arXiv subjects

M. Monelli

Publications and source records attributed to M. Monelli.

At least 145 records · Page 8Linked to original sources

arXiv2009

The Young Stellar Population of IC1613. I. A New Catalogue of OB Associations

Context: Determining the parameters of massive stars is crucial to understand many processes in galaxies and the Universe, since these objects are important sources of ionization, chemical enrichment and momentum. 10m class telescopes enable us to perform detailed quantitative spectroscopic analyses of massive stars in other galaxies, sampling areas of different metallicity. Relating the stars to their environment is crucial to understand the physical processes ruling their formation and evolution. Aims: In preparation for the GTC, our goal is to build a catalogue of massive star candidates in the metal-poor irregular galaxy IC1613 with high astrometric accuracy, apt for the current generation of multi-object spectrographs. A census of OB associations in this galaxy is also needed, to provide important additional information about age and environment of the candidate OB stars. Methods: From INT-WFC observations, we have built an astrometric and photometric catalogue of stars in IC1613. Candidate blue massive stars are preselected from their colors. A friends-of-friends algorithm is developed to find their clustering in the galaxy. While a common physical origin for all the members of the associations cannot be ensured, this is a necessary first step to place candidate OB stars in a population context. Results: We have produced a deep catalogue of targets in IC1613 that covers a large field of view. To achieve high astrometric accuracy a new astrometric procedure is developed for the INT-WFC data. We have also built a catalogue of OB associations in IC1613. We have found that they concentrate in the central regions, specially in the HII bubbles. The study of extinction confirms that it is patchy, with local values of color-excess above the foreground value.

astro-ph.SR↗

arXiv2009

Young stellar populations and star clusters in NGC 1705

(Abridged). We present HST photometry of the late-type dwarf galaxy NGC 1705 observed with the WFPC2 in the U and B bands, and with the ACS/HRC in the U, V, and I bands. We cross-correlate these data with previous ones acquired with the WFPC2 in V and I and derive multiband Color-Magnitude diagrams (CMDs) of the cross-identified individual stars and candidate star clusters. For the central regions of the galaxy, where HST-NICMOS J and H photometry is also available, we present U, B, V, I, J, H CMDs of the 256 objects with magnitudes measured in all bands. While our previous study based on V, I, J and H data allowed us to trace the star formation history of NGC 1705 back to a Hubble time, the new data provide a better insight on its recent evolution. With the method of the synthetic CMDs, we confirm the presence of two strong bursts of star formation (SF). The older of the two bursts (B1) occurred between ~ 10 and 15 Myr ago, while the younger burst (B2) started ~ 3 Myr ago, and it is still active. The stellar mass produced by B2 amounts to ~ 10^6 msun, and it is a factor of ~ 3 lower for B1. The interburst phase was likely characterized by a much lower level of SF rather than by its complete cessation. The two bursts show distinct spatial distributions: while B1 is centrally concentrated, B2 is more diffused, and presents ring and arc-like structures that remind of an expanding shell. This suggests a feedback mechanism, in which the expanding superbubble observed in NGC 1705, likely generated by the (10-15) Myr burst, triggered the current strong SF activity. From the HRC data, we identified 12 star clusters (plus the SSC) in the central region of NGC 1705, 10 of which have photometry in all the UBVIJH bands. Using the GALEV models, we derived ages from ~ 10 Myr to ~ 1 Gyr, and masses between ~ 10^4 and 10^5 msun for the clusters.

astro-ph.CO↗

arXiv2009

On the radial distribution of horizontal branch stars in ngc2808

We present accurate new ultraviolet and optical BVI photometry for the Galactic globular cluster ngc2808, based on both ground-based and archival HST imagery. From this we have selected a sample of ~2,000 HB stars; given the extensive wavelength range considered and the combination of both high-angular-resolution and wide-field photometric coverage, our sample should be minimally biased. We divide the HB stars into three radial bins and find that the relative fractions of cool, hot and extreme HB stars do not change radically when moving from the center to the outskirts of the cluster: the difference is typically smaller than ~2sigma. These results argue against the presence of strong radial differentiation among any stellar subpopulations having distinctly different helium abundances. The ratio between HB and RG stars brighter than the ZAHB steadly increases when moving from the innermost to the outermost cluster regions. The difference is larger than ~4sigma and indicates a deficiency of bright RGs in the outskirts of the cluster.

