures on an AGB star of Gaia DR2. Our result will help us to take a step forward in exploiting Gaia measurement uncertainties to extract the fundamental properties of AGB stars using appropriate RHD simulations.

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arXiv2020

Heavy element evolution in the inner regions of the Milky Way

We present results for the evolution of the abundances of heavy elements (O, Mg, Al, Si, K, Ca, Cr, Mn, Ni and Fe) in the inner Galactic regions ($R_{GC} < 4$kpc). We adopt a detailed chemical evolution model already tested for the Galactic bulge and compare the results with APOGEE data. We start with a set of yields from the literature which are considered the best to reproduce the abundance patterns in the solar vicinity. We find that in general the predicted trends nicely reproduce the data but in some cases either the trend or the absolute values of the predicted abundances need to be corrected, even by large factors, in order to reach the best agreement. We suggest how the current stellar yields should be modified to reproduce the data and we discuss whether such corrections are reasonable in the light of the current knowledge of stellar nucleosynthesis. However, we also critically discuss the observations. Our results suggest that Si, Ca, Cr and Ni are the elements for which the required corrections are the smallest, while for Mg and Al moderate modifications are necessary. On the other hand, O and K need the largest corrections to reproduce the observed patterns, a conclusion already reached for solar vicinity abundance patterns, with the exception of oxygen. For Mn we apply corrections already suggested in previous works. \end{abstract}

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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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arXiv2020

Detailed abundances in the Galactic center: Evidence of a metal-rich alpha-enhanced stellar population

We present a detailed study of the composition of 20 M giants in the Galactic center with 15 of them confirmed to be in the Nuclear Star Cluster. As a control sample we have also observed 7 M giants in the Milky Way Disk with similar stellar parameters. All 27 stars are observed using the NIRSPEC spectograph on the KECK II telescope in the K-band at a resolving power of R=23,000. We report the first silicon abundance trends versus [Fe/H] for stars in the Galactic center. While finding a disk/bulge like trend at subsolar metallicities, we find that [Si/Fe] is enhanced at supersolar metallicities. We speculate on possible enrichment scenarios to explain such a trend. However, the sample size is modest and the result needs to be confirmed by additional measurements of silicon and other \textalpha-elements. We also derive a new distribution of [Fe/H] and find the most metal rich stars at [Fe/H]=+0.5 dex, confirming our earlier conclusions that the Galactic center hosts no stars with extreme chemical composition.

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arXiv2019

The chemistry of stars in the bar of the Milky Way

We use a sample of 938 red clump giant stars located in the direction of the galactic long bar to study the chemistry of Milky Way bar stars. Kinematically separating stars on bar orbits from stars with inner disc orbits, we find that stars on bar-like orbits are more metal rich with a mean iron abundance of <[Fe/H]>=+0.30 compared to <[Fe/H]>=+0.03 for the inner disc. Spatially selecting bar stars is complicated by a strong vertical metallicity gradient of -1.1dex/kpc, but we find the metallicity distribution varies in a manner consistent with our orbital selection. Our results have two possible interpretations. The first is that the most metal rich stars in the inner Galaxy pre-existed the bar, but were kinematically cold at the time of bar formation and therefore more easily captured onto bar orbits when the bar formed. The second is that the most metal rich stars formed after the bar, either directly onto the bar following orbits or were captured by the bar after their formation.

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arXiv2019

Euclid preparation III. Galaxy cluster detection in the wide photometric survey, performance and algorithm selection

Galaxy cluster counts in bins of mass and redshift have been shown to be a competitive probe to test cosmological models. This method requires an efficient blind detection of clusters from surveys with a well-known selection function and robust mass estimates. The Euclid wide survey will cover 15000 deg$^2$ of the sky in the optical and near-infrared bands, down to magnitude 24 in the $H$-band. The resulting data will make it possible to detect a large number of galaxy clusters spanning a wide-range of masses up to redshift $\sim 2$. This paper presents the final results of the Euclid Cluster Finder Challenge (CFC). The objective of these challenges was to select the cluster detection algorithms that best meet the requirements of the Euclid mission. The final CFC included six independent detection algorithms, based on different techniques, such as photometric redshift tomography, optimal filtering, hierarchical approach, wavelet and friend-of-friends algorithms. These algorithms were blindly applied to a mock galaxy catalog with representative Euclid-like properties. The relative performance of the algorithms was assessed by matching the resulting detections to known clusters in the simulations. Several matching procedures were tested, thus making it possible to estimate the associated systematic effects on completeness to $<3$%. All the tested algorithms are very competitive in terms of performance, with three of them reaching $>80$% completeness for a mean purity of 80% down to masses of $10^{14}$ M$_{\odot}$ and up to redshift $z=2$. Based on these results, two algorithms were selected to be implemented in the Euclid pipeline, the AMICO code, based on matched filtering, and the PZWav code, based on an adaptive wavelet approach. [abridged]

