On the binarity of the classical Cepheid X Sgr from interferometric observations

Optical-infrared interferometry can provide direct geometrical measurements of the radii of Cepheids and/or reveal unknown binary companions of these stars. Such information is of great importance for a proper calibration of Period-Luminosity relations and for determining binary fraction among Cepheids. We observed the Cepheid X Sgr with VLTI/AMBER in order to confirm or disprove the presence of the hypothesized binary companion and to directly measure the mean stellar radius, possibly detecting its variation along the pulsation cycle. From AMBER observations in MR mode we performed a binary model fitting on the closure phase and a limb-darkened model fitting on the visibility. Our analysis indicates the presence of a point-like companion at a separation of 10.7 mas and 5.6 magK fainter than the primary, whose flux and position are sharply constrained by the data. The radius pulsation is not detected, whereas the average limb-darkened diameter results to be 1.48+/-0.08 mas, corresponding to 53+/-3 R_sun at a distance of 333.3 pc.Comment: 5 pages, 3 figures, research not

Galactic abundance gradients from Cepheids : On the iron abundance gradient around 10-12 kpc

Context: Classical Cepheids can be adopted to trace the chemical evolution of the Galactic disk since their distances can be estimated with very high accuracy. Aims: Homogeneous iron abundance measurements for 33 Galactic Cepheids located in the outer disk together with accurate distance determinations based on near-infrared photometry are adopted to constrain the Galactic iron gradient beyond 10 kpc. Methods: Iron abundances were determined using high resolution Cepheid spectra collected with three different observational instruments: ESPaDOnS@CFHT, Narval@TBL and [email protected] ESO/MPG telescope. Cepheid distances were estimated using near-infrared (J,H,K-band) period-luminosity relations and data from SAAO and the 2MASS catalog. Results: The least squares solution over the entire data set indicates that the iron gradient in the Galactic disk presents a slope of -0.052+/-0.003 dex/kpc in the 5-17 kpc range. However, the change of the iron abundance across the disk seems to be better described by a linear regime inside the solar circle and a flattening of the gradient toward the outer disk (beyond 10 kpc). In the latter region the iron gradient presents a shallower slope, i.e. -0.012+/-0.014 dex/kpc. In the outer disk (10-12 kpc) we also found that Cepheids present an increase in the spread in iron abundance. Current evidence indicates that the spread in metallicity depends on the Galactocentric longitude. Finally, current data do not support the hypothesis of a discontinuity in the iron gradient at Galactocentric distances of 10-12 kpc. Conclusions: The occurrence of a spread in iron abundance as a function of the Galactocentric longitude indicates that linear radial gradients should be cautiously treated to constrain the chemical evolution across the disk.Comment: 5 tables, 8 figures, Accepted in A&

Classical Cepheids as distance indicators and tracers of the disk abundance gradient

