Guido De Marchi - Current research

Ongoing research

Here is a list of some key papers recently published, submitted or in preparation. Click on the title to see the abstract. See also our star formation website at www.starformation.eu.

De Marchi, G., Panagia, N. 2012, in preparation
Probing interstellar extinction in local group galaxies with red giant stars

We show how multi-band stellar photometry can effectively be used to determine simultaneously the properties of a stellar population and the characteristics of the intervening dust. Our method exploits the fact that all stars in an external galaxy are at one and the same distance, and makes use of the properties of red giant stars of the Red Clump to complement the information provided by young B-type stars. Thus, our method is suitable to study populations comprising also stars older than about 0.5 Gyr. Moreover, our method works even when (a) there is differential or patchy extinction, (b) the stellar population is the result of several bursts of star formation, or (c) there is considerable nebular emission. As an application, we discuss in detail the case of a region in the Large Magellanic Cloud, observed with the HST-WFPC2 in the U, B, V, I and H-alpha bands as part of the WFPC2 pure parallel programme. This field is located about 6 arcmin SW of the Tarantula Nebula (30 Dor) and displays large variations of the extinction, by as much as 3 magnitudes in Av. Our method allows us to determine the relative three-dimensional distribution of the different populations as well as their location with respect to the dust clouds. We derive the extinction law for this region and use it to place constraints on the properties of the absorbing dust.

De Marchi, G., Panagia, N., Guarcello, M.G., Bonito, R. 2012, submitted
Evidence for a pre-main sequence population older than 8 Myr in the Eagle Nebula

Attention is given to a population of 110 stars with prominent near-infrared (NIR) excess in the NGC 6611 cluster of the Eagle Nebula that have optical colours typical of pre-main sequence (PMS) stars older than 8 Myr. In principle, their V-I colours 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 and their positions in the colour-magnitude diagram suggest that most of these stars 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 5 % to the disc frequency at ~16 Myr in NGC 6611. These values imply a characteristic exponential lifetime of ~5 Myr for disc dissipation.

Spezzi, L., De Marchi, G., Panagia, N., et al. 2012, MNRAS, 421, 78 [astro-ph]
Photometric determination of the mass accretion rates of pre-main sequence stars. III.
Results in the Large Magellanic Cloud

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 (Ṁ_acc). Our measurements increase by a factor of five the current sample of PMS stars with a measured Ṁ_acc and represent the largest Ṁ_acc dataset for 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 Ṁ_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 Ṁ_acc estimates, the difference in Ṁ_acc between the LMC and our Galaxy appears to be about an order of magnitude. We review the main mechanisms of disc dispersal and find indications that typically higher Ṁ_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 Ṁ_acc of PMS stars in the LMC increases with stellar mass as Ṁ_acc ∝ m^b, with 1 < b < 1.6, slightly slower than the second power low found for Galactic PMS stars in the same mass regime.

De Marchi, G., Panagia, N., Sabbi, E. 2011, ApJ, 740, 10 [astro-ph]
Clues to the star formation in NGC 346

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.

De Marchi, G., Paresce, F., Panagia, N., et al. 2011, ApJ, 739, 27 [astro-ph]
Star formation in 30 Doradus

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.

De Marchi, G., Panagia, N., Romaniello, M., et al. 2011, ApJ, 740, 11 [astro-ph]
Photometric determination of the mass accretion rates of pre-main sequence stars. II.
NGC 346 in the Small Magellanic Cloud

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 Ṁ_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 Ṁ_acc ≈ −0.6 × 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.

Spezzi, L., Beccari, G., De Marchi, G., et al. 2011, ApJ, 731, 1 [astro-ph]
Detection of Brown Dwarf Like Objects in the Core of NGC 3603

We used near-infrared data obtained with the Wide Field Camera 3 on the Hubble Space Telescope to identify objects having the colors of brown dwarfs (BDs) in the field of the massive galactic cluster NGC 3603. These are identified through a combination of narrow- and medium-band filters which span the J and H bands and are particularly sensitive to the presence of the 1.3-1.5 μm H₂O molecular band unique to BDs. We provide a calibration of the relationship between effective temperature and color for both field stars and BDs. This photometric method provides effective temperatures for BDs to an accuracy of ±350 K relative to spectroscopic techniques. This accuracy is shown to be not significantly affected by either stellar surface gravity or uncertainties in the interstellar extinction. We identify nine objects having effective temperatures between 1700 and 2200 K, typical of BDs, observed J-band magnitudes in the range 19.5-21.5, and that are strongly clustered toward the luminous core of NGC 3603. However, if these are located at the distance of the cluster, they are far too luminous to be normal BDs. We argue that it is unlikely that these objects are either artifacts of our data set, normal field BDs/M-type giants, or extragalactic contaminants and, therefore, might represent a new class of stars having the effective temperatures of BDs but with luminosities of more massive stars. We explore the interesting scenario in which these objects would be normal stars that have recently tidally ingested a hot Jupiter, the remnants of which are providing a short-lived extended photosphere to the central star. In this case, we would expect them to show the signature of fast rotation.

