arXiv subjects

A. Nota

Publications and source records attributed to A. Nota.

At least 37 records · Page 2Linked to original sources

arXiv2011

Is the young star cluster NGC 376 dissolving in the field of the SMC?

We use deep images acquired with the Advanced Camera for Surveys (ACS) on board of the Hubble Space Telescope (HST) in the filters F555W and F814W to characterize the properties of NGC 376, a young star cluster located in the wing of the Small Magellanic Cloud (SMC). Using isochrone fitting we derive for NGC 376 an age of 28+/-7 Myr, in good agreement with previous studies. The high spatial resolution ACS data allow us to determine the center of gravity of the cluster and to construct extended surface brightness and radial density profiles. Neither of these profiles can be fitted with a theoretical model, suggesting that the cluster is not in virial equilibrium. Considering the young age of the cluster, we speculate that the distortion of the radial profiles may be the result of the rapid gas dispersal that follows the initial phase of star formation. The cluster shows clear evidence of dynamical mass segregation. From the properties of the radial profiles and the present day mass function (PDMF) we conclude that NGC 376 appears to have already lost nearly 90% of its initial stellar mass, probably as a consequence of the sudden gas dispersal that follows the early phase of star formation (SF).

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arXiv2010

A Panchromatic View OF NGC 602: Time-Resolved Star Formation with the Hubble and Spitzer Space Telescopes

We present the photometric catalogs for the star-forming cluster NGC 602 in the wing of the Small Magellanic Cloud covering a range of wavelengths from optical HST/ACS (F555W, F814W) and SMARTS/ANDICAM (V, I) to infrared (Spitzer/IRAC 3.6, 4.5, 5.8, and 8 micron and MIPS 24 micron). Combining this with IRSF (InfraRed Survey Facility) near-infrared photometry (J, H, Ks), we compare the young main sequence (MS) and pre-main sequence (PMS) populations prominent in the optical with the current young stellar object (YSO) populations revealed by the infrared (IR). We analyze the MS and PMS population with isochrones in color-magnitude diagrams to derive ages and masses. The optical data reveal ~565 PMS candidates, low mass Stage III YSOs. We characterize ~40 YSOs by fitting their spectral energy distributions (SEDs) to a grid of models (Robitaille et al. 2007) to derive luminosities, masses and evolutionary phase (Stage I-III). The higher resolution HST images reveal that ~70% of the YSO candidates are either multiples or protoclusters. For YSOs and PMS sources found in common, we find a consistency in the masses derived. We use the YSO mass function to derive a present-day star-formation rate of ~0.2-1.0 Msun/yr/kpc^2, similar to the rate derived from the optical star formation history suggesting a constant star formation rate for this region. We demonstrate a progression of star formation from the optical star cluster center to the edge of the star forming dust cloud. We derive lifetimes of a few 10^5 years for the YSO Stages I and II.

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arXiv2010

History and modes of star formation in the most active region of the Small Magellanic Cloud, NGC 346

We discuss the star formation history of the SMC region NGC 346 based on Hubble Space Telescope images. The region contains both field stars and cluster members. Using a classical synthetic CMD procedure applied to the field around NGC 346 we find that there the star formation pace has been rising from a quite low rate 13 Gyr ago to \approx 1.4 \times 10^{-8} Mo yr^{-1}pc^{-2} in the last 100 Myr. This value is significantly higher than in other star forming regions of the SMC. For NGC 346 itself, we compare theoretical and observed Color-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified in the region, and we derive their basic evolution parameters. We find that NGC 346 experienced different star formation regimes, including a dominant and focused "high density mode", with the sub-clusters hosting both pre-main sequence (PMS) and upper main sequence (UMS) stars, and a diffuse "low density mode", as indicated by the presence of low-mass PMS sub-clusters. Quantitatively, the star formation in the oldest sub-clusters started about 6 Myr ago with remarkable synchronization, it continued at high rate (up to 2 \times 10^{-5} Mo yr^{-1} pc^{-2}) for about 3 Myr and is now progressing at a lower rate. Interestingly, sub-clusters mainly composed by low mass PMS stars seem to experience now the first episode of star formation, following multi-seeded spatial patterns instead of resulting from a coherent trigger. Two speculative scenarios are put forth to explain the deficiency of UMS stars: the first invokes under-threshold conditions of the parent gas; the second speculates that the initial mass function (IMF) is a function of time, with the youngest sub-clusters not having had sufficient time to form more massive stars.