Le Cefeidi Classiche sono gli indicatori primari di distanza piu' utilizzati. Relazioni ben definite intercorrono tra il loro periodo e la loro luminosita', sia nella banda ottica che in quella del vicino infrarosso. Grazie a queste relazioni e' possibile determinare le loro distanze con grade accuratezza. Inoltre sono i migliori traccianti di stelle di massa intermedia. Infatti, poiche' sono distribuite lungo tutto il disco Galattico, possono essere adottate per tracciare i gradienti radiali di abbondanza di ferro e elementi pesanti. Negli ultimi anni sono stati fatti molti progressi verso la comprensione e caratterizzazione dell'influenza che la composizione chimica potrebbe avere sulla relazione Periodo-Luminosita' (PL) delle Cefeidi, ma tale problema rimane ancora irrisolto. Inoltre, nonostante l'esistenza di un gradiente di abbondanza Galattico sia oggi largamente accettata, il suo preciso valore e' ancora da definire. Con lo scopo di comprendere meglio questi punti fondamentali, abbiamo rivolto la nostra attenzione su due obiettivi principali: i) valutare l'influenza della metallicita' sulla PL nelle bande V e K. ii) dare una stima accurata del gradiente di abbondanza Galattico tra 5 e 17 kpc, utilizzando misure omogenee di composizione chimica per un campione che comprende 270 Cefeidi situate lungo il disco e delle quali abbiamo ottenuto una buona stima delle distanze. Per valutare l'effetto che la metallicita' ha sulla PL, abbiamo messo in relazione, per le bande V e K, i residui dalla PL standard di Freedman et al. (2001) e Persson et al. (2004), rispettivamente, all'abbondanza di ferro ([Fe/H]). A tale scopo, abbiamo usato misure dirette del ferro per 68 Cefeidi Galattiche e Magellaniche ricavate dall'analisi di spettri ad alta risoluzione e alto rapporto segnale rumore (S/N) ottenuti con strumenti e telescopi ESO ([email protected] e UVES@VLT). In questo studio, abbiamo trovato una metallicita' media circa solare (sigma = 0.10) per il campione Galattico (32 stelle), -0.33 dex (sigma = 0.13) per quello della grande Nube di Magellano (LMC, 22 stelle) e -0.75 dex (sigma = 0.08) per la Piccola Nube di Magellano (SMC, 14 stelle). Il numero delle nostre misure di abbondanza per le Cefeidi Magellaniche risulta essere piu' del doppio di quello totale studiato fino ad ora ad alta risoluzione. I nostri risultati mostrano che la metallicita' influisce sulla PL delle Cefeidi e questa constatazione non dipende dal modulo di distanza adottato per LMC. Risultati basati sulla distanza canonica di LMC (DM_LMC = 18.5) indicano un ben definito effetto nella banda visuale V e un effetto piu' lieve in quella nel vicino infrarosso K. In dettaglio, abbiamo visto che i due bin di metallicita', quello povero di metalli e quello ricco, nella banda visuale, si trovano rispettivamente a ~ 2 sigma e a ~ 9 sigma, rispetto all'ipotesi di indipendenza e i due bins hanno una differenza di 3 sigma. Nella banda infrarossa, il bin povero di metalli si trova entro 1 sigma consistente con zero mentre quello ricco di metalli differisce dall'ipotesi di indipendenza di ~ 4 sigma. Inoltre, i due bins differiscono tra loro ~ 2 sigma. Questi risultati suggeriscono che la PL delle Cefeidi non puo' essere considerata Universale. Per indagare il gradiente di abbondanza Galattico, abbiamo fornito due nuove calibrazioni, una empirica e una teorica, di due indici di metallicita' basati sulla fotometria di Walraven. I migliori risultati sono stati ottenuti con le relazioni teoriche tra la metallicita' e gli Indici di Colore (MIC). Queste sono basate su un insieme omogeneo di tracce evolutive solar-scaled per stelle di massa intermedia e su previsioni pulsazionali riguardanti la topologia della striscia di instabilita'. Lo scatter intrinseco delle relazioni e' ~ 0.1 dex. Usando le relazioni calibrate, abbiamo ottenuto metallicita' fotometriche per l'intero campione di Walraven (122 stelle). Di queste, 51 disponevano di misure accurate di abbondanza di ferro in letteratura, per le rimanenti 71 abbiamo fornito noi una nuova stima. Abbiamo, inoltre, aggiunto le 4 per cui avevamo stimato le distanze con il metodo del Baade-Wesselink. Per aumentare ulteriormente il numero dei traccianti, abbiamo incluso anche 116 Cefeidi dal set di dati di Andrievsky et al. (2002b), 63 da quello di Lemasle et al. (2007), 6 dal campione di Szila'di et al. (2007) e 10 da quello di Romaniello et al. (2008). In questo modo siamo riusciti ad ottenere un campione di 270 stelle. Il risultato ottenuto facendo un fit lineare dell'intero dataset, indica che il gradiente di abbondanza Galattico presenta una pendenza di -0.048 +- 0.004 dex/kpc, in ottimo accordo con le stime piu' recenti. In ogni modo, la variazione dell'abbondanza di ferro lungo il disco, sembra essere meglio descritta da una distribuzione bimodale con una pendenza maggiore verso il bulge e un appiattimento del gradiente in direzione del disco piu' esterno con una discontinuita' attorno Rg ~ 10 kpc. Dall'analisi dei nostri dati non sembra emergere nessuna discontinuita' ma abbiamo trovato che le Cefeidi presentano un aumento dello spread in contenuto di ferro e che questo spread possa dipendere dalla