arXiv subjects

Alessio Mucciarelli

Publications and source records attributed to Alessio Mucciarelli.

60 records · Page 4Linked to original sources

arXiv2012

High resolution reddening map in the direction of the stellar system Terzan 5

We have used optical images acquired with the Hubble Space Telescope to construct the first high-resolution extinction map in the direction of Terzan 5, a peculiar stellar system in the inner bulge of our Galaxy. The map has a spatial resolution of 8" X 8", over a total FoV of 200" X 200". The absorption clouds show a patchy structure on a typical scale of 20" and extinction variations as large as delta E(B-V) = 0.67 mag, especially in the direction of the center of the system. These correspond to an absolute color excess ranging from E(B-V)=2.15 mag, up to 2.82 mag. After the correction for differential reddening, two distinct red giant branches become clearly visible in the color magnitude diagram of Terzan 5 and they well correspond to the two sub-populations with different iron abundances recently discovered in this system.

astro-ph.GA↗

arXiv2011

Microturbulent velocity from stellar spectra: a comparison between different approaches (Research Note)

Context --- The classical method to infer microturbulent velocity in stellar spectra requires that the abundances of the iron lines are not correlated with the observed equivalent widths. An alternative method, requiring the use of the expected line strength, is often used to by-pass the risk of spurious slopes due to the correlation between the errors in abundance and equivalent width. Aims --- To compare the two methods and identify pros and cons and applicability to the typical practical cases. Methods --- I performed a test with a grid of synthetic spectra, including instrumental broadening and Poissonian noise. For all these spectra, microturbulent velocity has been derived by using the two approaches and compared with the original value with which the synthetic spectra have been generated. Results --- The two methods provide similar results for spectra with SNR$ > 70, while for lower SNR both approaches underestimate the true microturbulent velocity, depending of the SNR and the possible selection of the lines based on the equivalent width errors. Basically, the values inferred by using the observed equivalent widths better agree with those of the synthetic spectra. In fact, the method based on the expected line strength is not totally free from a bias that can heavily affect the determination of microturbulent velocity. Finally, I recommend to use the classical approach (based on the observed equivalent widths) to infer this parameter. In cases of full spectroscopical determination of all the atmospherical parameters, the difference between the two approaches is reduced, leading to an absolute difference in the derived iron abundances of less than 0.1 dex.

astro-ph.SR↗

arXiv2008

Near-Infrared Photometry of Four Stellar Clusters in the Small Magellanic Cloud

We present high-quality J, H and K photometry of four Small Magellanic Cloud stellar clusters with intermediate ages in the 1-7 Gyr range (namely NGC 339, 361, 416 and 419) . We obtained deep Color-Magnitude Diagrams to study the evolved sequences and providing a detailed census of the Red Giant Branch (RGB), Asymptotic Giant Branch (AGB) and Carbon star populations in each cluster and their contribution to the total cluster light. We find that in the 5-7 Gyr old clusters AGB stars account for ~6 % of the total light in K-band, Carbon stars are lacking and RGB stars account for ~45 % of the total bolometric luminosity. These empirical findings are in good agreement with the theoretical predictions. Finally, we derived photometric metallicities computed by using the properties of the RGB and finding an iron content of [Fe/H]=-1.18, -1.08, -0.99 and -0.96 dex for NGC 339, 361, 416 and 419 respectively.

astro-ph↗

arXiv2007

The globular cluster NGC 1978 in the Large Magellanic Cloud

We have used deep high-resolution Hubble Space Telescope ACS observations to image the cluster NGC 1978 in the Large Magellanic Cloud. This high-quality photometric data set allowed us to confirm the high ellipticity (~0.30+-0.02) of this stellar system. The derived Color Magnitude Diagram allowed a detailed study of the main evolutionary sequences: in particular we have detected the so called Bump along the Red Giant Branch (at V_{555}=19.10+-0.10). This is the first detection of this feature in an intermediate-age cluster. Moreover the morphology of the evolutionary sequence and their population ratios have been compared with the expectations of different theoretical models (namely BaSTI, PEL and Padua) in order to quantify the effect of convective overshooting. The best agreement (both in terms of morphology and star counts) has been found the PEL (Pisa Evolutionary Library) isochrone with Z=0.008 (consistenly with the most recent determination of the cluster metallicity, [M/H]=-0.37 dex) and a mild overshooting efficiency (Lambda_{os}=0.1). By adopting this theoretical set an age of 1.9+-0.1 Gyr has been obtained.

astro-ph↗

arXiv2006

Infrared photometry of Cepheids in the LMC clusters NGC 1866 and NGC 2031

Near infrared (IR) studies of Cepheid variables in the LMC take advantage of the reduced light curve amplitude and metallicity dependence at these wavelengths. This work presents such photometry for two young clusters known to contain sizeable Cepheid populations: NGC 1866 and NGC 2031. Our goal is to determine light curves and period-luminosity (PL) relations in the near-IR, to assess the similarity between cluster and field pulsators, and to examine the predictive capability of current pulsation models. The light curves are obtained from multiwavelength broadband J,H,Ks photometry of Cepheids in both clusters, with periods previously established from optical photometry. Mean magnitudes for the Cepheids are used to construct PL relations in the near-IR. The properties in the PL planes are compared with the behavior of field Cepheids in the LMC and with the predictions of recent pulsational models, both canonical and overluminous. Cluster and field Cepheids are homogeneous and the inclusion of the cluster Cepheids in the field sample extends nicely the PL relation. The slope of the PL relation is constant over the whole period range and does not show -- at least in the adopted IR bands -- the break in slope at P ~ 10 d reported by some authors. A comparison with the predictions of pulsation models allows an estimate for the distance moduli of NGC 1866 and NGC 2031. The two clusters are found to lie at essentially the same distance. Fitting of theoretical models to the data gives, for the K filter, (m-M)_0 = 18.62+-0.10 if canonical models are used and (m-M)_0 = 18.42+-0.10 if overluminous models are used. On the basis of this result, some considerations on the relationship between the clusters and the internal structure of the LMC are presented.

astro-ph↗

arXiv2006

On the Iron content of NGC 1978 in the LMC: a metal rich, chemically homogeneous cluster

We present a detailed abundance analysis of giant stars in NGC 1978, a massive, intermediate-age stellar cluster in the Large Magellanic Cloud, characterized by a high ellipticity and suspected to have a metallicity spread. We analyzed 11 giants, all cluster members, by using high resolution spectra acquired with the UVES/FLAMES spectrograph at the ESO-Very Large Telescope. We find an iron content of [Fe/H]=-0.38 dex with very low (0.07 dex) dispersion, and a mean heliocentric radial velocity Vr=293.1 (with an error of 0.9 km/s) and a velocity dispersion (3.1 km/s), thus excluding the presence of a significant metallicity, as well as velocity, spread within the cluster.

astro-ph↗