On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk

Genovali, K.; Lemasle, B.; Bono, G.; Romaniello, M.; Fabrizio, M.; Ferraro, I.; Iannicola, G.; Laney, C. D.; Nonino, M.; Bergemann, M.; Buonanno, R.; François, P.; Inno, L.; Kudritzki, Rolf‐Peter; Matsunaga, Noriyuki; Pedicelli, Stefania; Primas, F.; Thévenin, F.

doi:10.1051/0004-6361/201323198

Cited by 188 publications

(298 citation statements)

References 139 publications

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“…As a result, the initial and the present-day gradient derived for the younger populations are very similar out to 12 kpc. This fairly explains the similarity in the radial gradients observed for the younger tracers such as OCs of age less than 2 Gyr ( Figure 4 and figure 5), field stars (Cheng et al 2012;Hayden et al 2014) and Cepheids (Luck & Lambert 2011;Genovali et al 2014) for Rgc < 12 kpc. The steep gradient of slope −0.058 dex kpc −1 for stars younger than 2 Gyr in the radial range 6−11 kpc and a shallow gradient of old stellar populations predicted after taking the radial migration into account is in fair agreement with our values of gradient measured for respective age groups.…”

Section: Comparison With Chemodynamical Modelssupporting

confidence: 72%

“…It is especially puzzling that the flattening of the metallicity distribution of OCs at large radii is not shown by the Galactic field stars (Cheng et al 2012;Hayden et al 2014) including Cepheids (Luck & Lambert 2011;Genovali et al 2014) where field stars suggest a constant steep decline of metallicity out to Rgc of 18 kpc. Another fascinating result, in common for both the field stars and OCs at large radii, is the evidence for enhanced [α/Fe] ratios in the outer Galactic disc (Yong et al 2005;Bensby et al 2011;Luck & Lambert 2011).…”

Section: Introductionmentioning

confidence: 99%

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The evolution of the Milky Way: new insights from open clusters

Reddy¹,

Lambert²,

Giridhar³

2016

Mon. Not. R. Astron. Soc.

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We have collected high-dispersion echelle spectra of red giant members in the twelve open clusters (OCs) and derived stellar parameters and chemical abundances for 26 species by either line equivalent widths or synthetic spectrum analyses. We confirm the lack of an age−metallicity relation for OCs but argue that such a lack of trend for OCs arise from the limited coverage in metallicity compared to that of field stars which span a wide range in metallicity and age. We confirm that the radial metallicity gradient of OCs is steeper (flatter) for R gc < 12 kpc (>12 kpc). We demonstrate that the sample of clusters constituting a steep radial metallicity gradient of slope −0.052±0.011 dex kpc −1 at R gc < 12 kpc are younger than 1.5 Gyr and located close to the Galactic midplane (| z| < 0.5 kpc) with kinematics typical of the thin disc. Whereas the clusters describing a shallow slope of −0.015±0.007 dex kpc −1 at R gc > 12 kpc are relatively old, thick disc members with a striking spread in age and height above the midplane (0.5 < | z| < 2.5 kpc). Our investigation reveals that the OCs and field stars yield consistent radial metallicity gradients if the comparison is limited to samples drawn from the similar vertical heights. We argue via the computation of Galactic orbits that all the outer disc clusters were actually born inward of 12 kpc but the orbital eccentricity has taken them to present locations very far from their birthplaces.

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Section: Comparison With Chemodynamical Modelssupporting

confidence: 72%

Section: Introductionmentioning

confidence: 99%

The evolution of the Milky Way: new insights from open clusters

Reddy¹,

Lambert²,

Giridhar³

2016

Mon. Not. R. Astron. Soc.

