arXiv subjects

Guido De Marchi

Publications and source records attributed to Guido De Marchi.

At least 37 records · Page 2Linked to original sources

arXiv2022

The 30 Doradus Molecular Cloud at 0.4 pc Resolution with the Atacama Large Millimeter/submillimeter Array: Physical Properties and the Boundedness of CO-emitting Structures

We present results of a wide-field (approximately 60 x 90 pc) ALMA mosaic of CO(2-1) and $^{13}$CO(2-1) emission from the molecular cloud associated with the 30 Doradus star-forming region. Three main emission complexes, including two forming a bowtie-shaped structure extending northeast and southwest from the central R136 cluster, are resolved into complex filamentary networks. Consistent with previous studies, we find that the central region of the cloud has higher line widths at fixed size relative to the rest of the molecular cloud and to other LMC clouds, indicating an enhanced level of turbulent motions. However, there is no clear trend in gravitational boundedness (as measured by the virial parameter) with distance from R136. Structures observed in $^{13}$CO are spatially coincident with filaments and are close to a state of virial equilibrium. In contrast, CO structures vary greatly in virialization, with low CO surface brightness structures outside of the main filamentary network being predominantly unbound. The low surface brightness structures constitute ~10% of the measured CO luminosity; they may be shredded remnants of previously star-forming gas clumps, or alternatively the CO-emitting parts of more massive, CO-dark structures.

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arXiv2021

Extinction in the Large Magellanic Cloud Bar around NGC1854, NGC1856, and NGC1858

We report on the extinction properties in the fields around the clusters NGC 1854, NGC 1856, and NGC 1858 in the bar of the Large Magellanic Cloud. The colour-magnitude diagrams of the stars in all these regions show an elongated red giant clump that reveals a variable amount of extinction across these fields, ranging from Av~0.2 to Av~1.9, including Galactic foreground extinction. The extinction properties nonetheless are remarkably uniform. The slope of the reddening vectors measured in the (V-I,V) and (B-I,B) colour-magnitude planes is fully in line with the Av/E(B-V)~5.5 value found in the outskirts of 30 Dor. This indicates the presence of an additional grey extinction component in the optical requiring big grains to be about twice as abundant as in the diffuse Galactic interstellar medium (ISM). Areas of higher extinction appear to be systematically associated with regions of more intense star formation, as measured by the larger number of stars more massive than 8 Msun, thus making injection of big grains into the ISM by SNII explosion the likely mechanism at the origin of the observed grey extinction component.

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arXiv2020

Anomalous extinction towards NGC 1938

Intrigued by the extended red-giant clump (RC) stretching across the colour-magnitude diagram of the stars in a 50x50 pc^2 region of the Large Magellanic Cloud (LMC) containing the clusters NGC 1938 and NGC 1939, we have studied the stellar populations to learn about the properties of the interstellar medium (ISM) in this area. The extended RC is caused by a large and uneven amount of extinction across the field. Its slope reveals anomalous extinction properties, with Av/E(B-V)=4.3, indicating the presence of an additional grey component in the optical contributing about 30% of the total extinction in the field and requiring big grains to be about twice as abundant as in the diffuse ISM. This appears to be consistent with the amount of big grains injected into the surrounding ISM by the about 70 SNII explosions estimated to have occurred during the lifetime of the ~120 Myr old NGC 1938. Although this cluster appears today relatively small and would be hard to detect beyond the distance of M 31, with an estimated initial mass of ~4800 Msun NGC 1938 appears to have seriously altered the extinction properties in a wide area. This has important implications for the interpretation of luminosities and masses of star-forming galaxies, both nearby and in the early universe.

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arXiv2019

Ultraviolet extinction properties of the 30 Dor Nebula and interpreting observations of starburst clusters

Recent investigation of the extinction law in 30 Dor and the Tarantula Nebula, at optical and near infrared (NIR) wavelengths, has revealed a ratio of total to selective extinction R_V=A_V/E(B-V) of about 4.5. This indicates a larger fraction of big grains than in the Galactic diffuse interstellar medium (ISM). Possible origins include coalescence of small grains, small grain growth, selective destruction of small grains, and fresh injection of big grains. From a study of the ultraviolet extinction properties of three massive stars in the 30 Dor nebula (R139, R140, R145), observed with the International Ultraviolet Explorer (IUE), we show that the excess of big grains does not come at the expense of small grains, which are still present and possibly even more abundant. Fresh injection of large grains appears the dominant mechanism. A process able to naturally account for this in environments such as the Tarantula nebula, where formation of massive stars has been ongoing for over ~20 Myr, is the explosion of massive stars as type-II supernovae (SN). The ensuing change in the conditions of the ISM is only temporary, lasting less than ~100 Myr, because shattering and shocks will eventually break and destroy the bigger grains. However, this is the only time when star-forming regions are detectable as such in starburst and high-redshift galaxies and we highlight the complexity inherent in interpreting observations of star-forming regions in these environments. If the extinction characteristics are not known properly, any attempts to derive quantitative physical parameters are bound to fail.