astro-ph.CO↗

arXiv2019

The inner two degrees of the Milky Way. Evidence of a chemical difference between the Galactic Center and the surrounding inner bulge stellar populations

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 $α$-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 $α$-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 $\rm [Fe/H]<0$, we find compelling evidence for a higher fraction ($\sim 80\%$) of metal-rich stars in the Galactic Center (GC) compared to the values (50-60\%) measured in the low latitude fields within the innermost 600 pc. The high fraction of metal-rich stars in the GC region implies a very high mean metallicity of +0.2 dex, while in the inner 600 pc of the bulge the mean metallicity is rather homogenous around the solar value. A vertical metallicity gradient of -0.27 dex/kpc in the inner 600 pc is only measured if the GC is included, otherwise the distribution is about flat and consistent with no vertical gradient. In addition to its high stellar density, the Galactic center/nuclear star cluster is also extreme in hosting high stellar abundances, compared to the surrounding inner bulge stellar populations; this has implications for formation scenarios and strengthens the case for the NSC being a distinct stellar system.

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arXiv2019

H-band discovery of additional Second-Generation stars in the Galactic Bulge Globular Cluster NGC 6522 as observed by APOGEE and Gaia

We present elemental abundance analysis of high-resolution spectra for five giant stars, deriving Fe, Mg, Al, C, N, O, Si and Ce abundances, and spatially located within the innermost regions of the bulge globular cluster NGC 6522, based on H-band spectra taken with the multi-object APOGEE-north spectrograph from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey. Of the five cluster candidates, two previously unremarked stars are confirmed to have second-generation (SG) abundance patterns, with the basic pattern of depletion in C and Mg simultaneous with enrichment in N and Al as seen in other SG globular cluster populations at similar metallicity. } In agreement with the most recent optical studies, the NGC 6522 stars analyzed exhibit (when available) only mild overabundances of the s-process element Ce, contradicting the idea of the NGC 6522 stars being formed from gas enriched by spinstars and indicating that other stellar sources such as massive AGB stars could be the primary intra-cluster medium polluters. The peculiar abundance signature of SG stars have been observed in our data, confirming the presence of multiple generations of stars in NGC 6522.

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arXiv2019

The bimodal [Mg/Fe] versus [Fe/H] bulge sequence as revealed by APOGEE DR14

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 $α$-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 $α$-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 $α$-abundance proxy for our clean sample as it has been proven to be the most accurate $α$-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.

astro-ph.GA↗

arXiv2019

Abundances of disk and Bulge giants from high-resolution optical spectra III. Sc, V, Cr, Mn, Co, Ni

Context. Recent observations of the Bulge, e.g., its X-shape, cylindrical stellar motions, and a potential fraction of young stars propose that it formed through secular evolution of the disk and not through gas dissipation and/or mergers, as thought previously. Aims. We measure abundances of six iron-peak elements (Sc, V, Cr, Mn, Co and Ni) in the local thin and thick disks as well as the Bulge to provide additional observational constraints for Galaxy formation and chemical evolution models. Methods. We use high-resolution optical spectra of 291 K giants in the local disk mostly obtained by the FIES at NOT (signal-to-noise (S/N) ratio of 80-100) and 45 K giants in the Bulge obtained by the UVES/FLAMES at VLT (S/N ratio of 10-80). We measure abundances in SME and apply NLTE corrections to the [Mn/Fe] and [Co/Fe] ratios. To discriminate between the thin and thick, we use stellar metallicity, [Ti/Fe]-ratios, and kinematics from Gaia DR2 (proper motions and the radial velocities). Results. The observed disk trend of V is more enhanced in the thick disk, while the Co disk trend shows a minor enhancement in the thick disk. The Bulge trends of V and Co appear even more enhanced w.r.t. the thick disk, but within the uncertainties. The [Ni/Fe] ratio seems slightly overabundant in the thick disk and the Bulge w.r.t. the thin disk, although the difference is minor. The disk and Bulge trends of Sc, Cr and Mn overlap strongly. Conclusions. The somewhat enhanced [(V,Co)/Fe] ratios observed in the Bulge suggest that the local thick disk and the Bulge might have experienced different chemical enrichment and evolutionary paths. However, we are unable to predict the exact evolutionary path of the Bulge solely based on these observations. Galactic chemical evolution models could, on the other hand, provide that using these results.

