G. De Marchi

The Photometric Performance and Calibration of the Hubble Space Telescope Advanced Camera for Surveys

ABSTRACT We present the photometric calibration of the Advanced Camera for Surveys (ACS). The ACS was installed in the Hubble Space Telescope (HST) in 2002 March. It comprises three cameras: the Wide Field Channel (WFC), optimized for deep near‐IR survey imaging programs; the High Resolution Channel (HRC), a high‐resolution imager that fully samples the HST point‐spread function (PSF) in the visible; and the Solar Blind Channel (SBC), a far‐UV imager. A significant amount of data has been collected to characterize the on‐orbit performance of the three channels. We give here an overview of the performance and calibration of the two CCD cameras (WFC and HRC) and a description of the best techniques for reducing ACS CCD data. The overall performance is as expected from prelaunch testing of the camera. Surprises were a better‐than‐predicted sensitivity in the visible and near‐IR for both the WFC and HRC and an unpredicted dip in the HRC UV response at ∼3200 A. On‐orbit observations of spectrophotometric stand...

Accurate determination of accretion and photospheric parameters in young stellar objects: The case of two candidate old disks in the Orion Nebula Cluster

Context. Current planet formation models are largely based on the observational constraint that protoplanetary disks have a lifetime of ~3 Myr. Recent studies, however, report the existence of pre-main-sequence stars with signatures of accretion (strictly connected with the presence of circumstellar disks) and photometrically determined ages of 30 Myr or more. Aims. Here, we present a spectroscopic study of two major age outliers in the Orion Nebula Cluster. We use broadband, intermediate resolution VLT/X-shooter spectra combined with an accurate method to determine the stellar parameters and the related age of the targets to confirm their peculiar age estimates and the presence of ongoing accretion. Methods. The analysis is based on a multicomponent fitting technique, which derives simultaneously spectral type, extinction, and accretion properties of the objects. With this method, we confirm and quantify the ongoing accretion. From the photospheric parameters of the stars, we derive their position on the H-R diagram and the age given by evolutionary models. With other age indicators like the lithium-equivalent width, we estimate the age of the objects with high accuracy. Results. Our study shows that the two objects analyzed are not older than the typical population of the Orion Nebula Cluster. While photometric determination of the photospheric parameters are an accurate method to estimate the parameters of the bulk of young stellar populations, our results show that those of individual objects with high accretion rates and extinction may be affected by large uncertainties. Broadband spectroscopic determinations should thus be used to confirm the nature of individual objects. Conclusions. The analysis carried out shows that this method allows us to obtain an accurate determination of the photospheric parameters of accreting young stellar objects in any nearby star-forming region. We suggest that our detailed, broadband spectroscopy method should be used to derive accurate properties of candidate old and accreting young stellar objects in star-forming regions. We also discuss how a similarly accurate determination of stellar properties can be obtained through a combination of photometric and spectroscopic data.

The Dynamical State of the Globular Cluster M10 (NGC 6254)

Studying the radial variation of the stellar mass function (MF) in globular clusters (GCs) has proved a valuable tool to explore the collisional dynamics leading to mass segregation and core collapse. Recently, Pasquato et al. (2009) used the mass segregation profile to investigate the presence of an intermediate-mass black hole (IMBH) in NGC?2298. As a relaxed cluster with a large core, M10 (NGC?6254) is suitable for a similar investigation. In order to study the radial dependence of the luminosity and MF of M10, we used deep high resolution archival images obtained with the Advanced Camera for Survey (ACS) on board the Hubble Space Telescope, reaching out to approximately the clusters half-mass radius (r hm), combined with deep Wide Field and Planetary Camera 2 (WFPC2) images that extend our radial coverage to more than 2 r hm. From our photometry, we derived a radial mass segregation profile and a global MF that we compared with those of simulated clusters containing different energy sources (namely hard binaries and/or an IMBH) able to halt core collapse and to quench mass segregation. A set of direct N-body simulations of GCs, with and without an IMBH of mass 1% of the total cluster mass, comprising different initial mass functions and primordial binary fractions, was used to predict the observed mass segregation profile and MF. The mass segregation profile of M10 is not compatible with cluster models without either an IMBH or primordial binaries, as a source of energy appears to be moderately quenching mass segregation in the cluster. Unfortunately, the present observational uncertainty on the binary fraction in M10 does not allow us to confirm the presence of an IMBH in the cluster, since an IMBH, a dynamically non-negligible binary fraction (~5%), or both can equally well explain the radial dependence of the cluster MF.

