Proceedings of the International Astronomical Union, 2009
For our understanding of the origin and evolution of baryonic matter in the Universe, the Protoso... more For our understanding of the origin and evolution of baryonic matter in the Universe, the Protosolar Cloud (PSC) is of unique importance in two ways: 1) Up to now, many of the naturally occurring nuclides have only been detected in the solar system. 2) Since the time of solar system formation, the Sun and planets have been virtually isolated from the galactic nuclear evolution, and thus the PSC is a galactic sample with a degree of evolution intermediate between the Big Bang and the present.The abundances of the isotopes of hydrogen and helium in the Protosolar Cloud are primarily derived from composition measurements in the solar wind, the Jovian atmosphere and “planetary noble gases” in meteorites, and also from observations of density profiles inside the Sun. After applying the changes in isotopic and elemental composition resulting from processes in the solar wind, the Sun and Jupiter, PSC abundances of the four lightest stable nuclides are given.
FUSE suitable for most abundant species-HI, OI, Ar I, Si II, P II, C II, NI, Fe II HI already enr... more FUSE suitable for most abundant species-HI, OI, Ar I, Si II, P II, C II, NI, Fe II HI already enriched in heavy elements, i.e. not primordial Existence of a metallicity offset between neutral and ionized gas, except for Fe SF at least ~ 1 Gyr old required to explain Fe in ISM Constraints to chemical evolution models
Proceedings of the International Astronomical Union, 2006
We present results from new deep HST/ACS photometry of I Zw 18, the most metalpoor blue compact d... more We present results from new deep HST/ACS photometry of I Zw 18, the most metalpoor blue compact dwarf galaxy in the nearby universe. It has been previously argued that this is a very young system that started forming stars only 500 Myr ago, but other work has hinted that older (1 Gyr) red giant branch (RGB) stars may exist in this galaxy. Our deeper data indeed reveal evidence for an RGB. Underlying old (1 Gyr) populations are therefore present in even the most metal-poor systems, implying that star formation started at z 0.1. The RGB tip (TRGB) magnitude and the properties of Cepheid variables identified from our program indicate that I Zw 18 is farther away (D = 19.0 ± 1.8 Mpc) than previously believed.
Proceedings of the International Astronomical Union, 2006
IZw18 has been recurrently claimed to be a young galaxy, but stars of increasingly older ages are... more IZw18 has been recurrently claimed to be a young galaxy, but stars of increasingly older ages are found every time deeper magnitude levels are reached with high-resolution photometry: from the original few Myrs to, possibly, several Gyrs. We summarize the history of IZw18's age and an HST project which will allow us to derive both its distance and age.
We describe the approach adopted by our group to derive the star formation history (SFH) of the c... more We describe the approach adopted by our group to derive the star formation history (SFH) of the chosen LMC field, as part of the experiment to compare the predictions obtained by different groups with the synthetic Colour-Magnitude diagram method. We point out what are the evolutionary characteristics of the observed stellar populations, and present the SFH scenario which appears to better account for them. We emphasize the importance of adopting a critical approach when dealing with this method.
The evolutionary properties and spatial distribution of I Zwicky 18 stellar populations are analy... more The evolutionary properties and spatial distribution of I Zwicky 18 stellar populations are analyzed by means of HST/ACS deep and accurate photometry. The comparison of the resulting Colour-Magnitude diagrams with stellar evolution models indicates that stars of all ages are present in all the system components, including objects possibly up to 13 Gyr old, intermediate age stars and very young ones. The Colour-Magnitude diagrams show evidence of thermally pulsing asymptotic giant branch and carbon stars. Classical and ultra-long period Cepheids, as well as long period variables have been measured. About 20 objects could be unresolved star clusters, and are mostly concentrated in the NorthWest (NW) portion of the Main Body (MB). If interpreted with simple stellar population models, these objects indicate a particularly active star formation over the past hundred Myr in IZw 18. The stellar spatial distribution shows that the younger ones are more centrally concentrated, while old and intermediate age stars are distributed homogeneously over the two bodies, although more easily detectable at the system periphery. The oldest stars are best visible in the Secondary Body (SB) and in the South East (SE) portion of the MB, where crowding is less severe, but are present also in the rest of the MB, although measured with larger uncertainties. The youngest stars are a few Myr old, are located predominantly in the MB and mostly concentrated in its NW portion. The SE portion of the MB appears to be in a similar, but not as young evolutionary stage as the NW, while the SB stars are older than at least 10 Myr. There is then a sequence of decreasing age of the younger stars from the Secondary Body to the SE portion of the MB to the NW portion. All our results suggest that IZw18 is not atypical compared to other BCDs.