Beccari, G., Spezzi, L., De Marchi, G., et al. 2010, ApJ, 720, 1108 [astro-ph]
Progressive star formation in the young Galactic super star cluster NGC 3603

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.

De Marchi, G., Paresce, F., Portegies Zwart, S. 2010, ApJ, 718, 105 [astro-ph]
On the temporal evolution of the stellar mass function in Galactic clusters

We show that we can obtain a good fit to the present day stellar mass functions (MFs) of a large sample of young and old Galactic clusters in the range 0.1 – 10 Msolar with a tapered power law distribution function with an exponential truncation of the form dN/dm ∝ m^α [1 − e^{−(m/m_c)^β}]. The average value of the power-law index α is −2, that of β is 2.5, whereas the characteristic mass m_c is in the range 0.1 – 0.8 Msolar and does not seem to vary in any systematic way with the present cluster parameters such as metal abundance, total cluster mass or central concentration. However, m_c shows a remarkable correlation with the dynamical age of the cluster, namely m_c/Msolar ≈ 0.15 + 0.5 τ_dyn^0.75, where τ_dyn is the dynamical age taken as the ratio of cluster age and dissolution time. The small scatter seen around this correlation is consistent with the uncertainties on the estimated value of τ_dyn. We attribute the observed trend to the onset of mass segregation via two-body relaxation in a tidal environment, causing the preferential loss of low-mass stars from the cluster and hence a drift of the characteristic mass m_c towards higher values. If dynamical evolution is indeed at the origin of the observed trend, it would seem plausible that high-concentration globular clusters, now with median m_c ∼ 0.33 Msolar, were born with a stellar MF very similar to that measured today in the youngest Galactic clusters and with a value of m_c ∼ 0.15 Msolar. This hypothesis is consistent with the absence of a turn-over in the MF of the Galactic bulge down to the observational limit at ∼0.2 Msolar and, if correct, it would carry the implication that the characteristic mass is not set by the thermal Jeans mass of the cloud.

De Marchi, G., Panagia, N., Romaniello, M. 2010, ApJ, 715, 1 [astro-ph]
Photometric determination of the mass accretion rates of pre-main sequence stars. I.
Method and application to the SN 1987A field

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) convert the excess Hα magnitude into 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 Ṁ_acc from L_acc and the stellar parameters (mass and radius). By selecting stars with an accuracy of 15% or better in the Hα photometry, the statistical uncertainty on the derived Ṁ_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 Ṁ_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 the accretion process in a field of 9.16 arcmin square around SN 1987A, using existing Hubble Space Telescope photometry. We identify as bona-fide PMS stars a total of 133 objects with a Hα excess above the 4 σ level and a median age of 13.5 Myr. Their median mass accretion rate of 2.6E-8 Msolar/yr is in excellent agreement with previous determinations based on the U-band excess of the stars in the same field, as well as with the value measured for G-type PMS stars in the Milky Way. The accretion luminosity of these PMS objects shows a strong dependence on their distance from a group of hot massive stars in the field and suggests that the ultraviolet radiation of the latter is rapidly eroding the circumstellar discs around PMS stars.

De Marchi, G., Paresce, F., Pulone, L. 2007, ApJ, 656, L65 [astro-ph]
Why haven't loose clusters collapsed yet?

We report on the discovery of a surprising observed correlation between the slope of the low-mass stellar global mass function (GMF) of globular clusters (GCs) and their central concentration parameter c=log(r_t/r_c). This result is based on the analysis of a sample of twenty Galactic GCs with solid GMF measurements from deep HST or VLT data. All the high-concentration clusters in the sample have a steep GMF, most likely reflecting their initial mass function (IMF). Low-concentration clusters, on the other hand, tend to have a flatter GMF than the high-concentration objects implying that they have lost a conspicuous fraction of their original stars via evaporation or tidal stripping. No GCs are found to have both a flat GMF and high central concentration. This finding appears counter-intuitive, since the same two-body relaxation mechanism that causes stars to evaporate and the cluster to eventually dissolve should also lead to higher central density and possibly core collapse. Therefore, more concentrated clusters should have lost a larger fraction of their stars and have a shallower GMF than low concentration clusters, contrary to what is observed. It is possible that the GCs that have lost a significant fraction of their original mass have also undergone core collapse and have already recovered a normal radial density profile. It is, however, more likely that GCs with a flat GMF have a much denser and smaller core than suggested by their surface brightness profile and may well be undergoing collapse at present. In either case, we may have so far seriously underestimated the number of post core-collapse clusters and an unknown number of them may be lurking in the Milky Way.