astro-ph.GA↗

arXiv2008

On the distance and reddening of the starburst galaxy IC10

We present deep and accurate optical photometry of the Local Group starburst galaxy IC10. The photometry is based on two sets of images collected with the Advanced Camera for Surveys and with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope. We provide new estimates of the Red Giant Branch tip (TRGB) magnitude, m_{F814W}^{TRGB}=21.90+-0.03, and of the reddening, E(B-V)=0.78+-0.06, using field stars in the Small Magellanic Cloud (SMC) as a reference. Adopting the SMC and two globulars, Omega Centauri and 47 Tucanae, as references we estimate the distance modulus to IC10: independent calibrations give weighted average distances of mu=24.51+-0.08 (TRGB) and mu=24.56+-0.08 (RR Lyrae). We also provide a new theoretical calibration for the TRGB luminosity, and using these predictions we find a very similar distance to IC10 (mu~24.60+-0.15). These results suggest that IC10 is a likely member of the M31 subgroup.

astro-ph↗

arXiv2008

Relative distances of Omega Centauri and 47 Tucanae

We present precise optical and near-infrared ground-based photometry of two Globular Clusters (GCs): Omega Cen and 47 Tuc. These photometric catalogs are unbiased in the Red Giant Branch (RGB) region close to the tip. We provide new estimates of the RGB tip (TRGB) magnitudes--m_I(TRGB)=9.84+/-0.05, Omega Cen; m_I(TRGB)=9.46+/-0.06, 47 Tuc--and use these to determine the relative distances of the two GCs. We find that distance ratios based on different calibrations of the TRGB, the RR Lyrae stars and kinematic distances agree with each other within one sigma. Absolute TRGB and RR Lyrae distance moduli agree within 0.10--0.15 mag, while absolute kinematic distance moduli are 0.2--0.3 mag smaller. Absolute distances to 47 Tuc based on the Zero-Age-Horizontal-Branch and on the white dwarf fitting agree within 0.1 mag, but they are 0.1--0.3 mag smaller than TRGB and RR Lyrae distances.

astro-ph↗

arXiv2008

Revised bolometric corrections and interstellar extinction coefficients for the ACS and WFPC2 photometric systems

We present extensive tables of bolometric corrections and interstellar extinction coefficients for the WFPC2 and ACS (both WFC and HRC) photometric systems. They are derived from synthetic photometry applied to a database of spectral energy distributions covering a large range of effective temperatures, surface gravity, and metal content. Carbon stars are also considered. The zero-points take into consideration the new high-accuracy Vega fluxes from Bohlin. These tables are employed to transform Padova isochrones into WFPC2 and ACS photometric systems using interstellar extinction coefficients on a star-to-star basis. All data are available either in tabular form or via an interactive web interface in the case of the isochrones. Preliminary tables for the WFC3 camera are also included in the database.

astro-ph↗

arXiv2008

MAD@VLT: Deep into the Madding Crowd of Omega Centauri

We present deep and accurate Near-Infrared (NIR) photometry of the Galactic Globular Cluster (GC) Omega Cen. Data were collected using the Multi-Conjugate Adaptive Optics Demonstrator (MAD) on VLT (ESO). The unprecedented quality of the images provided the opportunity to perform accurate photometry in the central crowded regions. Preliminary results indicate that the spread in age among the different stellar populations in Omega Cen is limited.

astro-ph↗

arXiv2007

On the radial distribution of white dwarfs in the Galactic globular cluster Omega Cen

We present deep and accurate photometry (F435W, F625W, F658N) of the Galactic Globular Cluster Omega Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We identified ~ 6,500 white dwarf (WD)candidates and compared their radial distribution with that of Main Sequence (MS) stars. We found a mild evidence that young WDs (0.1 < t < 0.6 Gyr) are less centrally concentrated when compared to MS stars in the magnitude range 25 < F435W < 26.5.