longitudine Galattocentrica. Il verificarsi di uno spread in metallicita' in funzione della longitudine Galactocentrica indica che il gradiente radiale dovrebbe essere utilizzato con cautela nel porre dei limiti per l'evoluzione chimica in tutto il disco. Infine, con questi risultati, abbiamo deciso di fare un fit esponenziale del gradiente ottenendo uno scarto quadratico medio di 0.129 dex.Classical Cepheids are the most popular primary distance indicator. They obey to well defined optical and near-infrared (NIR) Period-Luminosity (PL) relations and their distances can be estimated with an accuracy of a few percent. They also are, the best tracers of intermediate-mass stars. In particular, since they are distributed across the Galactic disk they can be adopted to trace the iron and heavy element radial gradients. In the past few years, significant progress has been made to understand and characterize the influence that chemical composition may have on the Cepheids Period-Luminosity relation but this issue is still unsettled. Although the existence of the Galactic abundance gradient is now widely accepted, its value still need to be established. To better understand these fundamental points, we focused our attenction on two main topics: i) the influence of the stellar iron content on the PL relation in the V and in the K band. ii) the behavior of the Galactic iron gradient between 5 and 17 kpc, using homogeneous iron abundances for 270 Galactic Cepheids located across the disk together with accurate distance determinations. To asses the effect of the metallicity on the PL relation, we have related the V and the K-band residuals from the standard PL relations of Freedman et al. (2001) and Persson et al. (2004), respectively, to iron abundance ([Fe/H]). We used iron measurements for 68 Galactic and Magellanic Cepheids based on high resolution and high signal-to-noise ratio (S/N) spectra collected with [email protected] and UVES@VLT (ESO, Telescopes). We have found a mean [Fe/H] ~ solar (sigma = 0.10) for our Galactic sample (32 stars), -0.33 dex (sigma = 0.13) for the Large Magellanic Cloud (LMC, 22 stars) and -0.75 dex (sigma = 0.08) for the Small Magellanic Cloud (SMC, 14 stars). Our abundance measurements of the Magellanic Cepheids double the number of stars studied up to now at high resolution. Our results show that the metallicity affects the Cepheid PL relation and this finding does not depend on the adopted LMC distance modulus. Results based on the canonical LMC distance (DM_LMC = 18.5) indicate a well defined effect in the V and a mild effect in the K-band PL relations. In particular, the metal-poor and the metal-rich bins are, in the visual band, at ~ 2 sigma and ~ 9 sigma, respectively, from the null hypothesis and the two bins differ at 3 sigma level. In the infrared band, the metal-poor bin is within 1 sigma consistent with zero while the metal-rich bin differs from the null hypothesis by ~ 4 sigma. Moreover, the magnitude residuals in the two metallicity bins differ by ~ 2 sigma. These findings support the evidence that the Cepheid PL relation is not Universal. To investigate the Galactic abundance gradient, we provided new calibrations of two photometric metallicity indices based on Walraven photometry. The best results have been obtained by the theoretical Metallicity-Index Color (MIC) relations. They rely on a homogeneous set of scaled-solar evolutionary tracks for intermediate-mass stars and on pulsation predictions concerning the topology of the instability strip. The intrinsic accuracy of the MIC relations are ~ 0.1 dex. Using the calibrated relations we have estimated photometric metallicities for the entire Walraven sample (122 Cepheids). Among them, 51 had accurate iron abundaces available in litterature and 71 are new estimates. We added four metal-rich Cepheids for which we collected high resolution spectra and for which we derived accurate distances based on the Baade-Wesselink technique. Moreover, to improve the number of tracers, we have also included 116 Cepheids from Andrievsky et al. (2002), 63 from Lemasle et al. (2007), 6 from Szila'di et al. (2007) and 10 from Romaniello et al. (2008) to end up with 270 Galactic Cepheids. Our result, over the entire data set, indicates that the iron gradient in the Galactic disk presents a slope of -0.048 +- 0.004 dex/kpc which is in very good agreement with the most recent literature values. However, the hypothesis of a linear gradient is still widely debated. Several investigations suggest a bimodal distribution with a steeper slope toward the bulge and a flattening of the gradient toward the outer disk with a discontinuity at Rg ~ 10 kpc. Our data do not support the discontinuity hypothesis but we have found that Cepheids present an increase in the spread in iron abundance and that it may depend on the Galactocentric longitude. The occurrence of this spread indicates that linear radial gradients should be cautiously treated to constrain the chemical evolution of the Galactic disk. Finally, with these results in hands, we decided to fit the Galactic abundance gradient with an exponential distribution, finding a good fit (rms=0.129 dex)