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arXiv2019

Photometric determination of the mass accretion rates of pre-main sequence stars. VI. The case of LH 95 in the Large Magellanic Cloud

We report on the accretion properties of low-mass stars in the LH95 association within the Large Magellanic Cloud (LMC). Using non-contemporaneous wide-band and narrow-band photometry obtained with the HST, we identify 245 low-mass pre-main sequence (PMS) candidates showing H$α$ excess emission above the 4$σ$ level. We derive their physical parameters, i.e. effective temperatures, luminosities, masses ($M_\star$), ages, accretion luminosities, and mass accretion rates ($\dot M_{\rm acc}$). We identify two different stellar populations: younger than ~8Myr with median $\dot M_{\rm acc}$~5.4x10$^{-8}M_\odot$/yr (and $M_\star$~0.15-1.8$M_\odot$) and older than ~8Myr with median $\dot M_{\rm acc}$~4.8x10$^{-9}M_\odot$/yr (and $M_\star$~0.6-1.2$M_\odot$). We find that the younger PMS candidates are assembled in groups around Be stars, while older PMS candidates are uniformly distributed within the region without evidence of clustering. We find that $\dot M_{\rm acc}$ in LH95 decreases with time more slowly than what is observed in Galactic star-forming regions (SFRs). This agrees with the recent interpretation according to which higher metallicity limits the accretion process both in rate and duration due to higher radiation pressure. The $\dot M_{\rm acc}-M_\star$ relationship shows different behaviour at different ages, becoming progressively steeper at older ages, indicating that the effects of mass and age on $\dot M_{\rm acc}$ cannot be treated independently. With the aim to identify reliable correlations between mass, age, and $\dot M_{\rm acc}$, we used for our PMS candidates a multivariate linear regression fit between these parameters. The comparison between our results with those obtained in other SFRs of our Galaxy and the MCs confirms the importance of the metallicity for the study of the $\dot M_{\rm acc}$ evolution in clusters with different environmental conditions.

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arXiv2018

A sextet of clusters in the Vela OB2 region revealed by Gaia

Using Gaia DR2 data, combined with OmegaCAM ground-based optical photometry from the AD-HOC survey, and detailed Radial Velocity measurements from ESO-Gaia, we analyse in detail a 10x5 deg region around the Wolf-Rayet star $γ^2$ Vel, including the previously known clusters Gamma Vel and NGC2547. Using clustering analysis that considers positions, proper motions and parallax, we discover 6 clusters or associations -- 4 of which appear new. Analysis of the colour-magnitude diagram for these clusters show that 4 of them formed coevally from the same molecular clouds 10 Myr ago, while NGC 2547 formed together with a newly discovered cluster 30 Myr ago. This study shows the incredible wealth of data provided by Gaia for the study of young stellar clusters.

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arXiv2017

Photometric determination of the mass accretion rates of pre-main sequence stars. V. Recent star formation in the 30 Dor nebula

We report on the properties of the low-mass stars that recently formed in the central ~ 2.7'x2.7' of 30 Dor including the R136 cluster. Using the photometric catalogue of De Marchi et al. (2011c), 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 Halpha excess emission at the 4 sigma level with Halpha equivalent width of 20 AA or more. We find a wide spread in age spanning the range ~ 0.1-50 Myr. We also find that the older PMS objects are placed in front of the R136 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. We derive physical parameters for all PMS stars, including masses m, ages t, and mass accretion rates M_acc. To identify reliable correlations between these parameters, which are intertwined, we use a multivariate linear regression fit of the type log M_acc = a log t + b log m + c. The values of a and b for 30 Dor are compatible with those found in NGC 346 and NGC 602. We extend the fit to a uniform sample of 1307 PMS stars with 0.5 < m/Msun < 1.5 and t < 16 Myr in six star forming regions in the Large and Small Magellanic Clouds and Milky Way with metallicities in the range 0.1-1.0 Zsun. We find a=-0.59+/-0.02 and b=0.78+/-0.08. The residuals are systematically different between the six regions and reveal a strong correlation with metallicity Z, of the type c = (-3.69+/-0.02) - (0.30+/-0.04) log Z/Zsun. 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.