We present high-resolution Hubble Space Telescope ultraviolet spectroscopy of NGC 3242, the only ... more We present high-resolution Hubble Space Telescope ultraviolet spectroscopy of NGC 3242, the only planetary nebula with a measured abundance of 3 He. The Space Telescope Imaging Spectrograph has been used to observe the C iii multiplet near 1908 Å. The presence of 12 C and 13 C lines at these wavelengths allows a direct estimate of the carbon isotopic ratio in the ionized gas of the nebula. We have detected the 12 C doublet and obtained an upper limit on the 13 C 1909.6 Å line, resulting in a carbon isotopic ratio of 12 C/ 13 , in agreement with C 1 38 standard stellar models. The lack of the 13 C line and the presence of the 3 He line in the spectrum indicate that the progenitor star did not undergo a phase of deep mixing during the last stages of its evolution. The significance of this result for studies of stellar nucleosynthesis and Galactic chemical evolution is discussed.
We report the discovery of a rich population of low mass stars in the young, massive star forming... more 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 Hα 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 M ⊙ , 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.
We infer the star formation history in different regions of the blue compact dwarf NGC 1705 by co... more We infer the star formation history in different regions of the blue compact dwarf NGC 1705 by comparing synthetic color-magnitude diagrams with HST optical and near-infrared photometry. We find that NGC 1705 is not a young galaxy because its star formation commenced at least 5 Gyr ago. On the other hand, we confirm the existence of a recent burst of star formation between 15 and 10 Myr ago. We also find evidence for new strong activity, which started 3 Myr ago and is still continuing. The old population is spread across the entire galaxy, while the young and intermediate stars are more concentrated in the central regions. We derive an almost continuous star formation with variable rate, and exclude the presence of long quiescent phases between the episodes during the last ≈ 1 Gyr. The central regions experienced an episode of star formation of ∼ 0.07 M ⊙ yr −1 (for a Salpeter initial mass function [IMF]) 15 to 10 Myr ago. This coincides with the strong activity in the central super star cluster. We find a rate of ∼ 0.3 M ⊙ yr −1 for the youngest ongoing burst which started ∼ 3 Myr ago. This is higher than in other dwarfs and comparable to the rate of NGC 1569. The star formation rate of earlier episodes is not especially high and falls in the range 10 −3 − 10 −1 M ⊙ yr −1. The IMF is close to the Salpeter value or slightly steeper.
We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (H... more We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I) broad-band filters, and in the F658N (Hα) narrow-band filter. Our photometry includes ≈ 300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V 28, and ≈ 400,000 objects in the (V, I) CMD, down to I 27. A subsample of ≈ 200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (0.2 dex). Also, we detect an interesting resolved star cluster on the West side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Hα emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D = 3.82 ± 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.
Monthly Notices of the Royal Astronomical Society, 2006
We present charge-coupled device UBVI photometry and high-resolution spectroscopy of the intermed... more We present charge-coupled device UBVI photometry and high-resolution spectroscopy of the intermediate-age open cluster NGC 3960. The colour-magnitude diagrams (CMDs) derived from the photometric data and interpreted with the synthetic CMD method allow us to estimate the cluster parameters. We derive the following: age τ = 0.9 or 0.6 Gyr (depending on whether or not overshooting from convective regions is included in the adopted stellar models); distance (m − M) 0 = 11.6 ± 0.1; reddening E(B − V) = 0.29 ± 0.02; differential reddening E(B − V) = 0.05; and approximate metallicity between solar and half-solar. We obtained highresolution spectra of three clump stars, and derived an average [Fe/H] = −0.12 (rms 0.04 dex), in very good agreement with the photometric determination. We also obtained the abundances of α-elements, of Fe-peak elements and of Ba. The reddenings towards individual stars derived from the spectroscopic temperatures and the calibrations of Alonso et al. give further support to the existence of significant variations across the cluster.