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“…Several studies of the Galactic abundance gradients in the thin disc using classical Cepheids have reported increased dispersion in the outer disc (Yong et al 2006;Lemasle et al 2008;Luck & Lambert 2011;Genovali et al 2014); however, these findings are hampered by the possible contamination of the current samples by misclassified Type II Cepheids that are thick-disc members. Including unrecognized Type II Cepheids modifies the abundance patterns not only because they belong to another stellar population but also because their current distances are computed with period-luminosity relations that are only valid for classical Cepheids.…”

Section: Discussionmentioning

confidence: 81%

Type II Cepheids in the Milky Way disc

Lemasle

Kovtyukh

Bono

et al. 2015

A&A

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Context. A robust classification of Cepheids into their different sub-classes and, in particular, between classical and Type II Cepheids, is necessary to properly calibrate the period-luminosity relations and for populations studies in the Galactic disc. Type II Cepheids are, however, very diverse, and classifications based either on intrinsic (period, light curve) or external parameters (e.g., [Fe/H], |z|) do not provide a unique classification. Aims. We want to ascertain the classification of two Cepheids, HQ Car and DD Vel, that are sometimes classified as classical Cepheids and sometimes as Type II Cepheids. Methods. To achieve this goal, we examine both their chemical composition and the presence of specific features in their spectra. Results. We find emission features in the Hα and in the 5875.64 Å He I lines that are typical of W Vir stars. The [Na/Fe] (or [Na/Zn]) abundances are typical of thick-disc stars, while BL Her stars are Na-overabundant ([Na/Fe] > +0.5 dex). Finally, the two Cepheids show a possible (HQ Car) or probable (DD Vel) signature of mild dust-gas separation that is usually observed only in long-period type II Cepheids and RV Tau stars. Conclusions. These findings clearly indicate that HQ Car and DD Vel are both Type II Cepheids from the W Vir sub-class. Several studies have reported an increase in the Cepheids' abundance dispersion towards the outer (thin) disc. A detailed inspection of the Cepheid classification, in particular for those located in the outer disc, will indicate whether this feature is real or simply an artefact of the inclusion of type II Cepheids belonging to the thick disc in the current samples.

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“…Interestingly, there are some intriguing young objects in the MW that might be related to the same phenomenon: a) the puzzling low-metallicity supergiants located near the end of the Galactic bar (Davies et al 2009a,b, see discussion in Genovali et al 2014 andOriglia et al 2013); b) the young [α/Fe]-enchanced stars reported by Cunha et al (2007) near the Galactic Centre, and; c) the unusual Cepheid BC Aql which, despite being young (Whitelock, priv. comm.)…”

Section: What Is Their Origin?mentioning

confidence: 96%

Young [α/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?

Chiappini

Anders

Rodrigues

et al. 2015

A&A

172

196

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We report the discovery of a group of apparently young CoRoT red-giant stars exhibiting enhanced [α/Fe] abundance ratios (as determined from APOGEE spectra) with respect to solar values. Their existence is not explained by standard chemical evolution models of the Milky Way, and shows that the chemical-enrichment history of the Galactic disc is more complex. We find similar stars in previously published samples for which isochrone-ages could be reliably obtained, although in smaller relative numbers. This might explain why these stars have not previously received attention. The young [α/Fe]-rich stars are much more numerous in the CoRoT-APOGEE (CoRoGEE) inner-field sample than in any other high-resolution sample available at present because only CoRoGEE can explore the inner-disc regions and provide ages for its field stars. The kinematic properties of the young [α/Fe]-rich stars are not clearly thick-disc like, despite their rather large distances from the Galactic mid-plane. Our tentative interpretation of these and previous intriguing observations in the Milky Way is that these stars were formed close to the end of the Galactic bar, near corotation -a region where gas can be kept inert for longer times than in other regions that are more frequently shocked by the passage of spiral arms. Moreover, this is where the mass return from older inner-disc stellar generations is expected to be highest (according to an inside-out disc-formation scenario), which additionally dilutes the in-situ gas. Other possibilities to explain these observations (e.g., a recent gas-accretion event) are also discussed.

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On the fine structure of the Cepheid metallicity gradient in the Galactic thin disk

Cited by 188 publications

References 139 publications

The evolution of the Milky Way: new insights from open clusters

The evolution of the Milky Way: new insights from open clusters

Type II Cepheids in the Milky Way disc

Young [α/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?

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