On the metallicity distribution of classical Cepheids in the Galactic inner disk

We present homogeneous and accurate iron abundances for almost four dozen (47) of Galactic Cepheids using high-spectral resolution (R$\sim$40,000) high signal-to-noise ratio (S/N $\ge$ 100) optical spectra collected with UVES at VLT. A significant fraction of the sample (32) is located in the inner disk (RG $le$ 6.9 kpc) and for half of them we provide new iron abundances. Current findings indicate a steady increase in iron abundance when approaching the innermost regions of the thin disk. The metallicity is super-solar and ranges from 0.2 dex for RG $\sim$ 6.5 kpc to 0.4 dex for RG $\sim$ 5.5 kpc. Moreover, we do not find evidence of correlation between iron abundance and distance from the Galactic plane. We collected similar data available in the literature and ended up with a sample of 420 Cepheids. Current data suggest that the mean metallicity and the metallicity dispersion in the four quadrants of the Galactic disk attain similar values. The first-second quadrants show a more extended metal-poor tail, while the third-fourth quadrants show a more extended metal-rich tail, but the bulk of the sample is at solar iron abundance. Finally, we found a significant difference between the iron abundance of Cepheids located close to the edge of the inner disk ([Fe/H]$\sim$0.4) and young stars located either along the Galactic bar or in the nuclear bulge ([Fe/H]$\sim$0). Thus suggesting that the above regions have had different chemical enrichment histories. The same outcome applies to the metallicity gradient of the Galactic bulge, since mounting empirical evidence indicates that the mean metallicity increases when moving from the outer to the inner bulge regions.Comment: 10 pages, 5 figures; Corrected typos, corrected Table

Ichthyosis Linearis Circumflexa as the Only Clinical Manifestation of Netherton Syndrome.

Ichthyosis linearis circumflexa (ILC) presents as serpiginous and migratory erythematous patches with double-edged scales. ILC is rarely an isolated skin manifestation, but most commonly a part of Netherton syndrome (NS). NS is caused by SPINK5 mutations, which lead to absent or sometimes reduced expression of the serine protease inhibitor LEKTI. NS is characterised by congenital ichthyosiform erytroderma, trichorrhexis invaginata (TI) and atopy. We report 2 children who presented since the first months of life cheek erythema followed by the appearance of sparse ILC lesions on the face, trunk and proximal extremities. Erythroderma at birth, TI and atopy were absent. LEKTI immunoreactivity was reduced in patient epidermis, and serine protease activity was modestly increased, while desmoglein-1 expression remained unaffected. SPINK5 mutation and expression analysis in patient keratinocytes revealed compound heterozygous splicing variants, which allowed residual LEKTI secretion. Our results show that ILC can be the only clinical manifestation of NS

New Baade-Wesselink distances and radii for four metal-rich Galactic Cepheids

We provided accurate estimates of distances, radii and iron abundances for four metal-rich Cepheids, namely V340 Ara, UZ Sct, AV Sgr and VY Sgr. The main aim of this investigation is to constrain their pulsation properties and their location across the Galactic inner disk. We adopted new accurate NIR (J,H,K) light curves and new radial velocity measurements for the target Cepheids to determinate their distances and radii using the Baade-Wesselink technique. In particular, we adopted the most recent calibration of the IR surface brightness relation and of the projection factor. Moreover, we also provided accurate measurements of the iron abundance of the target Cepheids. Current distance estimates agree within one sigma with similar distances based either on empirical or on theoretical NIR Period-Luminosity relations. However, the uncertainties of the Baade-Wesselink distances are on average a factor of 3-4 smaller when compared with errors affecting other distance determinations. Mean Baade-Wesselink radii also agree at one sigma level with Cepheid radii based either on empirical or on theoretical Period-Radius relations. Iron abundances are, within one sigma, similar to the iron contents provided by Andrievsky and collaborators, thus confirming the super metal-rich nature of the target Cepheids. We also found that the luminosity amplitudes of classical Cepheids, at odds with RR Lyrae stars, do not show a clear correlation with the metal-content. This circumstantial evidence appears to be the consequence of the Hertzsprung progression together with the dependence of the topology of the instability strip on metallicity, evolutionary effects and binaries.Comment: 9 pages, 7 figures, A&A accepte

CORS Baade-Wesselink method in the Walraven photometric system: the period-radius and the period-luminosity relation of classical Cepheids

