Recent papers

This page provides a selection of our recent papers. Some are published, others have been submitted and some are still being written. You can scroll down or use the navigation bar on the left to select the papers that you want to see. If you want to know more about a paper, please write to us at pic

Paper XIII   (2017)

Photometric determination of the mass accretion rates of pre-main sequence stars. V. The 30 Dor Nebula

Guido De Marchi (ESA), Nino Panagia (STScI, INAF-NA, Supernova Ltd), Giacomo Beccari (ESO)

We report on the properties of the low-mass stars that recently formed in the central ∼ 2.7' x 2.7' of 30 Dor including the R 136 cluster. Using the photometric catalogue of De Marchi et al. (2011a), based on observations with the Hubble Space Telescope (HST), and the most recent extinction law for this field, we identify 1035 bona-fide pre-main sequence (PMS) stars showing Hα excess emission at the 4 σ level with Hα equivalent width of 20Å or more. Analysis of the physical parameters and of the spatial distribution of these PMS stars indicates a spread in age spanning the range ∼ 0.1 – 50 Myr. The older PMS objects are placed in front of the R 136 cluster and are separated from it by a conspicuous amount of absorbing material, indicating that star formation has proceeded from the periphery into the interior of the region. From the measured Hα luminosity of these PMS stars we derive the accretion luminosities Lacc and, through the other stellar physical parameters, also their mass accretion rates Macc. We show that attempts to derive correlations between Macc and mass and age by simply fitting separately the observed dependence may introduce large biases and result in indices that do not have general validity. Since the effects are correlated, a multivariate fit is needed. We use a linear regression fit of the type log Macc = a x log t + b x log m + c, where t is the age of the star and m its mass. The values of a and b for 30 Dor are compatible with those found in NGC 346 and NGC 602. We apply the same regression fit to a uniform sample of 1063 PMS stars from five star forming regions in the Large and Small Magellanic Clouds and Milky Way and find a=-0.63 ± 0.02 and b=0.89 ± 0.08. The residuals are systematically different between the five regions and reveal a strong correlation with meallicity Z, of the type c=(-3.5±0.1)-(0.33±0.05) x log Z/Z. A possible interpretation of this trend is that when the metallicity is higher so is the radiation pressure and this limits the accretion process, both in its rate and duration.

This paper has been submitted to the Astrophysical Journal. It is not our practice to post manuscripts on preprint servers before they are accepted. However, if you think that reading the manuscript would help your research, please send us a message at pic


Paper XII   (2016)

Hubble Tarantula Treasury Project. IV. The extinction law

Guido De Marchi (ESA), Nino Panagia (STScI, INAF-NA, Supernova Ltd), Elena Sabbi (STScI), Daniel Lennon (ESA), Jay Anderson (STScI), Roeland van der Marel (STScI), Michele Cignoni (STScI), Eva Grebel (ARI Heidelberg), Søren Larsen (Radboud University Nijmegen), Dennis Zaritsky (Steward Observatory), Peter Zeidler (ARI Heidelberg), Dimitris Gouliermis (ITA Heidelberg), Alessandra Aloisi (STScI)

We report on the study of interstellar extinction across the Tarantula nebula (30 Doradus), in the Large Magellanic Cloud, using observations from the Hubble Tarantula Treasury Project in the 0.3 – 1.6 µm range. The considerable and patchy extinction inside the nebula causes about 3500 red clump stars to be scattered along the reddening vector in the colour-magnitude diagrams, thereby allowing an accurate determination of the reddening slope in all bands. The measured extinction law is remarkably flatter, in logarithmic terms, than in the the Galactic diffuse interstellar medium. At optical wavelengths, the larger ratio of total-to-selective extinction, namely RV = 4.5 ± 0.2, implies the presence of a grey component in the extinction law, due to a larger fraction of large grains. The extra large grains are most likely ices from supernova ejecta and will significanly alter the extinction properties of the region until they sublimate in 50 – 100 Myr. We discuss the implications of this extinction law for the Tarantula nebula and in general for regions of massive star formation in galaxies. Our results suggest that fluxes of strongly star forming regions are likely to be underestimated by a factor of about 2 in the optical.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (2016, MNRAS, 455, 4373). A copy of the preprint is available on the astro-ph server at this address arXiv:1510.8436.