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arXiv2016

Studying Relation Between Star Formation and Molecular Clumps on Subparsec Scales in 30 Doradus

We present $\mathrm{^{12}CO}$ and $\mathrm{^{13}CO}$ molecular gas data observed by ALMA, massive early stage young stellar objects identified by applying color-magnitude cuts to \textit{Spitzer} and \textit{Herschel} photometry, and low-mass late stage young stellar objects identified via H$\mathrmα$ excess. Using dendrograms, we derive properties for the molecular cloud structures. This is the first time a dendrogram analysis has been applied to extragalactic clouds. The majority of clumps have a virial parameter equal to unity or less. The size-linewidth relations of $\mathrm{^{12}CO}$ and $\mathrm{^{13}CO}$ show the clumps in this study have a larger linewidth for a given size (by factor of 3.8 and 2.5, respectively) in comparison to several, but not all, previous studies. The larger linewidths in 30 Doradus compared to typical Milky Way quiescent clumps are probably due to the highly energetic environmental conditions of 30 Doradus. The slope of the size-linewidth relations of $\mathrm{^{12}CO}$, 0.65 $\pm$ 0.04, and $\mathrm{^{13}CO}$, 0.97 $\pm$ 0.12, are on the higher end but consistent within 3$\mathrmσ$ of previous studies. Massive star formation occurs in clumps with high masses ($> 1.83 \times 10^{2}\;\mathrm{M_{\odot}}$), high linewidths (v $> 1.18\;\mathrm{km/s}$), and high mass densities ($> 6.67 \times 10^{2}\;\mathrm{M_{\odot}\;pc^{-2}}$). The majority of embedded, massive young stellar objects are associated with a clump. However the majority of more evolved, low-mass young stellar objects are not associated with a clump.

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arXiv2016

A model-based approach to the spatial and spectral calibration of NIRSpec onboard JWST

Context: The NIRSpec instrument for the James Webb Space Telescope (JWST) can be operated in multiobject (MOS), long-slit, and integral field (IFU) mode with spectral resolutions from 100 to 2700. Its MOS mode uses about a quarter of a million individually addressable minislits for object selection, covering a field of view of $\sim$9 $\mathrm{arcmin}^2$. Aims: The pipeline used to extract wavelength-calibrated spectra from NIRSpec detector images relies heavily on a model of NIRSpec optical geometry. We demonstrate how dedicated calibration data from a small subset of NIRSpec modes and apertures can be used to optimize this parametric model to the necessary levels of fidelity. Methods: Following an iterative procedure, the initial fiducial values of the model parameters are manually adjusted and then automatically optimized, so that the model predicted location of the images and spectral lines from the fixed slits, the IFU, and a small subset of the MOS apertures matches their measured location in the main optical planes of the instrument. Results: The NIRSpec parametric model is able to reproduce the spatial and spectral position of the input spectra with high fidelity. The intrinsic accuracy (1-sigma, RMS) of the model, as measured from the extracted calibration spectra, is better than 1/10 of a pixel along the spatial direction and better than 1/20 of a resolution element in the spectral direction for all of the grating-based spectral modes. This is fully consistent with the corresponding allocation in the spatial and spectral calibration budgets of NIRSpec.

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arXiv2016

Hubble Tarantula Treasury Project. IV. The extinction law

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 micron 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 slope of the reddening vector is remarkably steeper in all bands 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 significantly 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.

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arXiv2015

Do you know the extinction in your young massive cluster?

Up to ages of ~100 Myr, massive clusters are still swamped in large amounts of gas and dust, with considerable and uneven levels of extinction. At the same time, large grains (ices?) produced by type II supernovae profoundly alter the interstellar medium (ISM), thus resulting in extinction properties very different from those of the diffuse ISM. To obtain physically meaningful parameters of stars, from basic luminosities and effective temperatures to masses and ages, we must understand and measure the local extinction law. This problem affects all the massive young clusters discussed in his volume. We have developed a powerful method to unambiguously determine the extinction law in an uniform way across a cluster field, using multi-band photometry of red giant stars belonging to the red clump (RC). In the Large Magellanic Cloud, with about 20 RC stars per arcmin^2, we can easily derive a solid and self-consistent absolute extinction curve over the entire wavelength range of the photometry. Here, we present the extinction law of the Tarantula nebula (30 Dor) based on thousands of stars observed as part of the Hubble Tarantula Treasury Project.