We present a new derivation of the CORS Baade-Wesselink method in the Walraven photometric system. We solved the complete Baade-Wesselink equation by calibrating the surface brightness function with a recent grid of atmosphere models. The new approach was adopted to estimate the mean radii of a sample of Galactic Cepheids for which are available precise light curves in the Walraven bands. Current radii agree, within the errors, quite well with Cepheid radii based on recent optical and near-infrared interferometric measurements. We also tested the impact of the projection factor on the Period-Radius relation using two different values (p=1.36, p=1.27) that bracket the estimates available in the literature. We found that the agreement of our Period-Radius relation with similar empirical and theoretical Period-Radius relations in the recent literature, improves by changing the projection factor from p=1.36 to p=1.27. Our Period-Radius relation is log(R)=(0.75\pm 0.03)log(P)+(1.10 \pm 0.03), with a rms=0.03 dex. Thanks to accurate estimates of the effective temperature of the selected Cepheids, we also derived the Period-Luminosity relation in the V band and we found Mv=(-2.78 \pm 0.11)log(P)+(-1.42 \pm 0.11) with rms=0.13 mag, for p=1.27. It agrees quite well with recent results in the literature, while the relation for p=1.36 deviates by more than 2sigma. We conclude that, even taking into account the intrinsic dispersion of the obtained Period-Luminosity relations, that is roughly of the same order of magnitude as the effect of the projection factor, the results of this paper seem to favour the value p = 1.27.Comment: 17 pages, 14 figures, accepted for publication in MNRA

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

We present homogeneous and accurate iron abundances for 42 Galactic Cepheids based on high-spectral resolution (R~38,000) high signal-to-noise ratio (SNR>100) optical spectra collected with UVES at VLT (128 spectra). The above abundances were complemented with high-quality iron abundances provided either by our group (86) or available in the literature. We paid attention in deriving a common metallicity scale and ended up with a sample of 450 Cepheids. We also estimated for the entire sample accurate individual distances by using homogeneous near-infrared photometry and the reddening free Period-Wesenheit relations. The new metallicity gradient is linear over a broad range of Galactocentric distances (Rg~5-19 kpc) and agrees quite well with similar estimates available in the literature (-0.060+/-0.002 dex/kpc). We also uncover evidence which suggests that the residuals of the metallicity gradient are tightly correlated with candidate Cepheid Groups (CGs). The candidate CGs have been identified as spatial overdensities of Cepheids located across the thin disk. They account for a significant fraction of the residual fluctuations, and in turn for the large intrinsic dispersion of the metallicity gradient. We performed a detailed comparison with metallicity gradients based on different tracers: OB stars and open clusters. We found very similar metallicity gradients for ages younger than 3 Gyrs, while for older ages we found a shallower slope and an increase in the intrinsic spread. The above findings rely on homogeneous age, metallicity and distance scales. Finally we found, by using a large sample of Galactic and Magellanic Cepheids for which are available accurate iron abundances, that the dependence of the luminosity amplitude on metallicity is vanishing.Comment: Accepted in A&A, 11 figures, 7 table

On the metallicity gradient of the Galactic disk

Aims: The iron abundance gradient in the Galactic stellar disk provides fundamental constraints on the chemical evolution of this important Galaxy component. However the spread around the mean slope is, at fixed Galactocentric distance, larger than estimated uncertainties. Methods: To provide quantitative constraints on these trends we adopted iron abundances for 265 classical Cepheids (more than 50% of the currently known sample) based either on high-resolution spectra or on photometric metallicity indices. Homogeneous distances were estimated using near-infrared Period-Luminosity relations. The sample covers the four disk quadrants and their Galactocentric distances range from ~5 to ~17 kpc. Results: A linear regression over the entire sample provides an iron gradient of -0.051+/-0.004 dex/kpc. The above slope agrees quite well, within the errors, with previous estimates based either on Cepheids or on open clusters covering similar Galactocentric distances. However, once we split the sample in inner (Rg < 8 kpc) and outer disk Cepheids we found that the slope (-0.130+/-0.015 dex/kpc) in the former region is ~3 times steeper than the slope in the latter one (-0.042+/-0.004 dex/kpc). We found that in the outer disk the radial distribution of metal-poor (MP, [Fe/H]<-0.02 dex) and metal-rich (MR) Cepheids across the four disk quadrants does not show a clear trend when moving from the innermost to the external disk regions. We also found that the relative fractions of MP and MR Cepheids in the 1st and in the 3rd quadrant differ at 8 sigma (MP) and 15 sigma (MR) level.Comment: 6 pages, 6 figures, A&A accepte