Monthly Notices of the Royal Astronomical Society, 2002
Time-series of B, V, I CCD photometry and radial velocity measurements from highresolution spectr... more Time-series of B, V, I CCD photometry and radial velocity measurements from highresolution spectroscopy (R = 30 000) covering the full pulsation cycle are presented for the field RR Lyrae star CM Leonis. The photometric data span a 6-yr interval from 1994 to 1999, and allow us to firmly establish the pulsation mode and periodicity of the variable. The derived period P = 0.361 699 d (± 0.000001) is very close to the value published in the Fourth Edition of the General Catalogue of Variable Stars (P = 0.361 732 d). However, contrary to what was previously found, the amplitude and shape of the light curve qualify CM Leo as a very regular first overtone pulsator with a prominent hump on the rising branch of its multicolour light curves. According to an abundace analysis performed on three spectra taken near minimum light (0.42 < φ < 0.61), CM Leo is a metal-poor star with metal abundance [Fe/H] = −1.93 ± 0.20. The photometric and radial velocity curves of CM Leo have been compared with the predictions of suitable pulsational models to infer tight constraints on the stellar mass, effective temperature, and distance modulus of the star. We derive a true distance modulus of CM Leo of µ 0 = 13.11 ± 0.02 mag and a corresponding absolute magnitude of M V = 0.47 ± 0.04. This absolute magnitude, once corrected for evolutionary and metallicity effects, leads to a true distance modulus of the Large Magellanic Cloud of µ 0 = 18.43 ± 0.06 mag, in better agreement with the long astronomical distance scale.
Monthly Notices of the Royal Astronomical Society, 1997
UBGVRI photometry for the open cluster NGC 2506 is presented. From comparison of the observed col... more UBGVRI photometry for the open cluster NGC 2506 is presented. From comparison of the observed colour-magnitude diagrams with simulations based on stellar evolutionary models we derive in a self-consistent way the reddening, distance and age of the cluster: E (B-V) = 0-0.07, (m-M)o = 12.6 ± 0.1, 'r = 1.5-2.2 Gyr. The cluster shows a well-defined secondary sequence, suggesting that binary systems constitute ~20 per cent of the cluster members visible in the colour-magnitude diagram.
Monthly Notices of the Royal Astronomical Society, 2001
We present CCD BV I photometry of the intermediate age open cluster Pismis 2, covering from the b... more We present CCD BV I photometry of the intermediate age open cluster Pismis 2, covering from the brighter red giants to about 5 magnitudes below the main sequence turn-off. The cluster is heavily reddened and is possibly affected by a differential reddening of ∆E(B − V) ≃ 0.04-0.06. Using the synthetic Colour-Magnitude Diagram method, we estimate in a selfconsistent way distance modulus, (m-M) 0 ≃ 12.5-12.7, and age, τ ≃ 1.1-1.2 Gyr. The cluster probably contains a significant fraction of binary systems. The metallicity is most likely solar and the reddening E(B − V) ranges between 1.26 and 1.32 depending on the cluster region.
Monthly Notices of the Royal Astronomical Society, 2006
We have obtained BVI CCD imaging of Berkeley 17, an anticentre open cluster that competes with NG... more We have obtained BVI CCD imaging of Berkeley 17, an anticentre open cluster that competes with NGC 6791 as the oldest known open cluster. Using the synthetic colour-magnitude diagrams (CMDs) technique with three sets of evolutionary tracks, we have determined that its age is 8.5-9.0 Gyr, it distance modulus is (m − M) 0 = 12.2, with a reddening of E(B − V) = 0.62-0.60. Differential reddening, if present, is at the 5 per cent level. All these values have been obtained using models with metallicity about half of solar (Z = 0.008 or 0.01 depending on the stellar evolution tracks), which allows us to reproduce the features of the cluster CMD better than other metallicities. Finally, from the analysis of a nearby comparison field, we think to have intercepted a portion of the disrupting Canis Major dwarf galaxy.
Monthly Notices of the Royal Astronomical Society, 1998
We present CCD BVI photometry of the old open cluster Berkeley 21, one of the most distant cluste... more We present CCD BVI photometry of the old open cluster Berkeley 21, one of the most distant clusters in the Galactic anticentre direction, and possibly the lowest metallicity object in the open clusters sample. Its position and metal abundance make it very important for the study of the Galactic disc. Using the synthetic colour-magnitude diagram method, we estimate values for the distance modulus ðm ¹ MÞ 0 ¼ 13:4-13.6, reddening EðB ¹ VÞ ¼ 0:74-0.78 (with possible differential absorption), and age ¼ 2:2-2.5 Gyr.