Paper XI   (2015)

Mass accretion rates from multi-band photometry in the Carina Nebula: the case of Trumpler 14

Giacomo Beccari (ESO), Guido De Marchi (ESA), Nino Panagia (STScI, INAF-NA, Supernova Ltd), Elena Valenti (ESO), Giovanni Carraro (ESO), Martino Romaniello (ESO), Manuela Zoccali (Pontificia Universidad de Chile), Carsten Weidner (IAC, ULL)

We present a study of the mass accretion rates of pre-main sequence (PMS) stars in the cluster Trumpler 14 (Tr14) in the Carina nebula. Using optical multi-band photometry we were able to identify 356 PMS stars showing Hα excess emission with equivalent width EW(Hα)>20 Å. We interpret this observational feature as indication that these objects are still actively accreting gas from their circumstellar medium. From a comparison of the Hertzsprung-Russell diagram with PMS evolutionary models, we derive ages and masses of the PMS stars. We find that most of the PMS objects are younger than 10 Myr with a median age of ~3 Myr. Surprisingly, we also find that ~20% of the mass accreting objects are older than 10 Myr. For each PMS star in Trumpler 14 we determine the mass accretion rate (Macc) and discuss its dependence on mass and age. We finally combine the optical photometry with near-IR observations to build the spectral energy distribution (SED) for each PMS star in Tr14. The analysis of the SEDs suggests the presence of transitional discs in which a large amount of gas is still present and sustains accretion onto the PMS object at ages older than 10 Myr. Our results, discussed in light of recent discoveries with Herschel of transitional discs containing a massive gas component around the relatively old PSM stars TW Hydrae, 49 Ceti, and HD 95086, support a new scenario in which old and evolved debris discs still host a significant amount of gas.

This paper has been published in Astronomy and Astrophysics (2015, A&A, 574, 44). A copy of the preprint is available on the astro-ph server at this address arXiv:1409.4370.


Paper X   (2014)

The extinction law inside the 30 Doradus nebula

Guido De Marchi (ESA), Nino Panagia (STScI, INAF-NA, Supernova Ltd)

We have studied the interstellar extinction in a field of ∼ 3' x 3' at the core of the 30 Dor nebula, including the central R 136 cluster, in the Large Magellanic Cloud. Observations at optical and near-infrared wavelengths, obtained with the WFC3 camera on board the Hubble Space Telescope, show that the stars belonging to the red giant clump are spread across the colour–magnitude diagrams because of the considerable and uneven levels of extinction in this region. Since these stars share very similar physical properties and are all at the same distance, they allow us to derive the absolute extinction in a straightforward and reliable way. Thus we have measured the extinction towards about 180 objects and the extinction law in the range 0.3 – 1.6 µm. At optical wavelengths, the extinction curve is almost parallel to that of the diffuse Galactic interstellar medium, but the value of RV = 4.5 ± 0.2 that we measure indicates that there is an extra grey component due to a larger fraction of large grains. At wavelengths longer than ∼ 1 µm, the contribution of the grey component tapers off as λ–1.5, like in the Milky Way, suggesting that the nature of the grains is otherwise similar to those in our Galaxy, but with a ∼ 2.2 times higher fraction of large grains. These results are consistent with the addition of "fresh" large grains by supernova explosions, as recently revealed by Herschel and ALMA observations of SN1987A.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (2014, MNRAS, 445, 93). A copy of the preprint is available on the astro-ph server at this address arXiv:1408.4786.


Paper IX   (2014)

Probing interstellar extinction near the 30 Doradus nebula with red giant stars

Guido De Marchi (ESA), Nino Panagia (STScI, INAF-NA, Supernova Ltd), Leo Girardi (INAF-PD)

We have studied the properties of the interstellar extinction in a field of ∼ 3' x 3' located about 6' SW of 30 Doradus in the Large Magellanic Cloud (LMC). The observations with with the WFPC2 camera on board the Hubble Space Telescope in the U, B, V, I and Hα bands show the presence of patchy extinction in this field. In particular, the colour-magnitude diagram (CMD) reveals an elongated stellar sequence, running almost parallel to the main sequence (MS), which is in reality made up of stars belonging to the red giant clump (RC) and spread across the CMD by the considerable and uneven extinction in this region. This allows us to derive in a quantitative way both the extinction law in the range 3000 – 8000 Å and the values of the absolute extinction towards more than 100 objects, thereby setting statistically significant constraints on the properties of the extinction in this area. We find an extinction curve considerably flatter than the standard Galactic one and than those derived before towards a sample of sight lines in the LMC. The value of RV = 5.6 that we find implies that in this region large grains dominate. Comparing the extinction towards the individual RC stars and a similar number of stars in the upper MS reveals that the latter span a narrower range of E(B–V) values, contrary to what has been found elsewhere in the LMC.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (2014, MNRAS, 438, 513). A copy of the preprint is also available on the astro-ph server at this address arXiv:1311.3659.