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arXiv2015

Star formation in the Local Group as seen by low-mass stars

We have undertaken a systematic study of pre-main sequence (PMS) stars spanning a wide range of masses (0.5 - 4 Msolar), metallicities (0.1 - 1 Zsolar) and ages (0.5 - 30 Myr). We have used the Hubble Space Telescope (HST) to identify and characterise a large sample of PMS objects in several star-forming regions in the Magellanic Clouds, namely 30 Dor and the SN 1987A field in the LMC, and NGC 346 and NGC 602 in the SMC, and have compared them to PMS stars in similar regions in the Milky Way, such as NGC 3603 and Trumpler 14, which we studied with the HST and Very Large Telescope (VLT). We have developed a novel method that combines broad-band (V, I) photometry with narrow-band Halpha imaging to determine the physical parameters (temperature, luminosity, age, mass and mass accretion rate) of more than 3000 bona-fide PMS stars still undergoing active mass accretion. This is presently the largest and most homogeneous sample of PMS objects with known physical properties and includes not only very young objects, but also PMS stars older than 10 - 20 Myr that are approaching the main sequence (MS). We find that the mass accretion rate scales roughly with the square root of the age, with the mass of the star to the power of 1.5, and with the inverse of the cube root of the metallicity. The mass accretion rates for stars of the same mass and age are thus systematically higher in the Magellanic Clouds than in the Milky Way. These results are bound to have important implications for, and constraints on our understanding of the star formation process.

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arXiv2014

The extinction law inside the 30 Doradus nebula

We have studied the interstellar extinction in a field of ~3' x 3' at the core of the 30 Doradus nebula, including the central R136 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 micron. At optical wavelengths, the extinction curve is almost parallel to that of the diffuse Galactic interstellar medium. Taking the latter as a template, the value of Rv = 4.5 +/- 0.2 that we measure indicates that in the optical there is an extra grey component due to a larger fraction of large grains. At wavelengths longer than ~1 micron, the contribution of this additional component tapers off as lambda^-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 SN 1987A.

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arXiv2013

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

We have studied the interstellar extinction in a field of 3' x 3' located about 6' SW of 30 Doradus in the Large Magellanic Cloud (LMC). Hubble Space Telescope observations in the U, B, V, I and Halpha bands reveal patchy extinction in this field. The colour-magnitude diagram (CMD) shows an elongated stellar sequence, almost parallel to the main sequence (MS), which is in reality made up of stars of the red giant clump (RC) spread across the CMD by the uneven levels of extinction in this region. Since these objects are all at the same distance from us and share very similar physical properties, we can derive quantitatively both the extinction law in the range 3000 - 8000 Angstrom and the absolute extinction towards about 100 objects, setting statistically significant constraints on the dust grains properties in this area. We find an extinction curve considerably flatter than the standard Galactic one and than those obtained before for the LMC. The derived value of Rv = 5.6 +/- 0.3 implies that in this region larger grains dominate. Upper MS stars span a narrower range of E(B-V) values than RC objects, at variance with what has been found elsewhere in the LMC.

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arXiv2013

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

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 Halpha 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 arcsec 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 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.

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arXiv2013

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

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 Halpha 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.

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arXiv2012

Star Formation in the Starburst Cluster in NGC 3603

We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (~ 1 - 3 M_sun) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ~ 2 to ~ 30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20-30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East - North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction.

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arXiv2011

The non-segregated population of blue straggler stars in the remote globular cluster Palomar 14

We used deep wide-field observations obtained with the Canada-France-Hawaii Telescope to study the blue straggler star (BSS) population in the innermost five arcminutes of the remote Galactic globular cluster Palomar 14. The BSS radial distribution is found to be consistent with that of the normal cluster stars, showing no evidence of central segregation. Palomar 14 is the third system in the Galaxy (in addition to OmegaCentauri and NGC 2419) showing a population of BSS not centrally segregated. This is the most direct evidence that in Palomar 14 two-body relaxation has not fully established energy equipartition yet, even in the central regions (in agreement with the estimated half-mass relaxation time, which is significantly larger than the cluster age). These observational facts have important implications for the interpretation of the shape of the mass function and the existence of the tidal tails recently discovered in this cluster.

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