astro-ph↗

arXiv2007

On the white dwarf cooling sequence of the globular cluster Omega Centauri

We present deep and precise photometry (F435, F625W, F658N) of Omega Cen collected with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). We have identified ~ 6,500 white dwarf (WD) candidates, and the ratio of WD and Main Sequence (MS) star counts is found to be at least a factor of two larger than the ratio of CO-core WD cooling and MS lifetimes. This discrepancy is not explained by the possible occurrence of a He-enhanced stellar population, since the MS lifetime changes by only 15% when changing from a canonical (Y=0.25) to a He-enhanced composition (Y=0.42). The presence of some He-core WDs seems able to explain the observed star counts. The fraction of He WDs required ranges from 10% to 80% depending on their mean mass and it is at least five times larger than for field WDs. The comparison in the Color Magnitude Diagram between theory and observations also supports the presence of He WDs. Empirical evidence indicates that He WDs have been detected in stellar systems hosting a large sample of extreme horizontal branch stars, thus suggesting that a fraction of red giants might avoid the He-core flash.

astro-ph↗

arXiv2007

The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment

UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as results of the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population recently postulated to exist in some clusters. Moderately high resolution spectra of stars at the hot end of the blue HB in omega Cen were analysed for atmospheric parameters and abundances using LTE and Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14% are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step with helium enrichment, with a maximum carbon enrichment of 3% by mass. At least 14% of the hottest HB stars in omega Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y = 0.42 corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich stars show strong carbon enrichment as predicted by the late hot flasher scenario. We conclude that the helium-rich HB stars in omega Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence. (Abridged)

astro-ph↗

arXiv2007

The ACS LCID Project: data reduction strategy

During Cycle 14 a total of 113 HST orbits were secured to observe five isolated dwarf galaxies, namely Tucana, LGS3, LeoA, IC1613, and Cetus. The aim of the project is a full characterization of the stellar content of these galaxies, in term of their SFH, radial distributions, halo populations and variable stars. Deep (V~29) F475W, F814W data allowed us to fully sample all the evolutionary phases from the tip of the Red Giant Branch (RGB) to well below the old Main Sequence Turnoff (MSTO). Here we describe the observational design, and the reduction and calibration strategy adopted. A comparison of the results obtained using two different packages, ALLFRAME and Dolphot, is presented.

astro-ph↗

arXiv2007

Star Counts in the Globular Cluster Omega Centauri. I. Bright Stellar Components

We present an extensive photometry on HB, RGB, and MSTO stars in Omega Cen. The central regions of the cluster were covered with a mosaic of F435W, F625W, and F658N-band data collected with ACS/HST. The outer reaches were covered with a large set of U,B,V,I-band data collected with the WFI@2.2m ESO/MPI telescope. The final catalogue includes ~1.7 million stars. We identified ~3,200 likely HB stars and ~12,500 stars brighter than the subgiant branch and fainter than the RGB bumps. The HB morphology changes with the radial distance. The relative number of extreme HB stars decreases from ~30% to ~21% when moving from the center toward the outer regions of the cluster, while the fraction of less hot HB stars increases from ~62% to ~72%. We performed a detailed comparison between observed ratios of different stellar tracers and predictions based on canonical evolutionary models with a primordial helium (Y=0.23) content and metal abundances (Z=0.0002,0.001) that bracket the observed spread in metallicity of Omega Cen stars. We found that the empirical star counts of HB stars are on average larger (30%-40%) than predicted. Moreover, the rate of HB stars is 43% larger than the MSTO rate. The discrepancy between the rate of HB compared with the rate of RG and MSTO stars supports the evidence that we are facing a true excess of HB stars. The same comparison was performed by assuming a mix of stellar populations made with 70% of canonical stars and 30% of He-enhanced stars. The discrepancy between theory and observations decreases by a factor of two when compared with rates predicted by canonical He content models, but still 15%-25% (Y=0.42) and 15%-20% (Y=0.33) higher than observed. Furthermore, the ratio between HB and MSTO star counts are ~24% (Y=0.42) and 30% (Y=0.33) larger than predicted lifetime ratios.

astro-ph↗

arXiv2007

The Stellar Structures around Disk Galaxies

We present a brief summary of our current results on the stellar distribution and population gradients of the resolved stars in the surroundings of ~50 nearby disk galaxies, observed with space- (Hubble & Spitzer) and ground-based telescopes (Subaru, VLT, BTA, Palomar, CFHT & INT). We examine the radial (in-plane) and vertical (extraplanar) distributions of resolved stars as a function of stellar age and metallicity by tracking changes in the color-magnitude diagram of face-on and edge-on galaxies. Our data show, that the scale length and height of a stellar population increases with age, with the oldest detected stellar populations identified at a large galactocentric radius or extraplanar height, out to typically a few kpc. In the most massive of the studied galaxies there is evidence for a break in number density and color gradients of evolved stars, which plausibly correspond to the thick disk and halo components of the galaxies. The ratio of intermediate-age to old stars in the outermost fields correlate with the gas fraction, while relative sizes of the thick-to-thin disks anticorrelate with galactic circular velocity.