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arXiv2010

Pre-Main sequence Turn-On as a chronometer for young clusters: NGC346 as a benchmark

We present a novel approach to derive the age of very young star clusters, by using the Turn-On (TOn). The TOn is the point in the color-magnitude diagram (CMD) where the pre-main sequence (PMS) joins the main sequence (MS). In the MS luminosity function (LF) of the cluster, the TOn is identified as a peak followed by a dip. We propose that by combining the CMD analysis with the monitoring of the spatial distribution of MS stars it is possible to reliably identify the TOn in extragalactic star forming regions. Compared to alternative methods, this technique is complementary to the turn-off dating and avoids the systematic biases affecting the PMS phase. We describe the method and its uncertainties, and apply it to the star forming region NGC346, which has been extensively imaged with the Hubble Space Telescope (HST). This study extends the LF approach in crowded extragalactic regions and opens the way for future studies with HST/WFC3, JWST and from the ground with adaptive optics.

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arXiv2009

Star Formation History of the SMC: six HST/ACS

We observed six fields of the Small Magellanic Cloud (SMC) with the Advanced Camera for Survey on board the Hubble Space Telescope in the F555W and F814W filters. These fields sample regions characterized by very different star and gas densities, and, possibly, by different evolutionary histories. We find that the SMC was already forming stars ~12 Gyr ago, even if the lack of a clear horizontal branch suggests that in the first few billion years the star formation activity was low. Within the uncertainties of our two-band photometry, we find evidence of a radial variation in chemical enrichment, with the SMC outskirts characterized by lower metallicity than the central zones. From our CMDs we also infer that the SMC formed stars over a long interval of time until ~2-3 Gyr ago. After a period of modest activity, star formation increased again in the recent past, especially in the bar and the wing of the SMC, where we see an enhancement in the star-formation activity starting from ~500 Myr ago. The inhomogeneous distribution of stars younger than ~100 Myr indicates that recent star formation has mainly developed locally.

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arXiv2009

Star formation history in the SMC: the case of NGC602

Deep HST/ACS photometry of the young cluster NGC 602, located in the remote low density "wing" of the Small Magellanic Cloud, reveals numerous pre-main sequence stars as well as young stars on the main sequence. The resolved stellar content thus provides a basis for studying the star formation history into recent times and constraining several stellar population properties, such as the present day mass function, the initial mass function and the binary fraction. To better characterize the pre-main sequence population, we present a new set of model stellar evolutionary tracks for this evolutionary phase with metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a stellar population synthesis code, which takes into account a full range of stellar evolution phases to derive our best estimate for the star formation history in the region by comparing observed and synthetic color-magnitude diagrams. The derived present day mass function for NGC 602 is consistent with that resulting from the synthetic diagrams. The star formation rate in the region has increased with time on a scale of tens of Myr, reaching $0.3-0.7 \times 10^{-3} M_\odot yr^{-1}$ in the last 2.5 Myr, comparable to what is found in Galactic OB associations. Star formation is most complete in the main cluster but continues at moderate levels in the gas-rich periphery of the nebula.

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arXiv2008

SMC in space and time: a project to study the evolution of the prototype interacting late-type dwarf galaxy

We introduce the SMC in space and time, a large coordinated space and ground-based program to study star formation processes and history, as well as variable stars, structure, kinematics and chemical evolution of the whole SMC. Here, we present the Colour-Magnitude Diagrams(CMDs) resulting from HST/ACS photometry, aimed at deriving the star formation history (SFH) in six fields of the SMC. The fields are located in the central regions, in the stellar halo, and in the wing toward the LMC. The CMDs are very deep, well beyond the oldest Main Sequence Turn-Off, and will allow us to derive the SFH over the entire Hubble time.

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arXiv2008

NGC 602 Environment, Kinematics and Origins

The young star cluster NGC 602 and its associated HII region, N90, formed in a relatively isolated and diffuse environment in the Wing of the Small Magellanic Cloud. Its isolation from other regions of massive star formation and the relatively simple surrounding HI shell structure allows us to constrain the processes that may have led to its formation and to study conditions leading to massive star formation. We use images from Hubble Space Telescope and high resolution echelle spectrographic data from the Anglo-Australian Telescope along with 21-cm neutral hydrogen (HI) spectrum survey data and the shell catalogue derived from it to establish a likely evolutionary scenario leading to the formation of NGC 602. We identify a distinct HI cloud component that is likely the progenitor cloud of the cluster and HII region which probably formed in blister fashion from the cloud's periphery. We also find that the past interaction of HI shells can explain the current location and radial velocity of the nebula. The surrounding Interstellar Medium is diffuse and dust-poor as demonstrated by a low visual optical depth throughout the nebula and an average HI density of the progenitor cloud estimated at 1 cm^-3. These conditions suggest that the NGC 602 star formation event was produced by compression and turbulence associated with HI shell interactions. It therefore represents a single star forming event in a low gas density region.