Monthly Notices of the Royal Astronomical Society, 2006
We have obtained medium-and low-resolution spectroscopy and BVI CCD imaging of Berkeley 32, an ol... more We have obtained medium-and low-resolution spectroscopy and BVI CCD imaging of Berkeley 32, an old open cluster which lies in the anticentre direction. From the radial velocities of 48 stars in the cluster direction, we found that 31 of them, in crucial evolutionary phases, are probable cluster members, with an average radial velocity of +106.7 (σ = 8.5) km s −1. From isochrone fitting to the colour-magnitude diagrams of Berkeley 32, we have obtained an age of 6.3 Gyr, (m − M) 0 = 12.48 and E(B − V) = 0.10. The best fit is obtained with Z = 0.008. A consistent distance, (m − M) 0 12.6 ± 0.1, has been derived from the mean magnitude of red clump stars with confirmed membership; we may assume (m − M) 0 12.55 ± 0.1. The colour-magnitude diagram of the nearby field observed to check for field stars contamination looks intriguingly similar to that of the Canis Major overdensity.
In this paper we present some results concerning the effects of two instantaneous starbursts, sep... more In this paper we present some results concerning the effects of two instantaneous starbursts, separated by a quiescent period, on the dynamical and chemical evolution of Blue Compact Dwarf galaxies. In particular, we compare the model results to the galaxy IZw18, which is a very metal-poor, gas-rich dwarf galaxy, possibly experiencing its first or second burst of star formation. We follow the evolution of a first weak burst of star formation followed by a second more intense one occurring after several hundred million years. We find that a galactic wind develops only during the second burst and that metals produced in the burst are preferentially lost, relative to the hydrogen gas. We predict the evolution of several chemical abundances (H, He, C, N, O, α-elements, Fe) in the gas inside and outside the galaxy, by taking into account, in detail, the chemical and energetical contributions from type II and Ia supernovae. We find that the abundances predicted for the star forming region are in good agreement with the H ii region abundances derived for IZw18. We also predict the abundances of C, N and O expected for the H i gas to be compared with future FUSE abundance determinations. We conclude that IZw18 must have experienced two bursts of star formation, one occurred ∼300 Myr ago and a recent one with an age between 4-7 Myr. However, by taking into account also other independent estimates, such as the color-magnitude diagram and the spectral energy distribution of stars in IZw18, and the fact that real starbursts are not instantaneous, we suggest that it is more likely that the burst age is between 4 and 15 Myr.
We study the dynamical and chemical evolution of a galaxy similar to IZw18 under the assumption o... more We study the dynamical and chemical evolution of a galaxy similar to IZw18 under the assumption of a continuous star formation during bursts. We adopt a 2-D hydrocode coupled with detailed chemical yields originating from SNeII, SNeIa and from single intermediate-mass stars. Different nucleosynthetic yields and different IMF slopes are tested. In most of the explored cases, a galactic wind develops, mostly carrying out of the galaxy the metal-enriched gas produced by the burst itself. The chemical species with the largest escape probabilities are Fe and N. Consequently, we predict that the [α/Fe] and [α/N] ratios outside the galaxy are lower than inside. In order to reproduce the chemical composition of IZw18, the best choice seems to be the adoption of the yields of Meynet & Maeder (2002) which take into account stellar rotation, although these authors do not follow the whole evolution of all the stars. Models with a flat IMF (x = 0.5) seem to be able to better reproduce the chemical properties of IZw18, but they inject in the gas a much larger amount of energy and the resulting galactic wind is very strong, at variance with observations. We also predict the evolution of the abundances in the H medium and compare them with recent FUSE observations.
Proceedings of the International Astronomical Union, 2009
For our understanding of the origin and evolution of baryonic matter in the Universe, the Protoso... more For our understanding of the origin and evolution of baryonic matter in the Universe, the Protosolar Cloud (PSC) is of unique importance in two ways: 1) Up to now, many of the naturally occurring nuclides have only been detected in the solar system. 2) Since the time of solar system formation, the Sun and planets have been virtually isolated from the galactic nuclear evolution, and thus the PSC is a galactic sample with a degree of evolution intermediate between the Big Bang and the present.The abundances of the isotopes of hydrogen and helium in the Protosolar Cloud are primarily derived from composition measurements in the solar wind, the Jovian atmosphere and “planetary noble gases” in meteorites, and also from observations of density profiles inside the Sun. After applying the changes in isotopic and elemental composition resulting from processes in the solar wind, the Sun and Jupiter, PSC abundances of the four lightest stable nuclides are given.