astro-ph↗

arXiv2007

Short-Period Variables in the Local Group Dwarf Galaxies Tucana and LGS3

We present preliminary results concerning the search for short-period variable stars in Tucana and LGS3 based on very deep HST/ACS imaging. In the fraction of the observed field we studied in each galaxy, a total of 133 and 30 variables were found, respectively. For Tucana, we identified 76 of them as RR Lyrae (RRL) stars pulsating in the fundamental mode (RRab) and 32 in the first-overtone mode (RRc), as well as 2 anomalous Cepheids (AC). The mean period of the RRab and RRc is 0.59 and 0.35 days, respectively. In the case of LGS3, we found 24 RRab and 4 RRc, with mean periods 0.61and 0.39 days, respectively, plus two candidate ACs. These values place both galaxies in the Oosterhoff gap.

astro-ph↗

arXiv2006

A pulsational distance to Omega Centauri based on Near-Infrared Period-Luminosity relations of RR Lyrae stars

We present new Near-Infrared (J,K) magnitudes for 114 RR Lyrae stars in the globular cluster Omega Cen (NGC 5139) which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is presently the largest such sample in any stellar system. We also present updated predictions for J,K-band Period-Luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z=0.0001 to Z=0.004. By adopting for the Omega Cen variables with measured metal abundances an alpha-element enhancement of a factor of 3 (about 0.5 dex) with respect to iron we find a true distance modulus of 13.70 (with a random error of 0.06 and a systematic error of 0.06), corresponding to a distance d=5.5 Kpc (with both random and systematic errors equal to 0.03 Kpc). Our estimate is in excellent agreement with the distance inferred for the eclipsing binary OGLEGC-17, but differ significantly from the recent distance estimates based on cluster dynamics and on high amplitude Delta Scuti stars.

astro-ph↗

arXiv2006

The RR Lyrae distance scale from near-infrared photometry: current results

We present new observational results on the RR Lyrae K-band Period-Luminosity relation (PLK). Data on the Galactic globular clusters NGC 3201 and NGC 4590 (M68), and on the Large Magellanic Cloud cluster Reticulum are shown. We compare the observed slopes of the PLK relations for these three clusters with those predicted by pulsational and evolutionary models, finding a fair agreement. Trusting on this finding we decided to adopt these theoretical calibrations to estimate the distance to the target clusters,finding a good agreement with optical-based RR Lyrae distances, but with a smaller formal scatter.

astro-ph↗

arXiv2005

Reddening distribution across the center of the globular cluster Omega Centauri

We present new medium-band uvby Stromgren and broad-band VI photometry for the central regions of the globular cluster Omega Cen. From this photometry we have obtained differential reddening estimates relative to two other globular clusters (M13 and NGC288) using a metallicity-independent, reddening-free temperature index, [c]=(u-v)-(v-b) - 0.2(b-y), for hot horizontal-branch (HB) stars (T_e> 8,500K). We estimate color excesses of these hot HB stars using optical and near-infrared colors, and find clumpy extinction variations of almost a factor of two within the area of the cluster core. In particular, the greatest density of more highly reddened objects appears to be shifted along the right ascension axis when compared with less reddened ones. These findings complicate photometric efforts to investigate the star formation history of Omega Cen.

astro-ph↗

arXiv2005

The Carina dSph galaxy: where is the edge?

Recent cosmological N-body simulations suggest that current empirical estimates of tidal radii in dSphs might be underestimated by at least one order of magnitude. To constrain the plausibility of this theoretical framework, we undertook a multiband (U,B,V,I) survey of the Carina dSph. Deep B,V data of several fields located at radial distances from the Carina center ranging from 0.5 to 4.5 degrees show a sizable sample of faint blue objects with the same magnitudes and colors of old, Turn-Off stars detected across the center. We found that the (U-V,B-I) color-color plane is a robust diagnostic to split stars from background galaxies. Unfortunately, current U,I-band data are too shallow to firmly constrain the real extent of Carina.

astro-ph↗