Paper VIII   (2013)

Photometric determination of the mass accretion rates of pre-main sequence stars. IV. Recent star formation in NGC 602

Guido De Marchi (ESA), Giacomo Beccari (ESO), Nino Panagia (STScI, INAF-NA, Supernova Ltd)

We have studied the young stellar populations in NGC 602, in the Small Magellanic Cloud, using a novel method that we have developed to combine Hubble Space Telescope photometry in the V, I, and Hα bands. We have identified about 300 pre-main sequence (PMS) stars, all of which are still undergoing active mass accretion, and have determined their physical parameters (effective temperature, luminosity, age, mass and mass accretion rate). Our analysis shows that star formation has been present in this field over the last 60 Myr. In addition, we can recognise at least two clear, distinct, and prominent episodes in the recent past: one about 2 Myr ago, but still ongoing in regions of higher nebulosity, and one (or more) older than 30 Myr, encompassing both stars dispersed in the field and two smaller clusters located about 100" north of the centre of NGC 602. The relative locations of younger and older PMS stars do not imply a causal effect or triggering of one generation on the other. The strength of the two episodes appears to be comparable, but the episode occurring more than 30 Myr ago might have been even stronger than the current one. We have investigated the evolution of the mass accretion rate M_acc as a function of the stellar parameters finding that log M_acc ≈ −0.6 x log t + log m + c, where t is the age of the star, m its mass and c is a decreasing function of the metallicity.

This paper has been published in the Astrophysical Journal (2013, ApJ, 775, 68). A copy of the preprint is available on the astro-ph server at this address arXiv:1307.8445.


Paper VII   (2013)

Pre-main sequence stars older than 8 Myr in the Eagle Nebula

Guido De Marchi (ESA), Nino Panagia (STScI, INAF-NA, Supernova Ltd), M.G. Guarcello (Harvard-Smithsonian CfA), Rosaria Bonito (INAF-PA)

Attention is given to a population of 110 stars in the NGC 6611 cluster of the Eagle Nebula that have prominent near-infrared (NIR) excess and optical colours typical of pre-main sequence (PMS) stars older than 8 Myr. At least half of those for which spectroscopy exists have a Hα emission line profile revealing active accretion. In principle, the V-I colours of all these stars would be consistent with those of young PMS objects (< 1 Myr) whose radiation is heavily obscured by a circumstellar disc seen at high inclination and in small part scattered towards the observer by the back side of the disc. However, using theoretical models it is shown here that objects of this type can only account for a few percent of this population. In fact, the spatial distribution of these objects, their X-ray luminosities, their optical brightness, their positions in the colour-magnitude diagram and the weak Li absorption lines of the stars studied spectroscopically suggest that most of them are at least 8 times older than the ~1 Myr-old PMS stars already known in this cluster and could be as old as ~30 Myr. This is the largest homogeneous sample to date of Galactic PMS stars considerably older than 8 Myr that are still actively accreting from a circumstellar disc and it allows us to set a lower limit of 7% to the disc frequency at ~16 Myr in NGC 6611. These values imply a characteristic exponential lifetime of ~6 Myr for disc dissipation.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (2013, MNRAS, 435, 3058). A copy of the preprint is available on the astro-ph server at this address arXiv:1307.8446.