FUSE suitable for most abundant species-HI, OI, Ar I, Si II, P II, C II, NI, Fe II HI already enr... more FUSE suitable for most abundant species-HI, OI, Ar I, Si II, P II, C II, NI, Fe II HI already enriched in heavy elements, i.e. not primordial Existence of a metallicity offset between neutral and ionized gas, except for Fe SF at least ~ 1 Gyr old required to explain Fe in ISM Constraints to chemical evolution models
Proceedings of the International Astronomical Union, 2006
We present results from new deep HST/ACS photometry of I Zw 18, the most metalpoor blue compact d... more We present results from new deep HST/ACS photometry of I Zw 18, the most metalpoor blue compact dwarf galaxy in the nearby universe. It has been previously argued that this is a very young system that started forming stars only 500 Myr ago, but other work has hinted that older (1 Gyr) red giant branch (RGB) stars may exist in this galaxy. Our deeper data indeed reveal evidence for an RGB. Underlying old (1 Gyr) populations are therefore present in even the most metal-poor systems, implying that star formation started at z 0.1. The RGB tip (TRGB) magnitude and the properties of Cepheid variables identified from our program indicate that I Zw 18 is farther away (D = 19.0 ± 1.8 Mpc) than previously believed.
Proceedings of the International Astronomical Union, 2006
IZw18 has been recurrently claimed to be a young galaxy, but stars of increasingly older ages are... more IZw18 has been recurrently claimed to be a young galaxy, but stars of increasingly older ages are found every time deeper magnitude levels are reached with high-resolution photometry: from the original few Myrs to, possibly, several Gyrs. We summarize the history of IZw18's age and an HST project which will allow us to derive both its distance and age.
We describe the approach adopted by our group to derive the star formation history (SFH) of the c... more We describe the approach adopted by our group to derive the star formation history (SFH) of the chosen LMC field, as part of the experiment to compare the predictions obtained by different groups with the synthetic Colour-Magnitude diagram method. We point out what are the evolutionary characteristics of the observed stellar populations, and present the SFH scenario which appears to better account for them. We emphasize the importance of adopting a critical approach when dealing with this method.
The evolutionary properties and spatial distribution of I Zwicky 18 stellar populations are analy... more The evolutionary properties and spatial distribution of I Zwicky 18 stellar populations are analyzed by means of HST/ACS deep and accurate photometry. The comparison of the resulting Colour-Magnitude diagrams with stellar evolution models indicates that stars of all ages are present in all the system components, including objects possibly up to 13 Gyr old, intermediate age stars and very young ones. The Colour-Magnitude diagrams show evidence of thermally pulsing asymptotic giant branch and carbon stars. Classical and ultra-long period Cepheids, as well as long period variables have been measured. About 20 objects could be unresolved star clusters, and are mostly concentrated in the NorthWest (NW) portion of the Main Body (MB). If interpreted with simple stellar population models, these objects indicate a particularly active star formation over the past hundred Myr in IZw 18. The stellar spatial distribution shows that the younger ones are more centrally concentrated, while old and intermediate age stars are distributed homogeneously over the two bodies, although more easily detectable at the system periphery. The oldest stars are best visible in the Secondary Body (SB) and in the South East (SE) portion of the MB, where crowding is less severe, but are present also in the rest of the MB, although measured with larger uncertainties. The youngest stars are a few Myr old, are located predominantly in the MB and mostly concentrated in its NW portion. The SE portion of the MB appears to be in a similar, but not as young evolutionary stage as the NW, while the SB stars are older than at least 10 Myr. There is then a sequence of decreasing age of the younger stars from the Secondary Body to the SE portion of the MB to the NW portion. All our results suggest that IZw18 is not atypical compared to other BCDs.