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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

Evidence against anomalous compositions for giants in the Galactic Nuclear Star Cluster

Very strong Sc I lines have been found recently in cool M giants in the Nuclear Star Cluster in the Galactic Center. Interpreting these as anomalously high scandium abundances in the Galactic Center would imply a unique enhancement signature and chemical evolution history for nuclear star clusters, and a potential test for models of chemical enrichment in these objects. We present high resolution K-band spectra (NIRSPEC/Keck II) of cool M giants situated in the solar neighborhood and compare them with spectra of M giants in the Nuclear Star Cluster. We clearly identify strong Sc I lines in our solar neighborhood sample as well as in the Nuclear Star Cluster sample. The strong Sc I lines in M giants are therefore not unique to stars in the Nuclear Star Cluster and we argue that the strong lines are a property of the line formation process that currently escapes accurate theoretical modeling. We further conclude that for giant stars with effective temperatures below approximately 3800 K these Sc I lines should not be used for deriving the scandium abundances in any astrophysical environment until we better understand how these lines are formed. We also discuss the lines of vanadium, titanium, and yttrium identified in the spectra, which demonstrate a similar striking increase in strength below 3500 K effective temperature.

astro-ph.GA↗

arXiv2018

Heading Gaia to measure atmospheric dynamics in AGB stars

Context. Asymptotic Giant Branch stars are characterised by complex stellar surface dynamics that affect the measurements and amplify the uncertainties on stellar parameters. The resulting motion of the stellar photo-center could have adverse effects on the parallax determination with Gaia. Aims. We explore the impact of the convection-related surface structure in AGBs on the photocentric variability. We quantify these effects to characterise the observed parallax errors and estimate fundamental stellar parameters and dynamical properties. Methods. We use 3D radiative-hydrodynamics simulations of convection with CO5BOLD and the post-processing radiative transfer code Optim3D to compute intensity maps in the Gaia G band. From those maps, we calculate the intensity-weighted mean of all emitting points tiling the visible stellar surface and evaluate its motion as a function of time. We extract the parallax error from Gaia DR2 for a sample of semiregular variables in the solar neighbourhood and compare it to the synthetic predictions of photo-center displacements. Results. AGB stars show a complex surface morphology characterised by the presence of few large scale long-lived convective cells accompanied by short-lived and small scale structures. As a consequence, the position of the photo-center displays temporal excursions between 0.077 to 0.198 AU (5 to 11% of the corresponding stellar radius), depending on the simulation considered. We show that the convection-related variability accounts for a substantial part to the Gaia DR2 parallax error of our sample of semiregular variables. Finally, we put in evidence for a correlation between the mean photo-center displacement and the stellar fundamental parameters: surface gravity, radius, and pulsation. We denote that parallax variations could be exploited quantitatively using appropriate RHD simulations corresponding to the observed star.

astro-ph.SR↗

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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arXiv2018