HUBBLE TARANTULA TREASURY PROJECT: UNRAVELING TARANTULA'S WEB. I. OBSERVATIONAL OVERVIEW AND FIRST RESULTS*

The Hubble Tarantula Treasury Project (HTTP) is an ongoing panchromatic imaging survey of stellar populations in the Tarantula Nebula in the Large Magellanic Cloud that reaches into the sub-solar mass regime (<0.5 M ☉). HTTP utilizes the capability of the Hubble Space Telescope to operate the Advanced Camera for Surveys and the Wide Field Camera 3 in parallel to study this remarkable region in the near-ultraviolet, optical, and near-infrared spectral regions, including narrow-band Hα images. The combination of all these bands provides a unique multi-band view. The resulting maps of the stellar content of the Tarantula Nebula within its main body provide the basis for investigations of star formation in an environment resembling the extreme conditions found in starburst galaxies and in the early universe. Access to detailed properties of individual stars allows us to begin to reconstruct the temporal and spatial evolution of the stellar skeleton of the Tarantula Nebula over space and time on a sub-parsec scale. In this first paper we describe the observing strategy, the photometric techniques, and the upcoming data products from this survey and present preliminary results obtained from the analysis of the initial set of near-infrared observations.

The population of massive stars in R136 from faint object camera ultraviolet observations

New ultraviolet (λ≃1300 A, λ≃3400 A), HST Faint Object Camera observations have been used to derive the UV color-magnitude diagram (CMD) of R136. The main scientific goal is the study of the upper end of the stellar mass function at ultraviolet wavelengths where the color degeneracy encountered in visual CMDs is less severe. The CMD has been compared to a set of theoretical isochrones, which have been computed using the latest generation of evolutionary models and model atmospheres for early-type stars. Wolf-Rayet stars are included, and their emergent fluxes are calculated with the theoretical continuum energy distributions of Schmutz et al. Comparison of the theoretical and observed CMD suggests that there are no stars brighter than M 130 ≃-11

NGC 2298 : a globular cluster on its way to disruption

We have studied the stellar main sequence (MS) of the globular cluster NGC 2298 using deep HST/ACS observations in the F606W and F814W bands covering an area of 3. 4× 3. 4 around the cluster centre or about twice the cluster’s half -mass radius. The colourmagnitude diagram that we derive in this way reveals a narrow and well defined MS extending down to the 10σ detection limit at m606≃ 26.5, m814≃ 25, corresponding to stars of∼ 0.2 M⊙. The luminosity function (LF) obtained with these data, once corrected for the limited effects of photometric incompleteness, reveals a remarkable deficiency of low-mass stars as well as a radial gradient, in th at the LF becomes progressively steeper with radius. Using the mass‐luminosity relation appropriate for the metallicity of NGC 2298, we derive the cluster’s global mass function (GMF) by using a multi-mass Michie‐King model. Over the range 0.8− 0.2 M⊙, the number of stars per unit mass decreases following a power-law distribution of the type dN/dm∝ m 0.5 , where, for comparison, typical halo clusters have dN/dm∝ m −1.5 . If the IMF of NGC 2298 was similar to that of other metal poor halo clusters, like e.g. NGC 6397, the present GMF that we obtain implies that this object must have lost of order 85 % of its original mass, at a rate much higher than that suggested by current models based on the available cluster orbit. The latter may, therefore, need revision.

Why is the mass function of NGC 6218 flat

We have used the FORS-1 camera on the VLT to study the main sequence (MS) of the globular cluster NGC 6218 in the V and R bands. The observations cover an area of 3. 4 × 3. 4 around the cluster centre and probe the stellar population out to the cluster’s half-mass radius (rh ≃ 2. 2). The colour-magnitude diagram (CMD) that we derive in this way reveals a narrow and well defined MS extending down to the 5� detection limit at V ≃ 25, or about 6 magnitudes below the turn-off, corresponding to stars of ∼ 0.25M⊙. The luminosity function (LF) obtained with these data shows a marked radial gradient, in that the ratio of lower- and higher-mass stars increases monotonically with radius. The mass function (MF) measured at the half-mass radius, and as such representative of the cluster’s global properties, is surprisingly flat. Over the range 0.4 − 0.8M⊙, the number of stars per unit mass follows a power-law distribution of the type dN/dm ∝ m 0 , where, for comparison, Salpeter’s IMF would be dN/dm ∝ m −2.35 . We expect that such a flat MF does not represent the cluster’s IMF but is the result of severe tidal stripping of the stars from the cluster due to its interaction with the Galaxy’s gravitational field. Our results cannot be reconciled with the predictions of recent theoretical models that imply a relatively insignificant loss of stars from NGC6218 as measured by its expected very long time to disruption. They are more consistent with the orbital parameters based on the Hipparcos reference system that imply a much higher degree of interaction of this cluster with the Galaxy than assumed by those models. Our results indicate that, if the orbit of a cluster is known, the slope of its MF could be useful in discriminating between the various models of the Galactic potential.