We present high-resolution Hubble Space Telescope ultraviolet spectroscopy of NGC 3242, the only ... more We present high-resolution Hubble Space Telescope ultraviolet spectroscopy of NGC 3242, the only planetary nebula with a measured abundance of 3 He. The Space Telescope Imaging Spectrograph has been used to observe the C iii multiplet near 1908 Å. The presence of 12 C and 13 C lines at these wavelengths allows a direct estimate of the carbon isotopic ratio in the ionized gas of the nebula. We have detected the 12 C doublet and obtained an upper limit on the 13 C 1909.6 Å line, resulting in a carbon isotopic ratio of 12 C/ 13 , in agreement with C 1 38 standard stellar models. The lack of the 13 C line and the presence of the 3 He line in the spectrum indicate that the progenitor star did not undergo a phase of deep mixing during the last stages of its evolution. The significance of this result for studies of stellar nucleosynthesis and Galactic chemical evolution is discussed.
We report the discovery of a rich population of low mass stars in the young, massive star forming... more 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 Hα 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 M ⊙ , 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.
We infer the star formation history in different regions of the blue compact dwarf NGC 1705 by co... more We infer the star formation history in different regions of the blue compact dwarf NGC 1705 by comparing synthetic color-magnitude diagrams with HST optical and near-infrared photometry. We find that NGC 1705 is not a young galaxy because its star formation commenced at least 5 Gyr ago. On the other hand, we confirm the existence of a recent burst of star formation between 15 and 10 Myr ago. We also find evidence for new strong activity, which started 3 Myr ago and is still continuing. The old population is spread across the entire galaxy, while the young and intermediate stars are more concentrated in the central regions. We derive an almost continuous star formation with variable rate, and exclude the presence of long quiescent phases between the episodes during the last ≈ 1 Gyr. The central regions experienced an episode of star formation of ∼ 0.07 M ⊙ yr −1 (for a Salpeter initial mass function [IMF]) 15 to 10 Myr ago. This coincides with the strong activity in the central super star cluster. We find a rate of ∼ 0.3 M ⊙ yr −1 for the youngest ongoing burst which started ∼ 3 Myr ago. This is higher than in other dwarfs and comparable to the rate of NGC 1569. The star formation rate of earlier episodes is not especially high and falls in the range 10 −3 − 10 −1 M ⊙ yr −1. The IMF is close to the Salpeter value or slightly steeper.
We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (H... more We present photometry with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) of stars in the Magellanic starburst galaxy NGC 4449. The galaxy has been imaged in the F435W (B), F555W (V) and F814W (I) broad-band filters, and in the F658N (Hα) narrow-band filter. Our photometry includes ≈ 300,000 objects in the (B, V) color-magnitude diagram (CMD) down to V 28, and ≈ 400,000 objects in the (V, I) CMD, down to I 27. A subsample of ≈ 200,000 stars has been photometrized in all the three bands simultaneously. The features observed in the CMDs imply a variety of stellar ages up to at least 1 Gyr, and possibly as old as a Hubble time. The spatial variation of the CMD morphology and of the red giant branch colors point toward the presence of an age gradient: young and intermediate-age stars tend to be concentrated toward the galactic center, while old stars are present everywhere. The spatial variation in the average luminosity of carbon stars suggests that there is not a strong metallicity gradient (0.2 dex). Also, we detect an interesting resolved star cluster on the West side of the galaxy, surrounded by a symmetric tidal or spiral feature consisting of young stars. The positions of the stars in NGC 4449 younger than 10 Myr are strongly correlated with the Hα emission. We derive the distance of NGC 4449 from the tip of the red giant branch to be D = 3.82 ± 0.27 Mpc. This result is in agreement with the distance that we derive from the luminosity of the carbon stars.
Monthly Notices of the Royal Astronomical Society, 2006
We present charge-coupled device UBVI photometry and high-resolution spectroscopy of the intermed... more We present charge-coupled device UBVI photometry and high-resolution spectroscopy of the intermediate-age open cluster NGC 3960. The colour-magnitude diagrams (CMDs) derived from the photometric data and interpreted with the synthetic CMD method allow us to estimate the cluster parameters. We derive the following: age τ = 0.9 or 0.6 Gyr (depending on whether or not overshooting from convective regions is included in the adopted stellar models); distance (m − M) 0 = 11.6 ± 0.1; reddening E(B − V) = 0.29 ± 0.02; differential reddening E(B − V) = 0.05; and approximate metallicity between solar and half-solar. We obtained highresolution spectra of three clump stars, and derived an average [Fe/H] = −0.12 (rms 0.04 dex), in very good agreement with the photometric determination. We also obtained the abundances of α-elements, of Fe-peak elements and of Ba. The reddenings towards individual stars derived from the spectroscopic temperatures and the calibrations of Alonso et al. give further support to the existence of significant variations across the cluster.