Paper VI   (2012)

Photometric determination of the mass accretion rates of pre-main sequence stars. III. Results in the Large Magellanic Cloud

Loredana Spezzi (ESA), Guido De Marchi (ESA), Nino Panagia (STScI, INAF-CT, Supernova Ltd), Aurora Sicilia-Aguilar (MPIA), Barbara Ercolano (IoA Cambridge)

We present a multi-wavelength study of three star forming regions, spanning the age range 1-14 Myr, located between the 30 Doradus complex and supernova SN1987A in the Large Magellanic Cloud (LMC). We reliably identify pre-main sequence (PMS) objects actively undergoing mass accretion and estimate their stellar properties and mass accretion rate (M_acc). Our measurements represent the largest Macc dataset of low-metallicity stars presented so far. As such, they offer a unique opportunity to study on a statistical basis the mass accretion process in the LMC and, more in general, the evolution of the mass accretion process around low-metallicity stars. We find that the typical M_acc of PMS stars in the LMC is higher than for Galactic PMS stars of the same mass, independently of their age. Taking into account the caveats of isochronal age and M_acc estimates, the difference in M_acc between the LMC and our Galaxy appears to be about an order of magnitude. We review the main mechanisms of disk dispersal and find indications that typically higher M_acc are to be expected in low-metallicity environments. However, many issues of this scenario need to be clarified by future observations and modeling. We also find that, in the mass range 1-2 Msolar, the M_acc of PMS stars in the LMC increases with stellar mass as M_acc \propto m^b, with b ~ 1, i.e. slower than the second power low found for Galactic PMS stars in the same mass regime.

This paper has been published in the Monthly Notices of the Royal Astronomical Society (2012, MNRAS, 421, 78). A copy of the preprint is available on the astro-ph server at this address arXiv:1111.0835.


Paper V   (2011)

Star formation in 30 Doradus

Guido De Marchi, Francesco Paresce, Nino Panagia, Giacomo Beccari, Loredana Spezzi, Marco Sirianni, Morten Andersen, Max Mutchler, Bruce Balick, Michael Dopita, Jay Frogel, Bradley Whitmore, Howard Bond, Daniela Calzetti, Marcella Carollo, Michael Disney, Donald Hall, Jon A. Holtzman, Randy A. Kimble, Patrick McCarthy, Robert O'Connell, Abhijit Saha, Joseph Silk, John Trauger, Alistair Walker, Rogier Windhorst, Erick Young

Using observations recently obtained with the WFC3 camera on board the Hubble Space Telescope, we have studied the star formation properties in the 30 Dor region, in the Large Magellanic Cloud. The observations clearly reveal the presence of considerable differential extinction across the field. We characterise and quantify this effect using young massive main sequence stars to derive a statistical reddening correction for most objects in the field. We then search for pre-main sequence (PMS) stars by looking for objects with a strong (> 4 σ) Hα excess emission and find about 1,150 of them over the entire field. Comparison of their location in the Hertzprung--Russel diagram with theoretical PMS evolutionary tracks reveals that about one third of these objects are younger than ~4 Myr, compatible with that of the massive stars in the central ionising cluster R 136, whereas the rest have an age of up to ~30 Myr, with a median of ~12 Myr. This indicates that star formation has proceeded over an extended period of time, although we cannot discriminate between an extended episode and a series of short and frequent bursts that are not resolved in time. While the younger PMS population preferentially occupies the central regions of the cluster, older PMS objects are more uniformly distributed across the field and are remarkably few at the very centre of the cluster. We attribute this latter effect to photoevaporation of the older circumstellar discs caused by the massive ionising members of R 136.

This paper has been published in the Astrophysical Journal (2011, ApJ, 739, 27). A copy of the preprint is also available on the astro-ph server at this address arXiv:1106.2801.


Paper IV   (2011)

Clues to the star formation in NGC 346 across time and space

Guido De Marchi (ESA), Nino Panagia (STScI, INAF-CT, Supernova Ltd) and Elena Sabbi (STScI)

We have studied the properties of the stellar populations in the field of the NGC 346 cluster in the Small Magellanic Cloud, using the results of a novel self-consistent method that provides a reliable identification of pre-main sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The 680 identified bona-fide PMS stars show a bimodal age distribution, with two roughly equally numerous populations peaked respectively at ~1 Myr, and ~20 Myr. We use the age and other physical properties of these PMS stars to study how star formation has proceeded across time and space in NGC 346. We find no correlation between the locations of young and old PMS stars, nor do we find a correspondence between the positions of young PMS stars and those of massive OB stars of similar age. Furthermore, the mass distribution of stars with similar age shows large variations throughout the region. We conclude that, while on a global scale it makes sense to talk about an initial mass function, this concept is not meaningful for individual star-forming regions. An interesting implication of the separation between regions where massive stars and low-mass objects appear to form is that high-mass stars might not be "perfect" indicators of star formation and hence a large number of low-mass stars formed elsewhere might have so far remained unnoticed. For certain low surface density galaxies this way of preferential low-mass star formation may be the predominant mechanism, with the consequence that their total mass as derived from the luminosity may be severely underestimated and that their evolution is not correctly understood.