HST observations of the metal rich globular clusters NGC 6496 and NGC 6352

Deep exposures of the metal-rich globular clusters NGC6496 and NGC6352 were obtained with the WFPC2camera on board the Hubble Space Telescope (HST) through the F606W and F814W filters. The resulting colour-magnitude diagrams (CMD) reach down to absolute magnitude M814 � 10−10.5, approximately 5 magnitudes below the main sequence (MS) turn- off (TO). The MS of the two clusters are sharp and well defined and their fiducial lines overlap almost exactly throughout this range. Their colour is, however, more than 0.1mag redder than the MS fiducial line of the prototype metal-rich globular cluster 47Tuc (NGC104), after proper correction for the relative distances and reddening. This provides solid empirical evidence of a higher metal content, which is not surprising if these objects belong indeed to the bulge as their present location suggests. A good fit to the upper part of the MS of both clusters is obtained with a 10Gyr-old theoretical isochrone from Baraffe et al. (1998) for a metallicity of (M/H) = −0.5, but at lower luminosities all models depart considerably from the observations, probably because of a deficiency in the treatment of the TiO opacity. The luminosity functions (LF) obtained from the observed CMD are rather similar to one another and show a peak at M814 � 9. The present day mass functions (PDMF) of both clusters are derived down toM814 � 10. 5o rm � 0.2 Mand are consistent with power-law indices α = 0.7 for NGC6496 and α = 0.6 for NGC6352. The PDMF of NGC104 is twice as steep in the same mass range (α = 1.4). We investigate the origin of this discrepancy and show that it can be understood if the two clusters contain a considerably higher fraction of primordial binaries amongst their MS population, similar to that expected in the bulge. We briefly discuss the implications of this finding on the process of star and binary formation and on the universality of the IMF.

HUBBLE TARANTULA TREASURY PROJECT. II. the STAR-FORMATION HISTORY of the STARBURST REGION NGC 2070 in 30 DORADUS

We present a study of the recent star formation of 30 Doradus in the Large Magellanic Cloud (LMC) using the panchromatic imaging survey Hubble Tarantula Treasury Project (HTTP). In this paper we focus on the stars within 20 pc of the center of the massive ionizing cluster of 30 Doradus, NGC 2070. We recovered the star formation history by comparing deep optical and NIR color-magnitude diagrams (CMDs) with state-of-the-art synthetic CMDs generated with the latest PARSEC models, which include all stellar phases from pre-main sequence to post- main sequence. For the first time in this region we are able to measure the star formation using intermediate and low mass stars simultaneously. Our results suggest that NGC2070 experienced a prolonged activity. In particular, we find that the star formation in the region: i) exceeded the average LMC rate ~ 20 Myr ago; ii) accelerated dramatically ~ 7 Myr ago; and iii) reached a peak value 1-3 Myr ago. We did not find significant deviations from a Kroupa initial mass function down to 0.5 Msun. The average internal reddening E(B-V) is found to be between 0.3 and 0.4 mag.

The global mass function of M 15

Data obtained with the NICMOS instrument on board the Hubble Space Telescope (HST) have been used to deter- mine the H-band luminosity function (LF) and mass function (MF) of three stellar fields in the globular cluster M 15, located ∼7 � from the cluster centre. The data confirm that the cluster MF has a characteristic mass of ∼0.3 M� , as obtained by Paresce & De Marchi (2000) for a stellar field at 4. 6 from the centre. By combining the present data with those published by other authors for various radial distances (near the centre, at 20 �� and at 4. 6), we have studied the radial variation of the LF due to the effects of mass segregation and derived the global mass function (GMF) using the Michie-King approach. The model that simulta- neously best fits the LF at various locations, the surface brightness profile and the velocity dispersion profile suggest that the GMF should resemble a segmented power-law with the following indices: x � 0.8 for stars more massive than 0.8 M� , x � 0.9 for 0.3−0.8 Mand x �− 2.2 at smaller masses (Salpeters IMF would have x = 1.35). The best fitting model also suggests that the cluster mass is ∼5.4 × 10 5 Mand that the mass-to-light ratio is on average M/LV � 2.1, with M/LV � 3. 7i n the core. A large amount of mass (∼44%) is found in the cluster core in the form of stellar heavy remnants, which may be sufficient to explain the mass segregation in M 15 without invoking the presence of an intermediate-mass black hole.