Monthly Notices of the Royal Astronomical Society, 2002
Time-series of B, V, I CCD photometry and radial velocity measurements from highresolution spectr... more Time-series of B, V, I CCD photometry and radial velocity measurements from highresolution spectroscopy (R = 30 000) covering the full pulsation cycle are presented for the field RR Lyrae star CM Leonis. The photometric data span a 6-yr interval from 1994 to 1999, and allow us to firmly establish the pulsation mode and periodicity of the variable. The derived period P = 0.361 699 d (± 0.000001) is very close to the value published in the Fourth Edition of the General Catalogue of Variable Stars (P = 0.361 732 d). However, contrary to what was previously found, the amplitude and shape of the light curve qualify CM Leo as a very regular first overtone pulsator with a prominent hump on the rising branch of its multicolour light curves. According to an abundace analysis performed on three spectra taken near minimum light (0.42 < φ < 0.61), CM Leo is a metal-poor star with metal abundance [Fe/H] = −1.93 ± 0.20. The photometric and radial velocity curves of CM Leo have been compared with the predictions of suitable pulsational models to infer tight constraints on the stellar mass, effective temperature, and distance modulus of the star. We derive a true distance modulus of CM Leo of µ 0 = 13.11 ± 0.02 mag and a corresponding absolute magnitude of M V = 0.47 ± 0.04. This absolute magnitude, once corrected for evolutionary and metallicity effects, leads to a true distance modulus of the Large Magellanic Cloud of µ 0 = 18.43 ± 0.06 mag, in better agreement with the long astronomical distance scale.
Monthly Notices of the Royal Astronomical Society, 1997
UBGVRI photometry for the open cluster NGC 2506 is presented. From comparison of the observed col... more UBGVRI photometry for the open cluster NGC 2506 is presented. From comparison of the observed colour-magnitude diagrams with simulations based on stellar evolutionary models we derive in a self-consistent way the reddening, distance and age of the cluster: E (B-V) = 0-0.07, (m-M)o = 12.6 ± 0.1, 'r = 1.5-2.2 Gyr. The cluster shows a well-defined secondary sequence, suggesting that binary systems constitute ~20 per cent of the cluster members visible in the colour-magnitude diagram.
Monthly Notices of the Royal Astronomical Society, 2001
We present CCD BV I photometry of the intermediate age open cluster Pismis 2, covering from the b... more We present CCD BV I photometry of the intermediate age open cluster Pismis 2, covering from the brighter red giants to about 5 magnitudes below the main sequence turn-off. The cluster is heavily reddened and is possibly affected by a differential reddening of ∆E(B − V) ≃ 0.04-0.06. Using the synthetic Colour-Magnitude Diagram method, we estimate in a selfconsistent way distance modulus, (m-M) 0 ≃ 12.5-12.7, and age, τ ≃ 1.1-1.2 Gyr. The cluster probably contains a significant fraction of binary systems. The metallicity is most likely solar and the reddening E(B − V) ranges between 1.26 and 1.32 depending on the cluster region.
Monthly Notices of the Royal Astronomical Society, 2006
We have obtained BVI CCD imaging of Berkeley 17, an anticentre open cluster that competes with NG... more We have obtained BVI CCD imaging of Berkeley 17, an anticentre open cluster that competes with NGC 6791 as the oldest known open cluster. Using the synthetic colour-magnitude diagrams (CMDs) technique with three sets of evolutionary tracks, we have determined that its age is 8.5-9.0 Gyr, it distance modulus is (m − M) 0 = 12.2, with a reddening of E(B − V) = 0.62-0.60. Differential reddening, if present, is at the 5 per cent level. All these values have been obtained using models with metallicity about half of solar (Z = 0.008 or 0.01 depending on the stellar evolution tracks), which allows us to reproduce the features of the cluster CMD better than other metallicities. Finally, from the analysis of a nearby comparison field, we think to have intercepted a portion of the disrupting Canis Major dwarf galaxy.