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arXiv2008

An Accurate Age Determination for the SMC Star Cluster NGC121 with HST/ACS

As first Paper of a series devoted to study the old stellar population in clusters and fields in the Small Magellanic Cloud, we present deep observations of NGC121 in the F555W and F814W filters, obtained with the Advanced Camera for Surveys on the Hubble Space Telescope. The resulting color-magnitude diagram reaches ~3.5 mag below the main-sequence turn-off; deeper than any previous data. We derive the age of NGC121 using both absolute and relative age-dating methods. Fitting isochrones in the ACS photometric system to the observed ridge line of NGC121, gives ages of 11.8 +- 0.5 Gyr (Teramo), 11.2 +- 0.5 Gyr (Padova) and 10.5 +- 0.5 Gyr (Dartmouth). The cluster ridge line is best approximated by the alpha-enhanced Dartmouth isochrones. Placing our relative ages on an absolute age scale, we find ages of 10.9 +- 0.5 Gyr (from the magnitude difference between the main-sequence turn-off and the horizontal branch) and 11.5 +- 0.5 Gyr (from the absolute magnitude of the horizontal branch), respectively. These five different age determinations are all lower by 2 - 3 Gyr than the ages of the oldest Galactic globular clusters of comparable metallicity. Therefore we confirm the earlier finding that the oldest globular cluster in the Small Magellanic Cloud, NGC121, is a few Gyr younger than its oldest counterparts in the Milky Way and in other nearby dwarf galaxies such as the Large Magellanic Cloud, Fornax, and Sagittarius. If it were accreted into the Galactic halo, NGC121 would resemble the ''young halo globulars'', although it is not as young as the youngest globular clusters associated with the Sagittarius dwarf. The young age of NGC121 could result from delayed cluster formation in the Small Magellanic Cloud or result from the random survival of only one example of an initially small number star clusters.

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arXiv2007

The Stellar Mass Distribution in the Giant Star Forming Region NGC 346

Deep F555W and F814W Hubble Space Telescope ACS images are the basis for a study of the present day mass function (PDMF) of NGC346, the largest active star forming region in the Small Magellanic Cloud (SMC). We find a PDMF slope of Gamma=-1.43+/-0.18 in the mass range 0.8-60 Mo, in excellent agreement with the Salpeter Initial Mass Function (IMF) in the solar neighborhood. Caveats on the conversion of the PDMF to the IMF are discussed. The PDMF slope changes, as a function of the radial distance from the center of the NGC 346 star cluster, indicating a segregation of the most massive stars. This segregation is likely primordial considering the young age (~3 Myr) of NGC346, and its clumpy structure which suggests that the cluster has likely not had sufficient time to relax. Comparing our results for NGC346 with those derived for other star clusters in the SMC and the Milky Way (MW), we conclude that, while the star formation process might depend on the local cloud conditions, the IMF does not seem to be affected by general environmental effects such as galaxy type, metallicity, and dust content.

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arXiv2007

Progressive star formation in the young SMC cluster NGC 602

NGC 602 is a young stellar cluster located in a peripheral region of the Small Magellanic Cloud known as the wing. Far from the main body of the galaxy and abutting the Magellanic Bridge, the SMC's wing is characterized by low gas and stellar content. With deep optical imaging from the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST), we have discovered an extensive pre-Main Sequence (PMS) population, with stellar masses in the range 0.6-3 Solar masses. These low mass PMS stars formed coevally with the central cluster about 4 Myr ago. Spitzer Space Telescope (Spitzer) images of the same region from the Infrared Array Camera (IRAC) also reveal a population of Young Stellar Objects (YSOs), some of which are still embedded in nebular material and most of which likely formed even more recently than the young stars detected with HST/ACS imaging. We infer that star formation started in this region about 4 Myr ago with the formation of the central cluster and gradually propagated towards the outskirts where star formation is presently ongoing.