This paper has been published in the Astrophysical Journal (2011, ApJ, 740, 10). A copy of the preprint is also available on the astro-ph server at this address arXiv:1106.5780.



Paper III   (2010)

Progressive star formation in the young Galactic starburst cluster NGC 3603

Giacomo Beccari (ESA), Loredana Spezzi (ESA), Guido De Marchi (ESA), Francesco Paresce (IAFS-BO, INAF), Erik Young (Univ. Arizona), Morten Andersen (ESA), Nino Panagia (STScI, INAF-CT, Supernova Ltd), and the WFC3-SOC

Early release science observations of the cluster NGC3603 with the WFC3 on the refurbished HST allow us to study its recent star formation history. Our analysis focuses on stars with Hα excess emission, a robust indicator of their premain sequence (PMS) accreting status. The comparison with theoretical PMS isochrones shows that 2/3 of the objects with Hα excess emission have ages from 1 to 10 Myr, with a median value of 3 Myr, while a surprising 1/3 of them are older than 10 Myr. The study of the spatial distribution of these PMS stars allows us to confirm their cluster membership and to statistically separate them from field stars. This result establishes unambiguously for the first time that star formation in and around the cluster has been ongoing for at least 10-20 Myr, at an apparently increasing rate.

This paper has been published in the Astrophysical Journal (2010, ApJ, 720, 1108). A copy of the preprint is also available on the astro-ph server at this address arXiv:1007.2795



Paper II   (2011)

Photometric determination of the mass accretion rates of pre-main sequence stars. II. NGC 346 in the Small Magellanic Cloud

Guido De Marchi (ESA), Nino Panagia (STScI, INAF-CT, Supernova Ltd), Martino Romaniello (ESO), Elena Sabbi (STScI), Marco Sirianni (ESA), Pier Giorgio Prada Moroni (Univ. Pisa, INFN), Scilla Degl'Innocenti (Univ. Pisa, INFN)

We have studied the properties of the stellar populations in the field of the NGC 346 cluster in the Small Magellanic Cloud, using a novel self-consistent method that allows us to reliably identify pre-main sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The method does not require spectroscopy and combines broad-band V and I photometry with narrow-band Hα imaging to identify all stars with excess Hα emission and derive the accretion luminosity L_acc and mass accretion rate M_acc for all of them. The application of this method to existing HST/ACS photometry of the NGC 346 field has allowed us to identify and study 680 bona-fide PMS stars with masses from ~0.4 Msolar to ~4 Msolar and ages in the range from ~1 Myr to ~30 Myr. Previous investigations of this region, based on the same data, had identified young (~3 Myr old) candidate PMS stars on the basis of their broad-band colours. In this study we show that there are at least two, almost equally numerous, young populations with distinct ages of respectively ~1 and ~16 Myr. We provide for all of them accurate physical parameters.
We take advantage of the unprecedented size of our PMS sample and of its spread in mass and age to study the evolution of the mass accretion rate as a function of stellar parameters. We find that, regardless of stellar mass, the mass accretion rate decreases with roughly the square root of the age, or about three times slower than predicted by current models of viscous disc evolution, and that more massive stars have systematically higher mass accretion rate in proportion to their mass. A multivariate linear regression fit reveals that log M_acc ≈ −0.6 x log t + log m + c, where t is the age of the star, m its mass and c a quantity that is higher at lower metallicity. This result is consistent with measurements of the mass accretion rate in the 30 Dor region and in the Milky Way and suggests that longer duration for mass accretion could be related to lower metallicity. The high mass accretion rates that we find suggest that a considerable fraction of the stellar mass is accreted during the PMS phase,of order ~0.2 Msolar or possibly ~20 % of the final mass for stars with mass m < 1 Msolar if their discs are eroded by 20 Myr, i.e. before they reach the main sequence. Therefore, PMS evolutionary models that do not account for this effect will systematically underestimate the true age when compared with the observations.

This paper has been published in the Astrophysical Journal (2011, ApJ, 740, 11). A copy of the preprint is also available on the astro-ph server at this address arXiv:1104.4494.