Chemical Characterization of the Inner Galactic bulge:North-South Symmetry

While the number of stars in the Galactic bulge with detailed chemical abundance measurements is increasingly rapidly, the inner Galactic bulge ( |b| < 2$^\circ$) remains poorly studied, due to heavy interstellar absorption and photometric crowding. We have carried out a high-resolution IR spectroscopic study of 72 M giants in the inner bulge using the CRIRES (ESO/VLT) facility. Our spectra cover the wavelength range of 2.0818 - 2.1444 $μ$m with the resolution of R$\sim$50,000 and have signal-to-noise ratio of 50 - 100. Our stars are located along the bulge minor axis at l = 0$^\circ$, b = $\pm$0$^\circ$, $\pm$1$^\circ$, $\pm$2$^\circ$ and +3$^\circ$, selected with the aim of investigating any North-South asymmetries in the metallicity distribution function and composition, and comparing them to the outer bulge fields. Our sample was analysed in a homogeneous way using the most current K-band line list. We clearly detect a bimodal MDF with a metal-rich peak at $\rm \sim +0.3\,dex$ and a metal-poor peak at $\rm \sim -0.5\,dex$. Only a single star is found to exceed $\rm [Fe/H]=+0.5\,dex$. They show a symmetric behaviour along the $\pm$1$^\circ$, $\pm$2$^\circ$ fields. The Galactic Center field reveals in contrast a mainly metal-rich population with a mean metallicity of $\rm +0.3\,dex$. We derived $\rm [Mg/Fe]$ and $\rm [Si/Fe]$ abundances which are consistent with trends from the outer bulge, with a gradually decreasing trend with increasing metallicity. We confirm for the supersolar metallicity stars the decreasing trend in \mgfe\ and \sife\ as expected from chemical evolution models. With the caveat of a relatively small sample, we do not find significant differences in the chemical abundances between the Northern and the Southern fields, hence the evidence is consistent with symmetry in chemistry between North and South.

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arXiv2018

The disc origin of the Milky Way bulge: Dissecting the chemo-morphological relations using N-body simulations and APOGEE

There is a long-standing debate on the origin of the metal-poor stellar populations of the Milky Way (MW) bulge, with the two leading scenarios being that these populations are either i) part of a classical metal-poor spheroid or ii) the same population as the chemically defined thick disc seen at the Solar neighbourhood. Here we test whether the latter scenario can reproduce the observed chemical properties of the MW bulge. To do so we compare an N-body simulation of a composite (thin+thick) stellar disc -- which evolves secularly to form a bar and a boxy/peanut (b/p) bulge -- to data from APOGEE DR13. This model, in which the thick disc is massive and centrally concentrated, can reproduce the morphology of the metal-rich and metal-poor stellar populations in the bulge, as well as the mean metallicity and [$α$/Fe] maps as obtained from the APOGEE data. It also reproduces the trends, in both longitude and latitude, of the bulge metallicity distribution function (MDF). Additionally, we show that the model predicts small but measurable azimuthal metallicity variations in the inner disc due to the differential mapping of the thin and thick disc in the bar. We therefore see that the chemo-morphological relations of stellar populations in the MW bulge are naturally reproduced by mapping the thin and thick discs of the inner MW into a b/p.

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arXiv2017

Near-infrared spectroscopic observations of massive young stellar object candidates in the Central Molecular Zone

We present a spectroscopic follow-up of photometrically-selected young stellar object (YSO) candidates in the Central Molecular Zone of the Galactic center. Our goal is to quantify the contamination of this YSO sample by reddened giant stars with circumstellar envelopes and to determine the star formation rate in the CMZ. We obtained KMOS low-resolution near-infrared spectra (R ~4000) between 2.0 and 2.5 um of sources, many of them previously identified, by mid-infrared photometric criteria, as massive YSOs in the Galactic center. Our final sample consists of 91 stars with good signal-to-noise ratio. We separate YSOs from cool late-type stars based on spectral features of CO and Br_gamma at 2.3 um and 2.16 um respectively. We make use of SED model fits to the observed photometric data points from 1.25 to 24 um in order to estimate approximate masses for the YSOs. Using the spectroscopically identified YSOs in our sample, we confirm that existing colour-colour diagrams and colour-magnitude diagrams are unable to efficiently separate YSOs and cool late-type stars. In addition, we define a new colour-colour criterion that separates YSOs from cool late-type stars in the H-Ks vs H-[8.0] diagram. We use this new criterion to identify YSO candidates in the |l| < 1.5, |b|<0.5 degree region and use model SED fits to estimate their approximate masses. By assuming an appropriate initial mass function (IMF) and extrapolating the stellar IMF down to lower masses, we determine a star formation rate (SFR) of ~0.046 +/- 0.026 Msun/yr assuming an average age of 0.75 +/- 0.25 Myr for the YSOs. This value is lower than estimates found using the YSO counting method in the literature. Our SFR estimate in the CMZ agrees with the previous estimates from different methods and reaffirms that star formation in the CMZ is proceeding at a lower rate than predicted by various star forming models.

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