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arXiv2006

Past and present star formation in the SMC: NGC 346 and its neighborhood

In the quest of understanding how star formation occurs and propagates in the low metallicity environment of the Small Magellanic Cloud (SMC), we acquired deep F555W (~V), and F814W (~I) HST/ACS images of the young and massive star forming region NGC 346. These images and their photometric analysis provide us with a snapshot of the star formation history of the region. We find evidence for star formation extending from ~10 Gyr in the past until ~150 Myr in the field of the SMC. The youngest stellar population (~3 +/- 1 Myr) is associated with the NGC 346 cluster. It includes a rich component of low mass pre-main sequence stars mainly concentrated in a number of sub-clusters, spatially co- located with CO clumps previously detected by Rubio et al. (2000). Within our analysis uncertainties, these sub-clusters appear coeval with each other. The most massive stars appear concentrated in the central sub-clusters, indicating possible mass segregation. A number of embedded clusters are also observed. This finding, combined with the overall wealth of dust and gas, could imply that star formation is still active. An intermediate age star cluster, BS90, formed ~4.3 +/-0.1 Gyr ago, is also present in the region. Thus, this region of the SMC has supported star formation with varying levels of intensity over much of the cosmic time.

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arXiv2006

Spitzer SAGE survey of the Large Magellanic Cloud II: Evolved Stars and Infrared Color Magnitude Diagrams

Color-magnitude diagrams (CMDs) are presented for the Spitzer SAGE (Surveying the Agents of a Galaxy's Evolution) survey of the Large Magellanic Cloud (LMC). IRAC and MIPS 24 um epoch one data are presented. These data represent the deepest, widest mid-infrared CMDs of their kind ever produced in the LMC. Combined with the 2MASS survey, the diagrams are used to delineate the evolved stellar populations in the Large Magellanic Cloud as well as Galactic foreground and extragalactic background populations. Some 32000 evolved stars brighter than the tip of the red giant branch are identified. Of these, approximately 17500 are classified as oxygen-rich, 7000 carbon-rich, and another 1200 as ``extreme'' asymptotic giant branch (AGB) stars. Brighter members of the latter group have been called ``obscured'' AGB stars in the literature owing to their dusty circumstellar envelopes. A large number (1200) of luminous oxygen--rich AGB stars/M supergiants are also identified. Finally, there is strong evidence from the 24 um MIPS channel that previously unexplored, lower luminosity oxygen-rich AGB stars contribute significantly to the mass loss budget of the LMC (1200 such sources are identified).

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arXiv2006

The Discovery of a Population of Pre-Main Sequence Stars in N66/NGC346 from Deep HST/ACS Image

We report the discovery of a rich population of low mass stars in the young, massive star forming region N66/NGC346 in the Small Magellanic Cloud, from deep V, I and H alpha images taken with the HST/ACS. These stars have likely formed together with the NGC346 cluster, ~3-5 Myr ago. Their magnitude and colors are those of pre-main sequence stars in the mass range 0.6-3 Mo, mostly concentrated in the main cluster, but with secondary subclusters spread over a region across ~45 pc. These subclusters appear to be spatially coincident with previously known knots of molecular gas identified in ground based and ISO observations. We show that N66/NGC346 is a complex region, being shaped by its massive stars, and the observations presented here represent a key step towards the understanding of how star formation occurred and has progressed in this low metallicity environment.

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arXiv2005

The Birth-Cluster of the Galactic Luminous Blue Variable WRA751

We present the results of NTT/VLT UBV imaging of a 260 square arcmin region containing the Galactic Luminous Blue Variable WRA751, in search for its birth-cluster, i.e. a cluster of young and massive stars spatially and physically associated with it. On the basis of the classical reddening-free parameter Q, we have identified a sample of 24 early-type stars with colours typical of spectral types earlier than B3. Interestingly, these stars are clustered within a radius of 1 arcmin from WRA751, corresponding to about 1% of the imaged field. These stars tightly distribute around (B-V) = 1.67, which in turn defines a mean extinction A(V) = 6.1 mag. The 5 brighter (V > 16.2) and bluer (Q < -0.9) stars of the sample have been subsequently observed with FORS1 and classified as 3 late O- and 2 early B- stars. The absence of stars earlier than O8 indicates an age of the cluster older than 4 Myr, although it could be due to an incomplete sampling of the upper end of the main sequence. Nevertheless, the detection of OB stars of class I certainly indicates an age of a few million years. At an assumed distance of 6 kpc, we estimate a cluster radius of 3.4 pc and a total mass of 2200 solar masses. Our discovery is only the second known instance of a Galactic Luminous Blue Variable associated with its birth-cluster.