Paper I   (2010)

Photometric determination of the mass accretion rates of pre-main sequence stars. I. Method and application to the SN1987A field

Guido De Marchi (ESA), Nino Panagia (STScI, INAF-CT, Supernova Ltd), Martino Romaniello (ESO)

We have developed and successfully tested a new self-consistent method to reliably identify pre-main sequence (PMS) objects actively undergoing mass accretion in a resolved stellar population, regardless of their age. The method does not require spectroscopy and combines broad-band V and I photometry with narrow-band Hα imaging to: (1) identify all stars with excess Hα emission; (2) derive their Hα luminosity L(Hα); (3) estimate the Hα emission equivalent width; (4) derive the accretion luminosity L_acc from L(Hα); and finally (5) obtain the mass accretion rate M_acc from L_acc and the stellar parameters (mass and radius). By selecting stars with photometric accuracy in Hα better than 15%, the statistical uncertainty on the derived M_acc is typically <17% and is dictated by the precision of the Hα photometry. Systematic uncertainties, of up to a factor of 3 on the value of M_acc, are caused by our incomplete understanding of the physics of the accretion process and affect all determinations of the mass accretion rate, including those based on a spectroscopic Hα line analysis.
As an application of our method, we study a field of 9.16 arcmin2 around SN1987A, using existing HST photometry. We identify as bona-fide PMS stars a total of 133 objects with a Hα excess above the 4 sigma level and a median age of 13.5 Myr. Their median mass accretion rate of 2.6x10-8 Msolar/yr is in excellent agreement with previous determinations based on the U-band excess of the stars in this field, and with the value measured for G-type PMS stars in the Milky Way. Their L_acc shows a strong dependence on their distance from a group of hot massive stars in the field and suggests that the UV radiation of the latter is rapidly eroding the circumstellar discs around PMS stars.

This paper has been published in the Astrophysical Journal (2010, ApJ, 715, 1). A copy of the preprint is also available on the astro-ph server at this address arXiv:1002.4864.



Older papers

Here is a selection of our recent papers. You can scroll down or use the navigation bar on the left to select the papers that you want to see. If you want to know more about a paper, please write to us at pic

(2004)

Low-Mass Pre-Main-Sequence Stars in the Large Magellanic Cloud. III. Accretion Rates from Hubble Space Telescope WFPC2 Observations

Martino Romaniello (ESO), Massimo Robberto (STScI, ESA), Nino Panagia (STScI, ESA)

We have measured the present accretion rate of roughly 800 low-mass (~1-1.4 Msolar) pre-main-sequence stars in the field of supernova 1987A in the Large Magellanic Cloud (Z~=0.3 Zsolar). It is the first time that this fundamental parameter for star formation has been determined for low-mass stars outside our Galaxy. The Balmer continuum emission used to derive the accretion rate positively correlates with the H# excess. Both these phenomena are believed to originate from accretion from a circumstellar disk, so their simultaneous detection provides an important confirmation of the pre-main-sequence nature of the H# and UV excess objects, which are likely to be the LMC equivalent of Galactic classical T Tauri stars. The stars with statistically significant excesses are measured to have accretion rates larger than ~1.510-8 Msolar yr-1 at an age of 12-16 Myr. For comparison, the timescale for disk dissipation observed in the Galaxy is of the order of 6 Myr. Moreover, the oldest classical T Tauri star known in the Milky Way (TW Hydrae, with 10 Myr of age) has a measured accretion rate of only 510-10 Msolar yr-1, i.e., 30 times less than what we measure for stars at a comparable age in the LMC. Our findings indicate that metallicity plays a major role in regulating the formation of low-mass stars.

This paper has been published in the Astrophysical Journal (2004, ApJ, 608, 220). A copy of the preprint is also available on the astro-ph server at this address astro-ph/0402459.



(2002)

Accurate Stellar Population Studies from Multiband Photometric Observations

Martino Romaniello (ESO), Nino Panagia (STScI, ESA), Salvatore Scuderi (INAF-CT), Robert Kirshner (CfA)

We present a new technique based on multiband near-ultraviolet and optical photometry to measure both the stellar intrinsic properties, i.e., luminosity and effective temperature, and the interstellar dust extinction along the line of sight to hundreds of stars per square arcminute. The yield is twofold. On the one hand, the resulting reddening map has a very high spatial resolution, of the order of a few arcseconds, and can be quite effectively used in regions where the interstellar material is patchy, thus producing considerable differential extinction on small angular scales. On the other hand, combining the photometric information over a wide baseline in wavelength provides an accurate determination of temperature and luminosity for thousands of stars. As a test case, we present the results for the region around SN 1987A in the Large Magellanic Cloud imaged with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope.