Monthly Notices of the Royal Astronomical Society, 1998
We present CCD BVI photometry of the old open cluster Berkeley 21, one of the most distant cluste... more We present CCD BVI photometry of the old open cluster Berkeley 21, one of the most distant clusters in the Galactic anticentre direction, and possibly the lowest metallicity object in the open clusters sample. Its position and metal abundance make it very important for the study of the Galactic disc. Using the synthetic colour-magnitude diagram method, we estimate values for the distance modulus ðm ¹ MÞ 0 ¼ 13:4-13.6, reddening EðB ¹ VÞ ¼ 0:74-0.78 (with possible differential absorption), and age ¼ 2:2-2.5 Gyr.
Monthly Notices of the Royal Astronomical Society, 2006
We have obtained medium-and low-resolution spectroscopy and BVI CCD imaging of Berkeley 32, an ol... more We have obtained medium-and low-resolution spectroscopy and BVI CCD imaging of Berkeley 32, an old open cluster which lies in the anticentre direction. From the radial velocities of 48 stars in the cluster direction, we found that 31 of them, in crucial evolutionary phases, are probable cluster members, with an average radial velocity of +106.7 (σ = 8.5) km s −1. From isochrone fitting to the colour-magnitude diagrams of Berkeley 32, we have obtained an age of 6.3 Gyr, (m − M) 0 = 12.48 and E(B − V) = 0.10. The best fit is obtained with Z = 0.008. A consistent distance, (m − M) 0 12.6 ± 0.1, has been derived from the mean magnitude of red clump stars with confirmed membership; we may assume (m − M) 0 12.55 ± 0.1. The colour-magnitude diagram of the nearby field observed to check for field stars contamination looks intriguingly similar to that of the Canis Major overdensity.
In this paper we present some results concerning the effects of two instantaneous starbursts, sep... more In this paper we present some results concerning the effects of two instantaneous starbursts, separated by a quiescent period, on the dynamical and chemical evolution of Blue Compact Dwarf galaxies. In particular, we compare the model results to the galaxy IZw18, which is a very metal-poor, gas-rich dwarf galaxy, possibly experiencing its first or second burst of star formation. We follow the evolution of a first weak burst of star formation followed by a second more intense one occurring after several hundred million years. We find that a galactic wind develops only during the second burst and that metals produced in the burst are preferentially lost, relative to the hydrogen gas. We predict the evolution of several chemical abundances (H, He, C, N, O, α-elements, Fe) in the gas inside and outside the galaxy, by taking into account, in detail, the chemical and energetical contributions from type II and Ia supernovae. We find that the abundances predicted for the star forming region are in good agreement with the H ii region abundances derived for IZw18. We also predict the abundances of C, N and O expected for the H i gas to be compared with future FUSE abundance determinations. We conclude that IZw18 must have experienced two bursts of star formation, one occurred ∼300 Myr ago and a recent one with an age between 4-7 Myr. However, by taking into account also other independent estimates, such as the color-magnitude diagram and the spectral energy distribution of stars in IZw18, and the fact that real starbursts are not instantaneous, we suggest that it is more likely that the burst age is between 4 and 15 Myr.
We study the dynamical and chemical evolution of a galaxy similar to IZw18 under the assumption o... more We study the dynamical and chemical evolution of a galaxy similar to IZw18 under the assumption of a continuous star formation during bursts. We adopt a 2-D hydrocode coupled with detailed chemical yields originating from SNeII, SNeIa and from single intermediate-mass stars. Different nucleosynthetic yields and different IMF slopes are tested. In most of the explored cases, a galactic wind develops, mostly carrying out of the galaxy the metal-enriched gas produced by the burst itself. The chemical species with the largest escape probabilities are Fe and N. Consequently, we predict that the [α/Fe] and [α/N] ratios outside the galaxy are lower than inside. In order to reproduce the chemical composition of IZw18, the best choice seems to be the adoption of the yields of Meynet & Maeder (2002) which take into account stellar rotation, although these authors do not follow the whole evolution of all the stars. Models with a flat IMF (x = 0.5) seem to be able to better reproduce the chemical properties of IZw18, but they inject in the gas a much larger amount of energy and the resulting galactic wind is very strong, at variance with observations. We also predict the evolution of the abundances in the H medium and compare them with recent FUSE observations.
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Papers by M. Tosi