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arXiv2002

Detection of CO emission 12 CO J=1->0 and 12 CO J=2->1 from the Luminous Blue Variable Star AG Carinae: circumstellar envelope or disk?

We present the first detection of 12 CO J=2->1 and 12 CO J=1->0 emission from the LBV AG Carinae. AG Carinae resides in a region which is very rich in molecular gas with complex motions. We find evidence of a slow outflow of molecular gas, expanding at ~ 7 km/s. This emission appears spatially unresolved. We argue that it is spatially localised, rather than extended, and possibly associated with the immediate circumstellar region of AG Carinae. Does it originate from a circumstellar envelope, similar to carbon stars, or from a circumstellar disk? The option of the circumstellar disk is preferable because it is consistent with additional independent indications for the existence of wind asymmetries in close proximity to the central star, found from spectropolarimetry and analysis of the UV and optical line profiles, and it provides the conditions of density and shielding necessary for the survival of the CO molecules in proximity to such a hot star (Teff ~ 14000 K - 20000 K). In the assumption that the CO emission originated when AG Carinae was in an evolved state, we derive a lower limit to the mass of molecular gas of 2.8 solar masses. This is smaller, but still comparable with the mass of ionized gas present in the circumstellar environment (4.2 solar masses), with the implication that the molecular gas fraction can contribute significantly to the overall mass lost from the central star in its post main sequence evolution.

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arXiv2002

Multi-wavelength study of the nebula associated with the galactic LBV candidate HD 168625

We present high resolution HST imaging of the nebula associated with the galactic LBV candidate HD 168625, together with ISO imaging and AAT echelle spectroscopy. The overall nebular morphology is elliptical with the major axis at PA ~ 120 deg. The dimensions of the nebula are 12'' x 16''.7 at Halpha and 15''.5 x 23''.5 at 4 micron. In the HST Halpha image, the nebula is resolved into a complex structure of filaments and arcs of different brightness. The asymmetry is lost in the HST continuum image where the nebula appears more diffuse and richer in filaments and clumps with the shape of cometary tails. At 11.3 micron the nebular emission peaks in two diametrically opposite lobes, placed on the nebula boundaries and along its major axis. A very faint loop is also visible at optical wavelengths, north and south of the shell. We suggest that the nebula is an ellipsoid with projected sizes of 14'' and 9'' (0.19 pc x 0.12 pc) along the RA and DEC directions, respectively. This ellipsoid is expanding at 19 km/s and is dynamically as old as ~ 4800 yrs; it probably interacts with the stellar wind and the loop so that PAH emission is detected from its caps, i.e. the lobes seen in the ISO images. The chemistry of the loop suggests that it is composed of un-processed material, probably from the local interstellar medium swept by the stellar wind.

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arXiv2000

The red stellar population in NGC 1569

We present HST NICMOS photometry of the resolved stellar population in NGC 1569. The CMD in the F110W and F160W photometric bands contains ~2400 stars with a formal photometric error < 0.1 mag down to J~23.5 and H~22.5. We describe the data processing which is required to calibrate the instrumental peculiarities of NICMOS. Two different packages for PSF-fitting photometry are used to strengthen the photometric results in the crowded stellar field of NGC 1569. The resulting CMD is discussed in terms of the major evolutionary properties of the resolved stellar populations. For a distance modulus of 26.71 and a reddening E(B-V)=0.56, our CMD samples stars down to ~0.8 Mo, corresponding to look-back times > 15 Gyr. This is clear indication of SF activity spanning an entire Hubble time. The metallicity of the reddest RGB stars is in better agreement with Z=0.004 as measured in HII regions, than with Z=0.0004 as expected from the stellar ages. The presence of - yet undetected - very metal-poor stars embedded in the stellar distribution around J=22.75 and J-H=1.15 is, however, not ruled out. The youngest stars (< 50 Myr) are preferentially found around the two central super star clusters, whereas the oldest population has a more uniform spatial distribution. A SFR per unit area of 1 Mo yr*(-1) kpc*(-2) and a mass formed in stars of ~ 1.4x10*6 Mo in the last 50 Myr are derived from the CMD. The NIR CMD places strong constraints on the lower limit of the onset of SF in NGC 1569. The exceptionally high crowding in the NICMOS images of NGC 1569 is a challenge for the photometric analysis. As a result, optical and NIR images of NGC 1569 sample different populations and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of the SF histories derived from the optical and NIR CMDs.

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