This paper has been published in the Astronomical Journal (2002, AJ, 123, 915). A copy of the preprint is also available on the astro-ph server at this address astro-ph/0111399.



(2000)

Young Stellar Populations around SN 1987A

Nino Panagia (STScI, ESA), Martino Romaniello (ESO), Salvatore Scuderi (INAF-CT), Robert Kirshner (CfA)

We present the first results of a study of the stellar population in a region of 30 pc radius around SN 1987A, based on an analysis of multiband Hubble Space Telescope (HST) WFPC2 images. The effective temperature, radius and, possibly, reddening of each star were determined by fitting the measured broadband magnitudes to the ones calculated with model atmospheres. In particular, we have determined effective temperatures and bolometric luminosities for 21,995 stars, and for a subsample of 2510 stars we also determined individual reddening corrections. In addition, we have identified all stars with H#alpha; equivalent widths in excess of 8 A˚, a total of 492 stars. An inspection of the H-R diagram reveals the presence of several generations of young stars, with ages between 1 and 150 Myr, superposed on a much older field population (0.6-6 Gyr). A substantial fraction of young stars with ages around 12 Myr make up the stellar generation coeval to SN 1987A progenitor. The youngest stars in the field appear to be strong-line T Tauri stars, identified on the basis of their conspicuous (Weq > 8 A˚) Hα excesses. This constitute the first positive detection of low-mass (about 1-2 Msolar) pre-main-sequence (PMS) stars outside the Milky Way. Their positions in the H-R diagram appear to require that star formation in the LMC occurs with accretion rates about 10 times higher than in the Milky Way, i.e., ~10-4 Msolar yr-1. SN 1987A appears to belong to a loose, young cluster 12+/-2 Myr old, in which the slope of the present mass function is almost identical to Salpeter's, i.e., Γ=dlogN/dlogM ≈ -1.25 for masses above 3 Msolar, but becomes much flatter for lower masses, i.e., Γ ≈ -0.5. On a large scale, we find that the spatial distributions of massive stars and low-mass PMS stars are conclusively different, indicating that different star formation processes operate for high- and low-mass stars. This results casts doubts on the validity of an initial mass function (IMF) concept on a small scale (say, less than 10 pc). Moreover, it appears that a determination of the low-mass end IMF in the LMC requires an explicit identification of PMS stars. A preliminary analysis, done for the whole field as a single entity, shows that the IMF slope for the young population present over the entire region is steeper than Γ ≈ -1.7.

This paper has been published in the Astrophysical Journal (2000, ApJ, 539, 197). A copy of the preprint is also available on the astro-ph server at this address astro-ph/0001476.



(1998)

Young Stellar Populations in the Large Magellanic Cloud: surprises from HST observations

Martino Romaniello (Scuola Normale Superiore, Pisa)

This PhD thesis has been successfully defended in 1998 at the Scuole Normale Superiore in Pisa, Italy, is listed in the ADS and is available in full here (large file, 16MB).



(1994)

WFPC2 observations of the double cluster NGC 1850 in the Large Magellanic Cloud

Roberto Gilmozzi (STScI, ESA), Elaine Kinney (STScI), Sean Ewald (STScI), Nino Panagia (STScI, ESA), Martino Romaniello (Pisa)

Hubble Space Telescope-Wide Field/Planetary Camera-2 (HST-WFPC2) optical and ultraviolet imaging observations of the young double cluster NGC 1850 in the Large Magellanic Cloud (LMC) are presented. The main cluster, NGC 1850A, is a globular-like cluster and has an age of 50 ± 10 Myr, while the subcluster, NGC 1850B, which is more loosely distributed, is very young at 4.3 ± 0.9 Myr. Its young age is confirmed by the detection of a pre-main-sequence population of stars associated to it. The two clusters have considerably different IMF slopes, with the main cluster having a flat slope (f(m) propto m^(-1.4 ± 0.2)) and the young cluster a much steeper one (f(m) propto m^(-2.6 ± 0.1)). The LMC field star population displays a broad range of ages, from approximately 0.5 Gyr up to more than 4 Gyr.

This paper has been published in the Astrophysical Journal (1994, ApJ, 435, L43).