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Papers by Sergio Ortolani

Research paper thumbnail of 1.9. Globular clusters within 5° of the Galactic center

Symposium - International Astronomical Union, 1998

The knowledge of age and spatial distribution of stars in the Galactic bulge require observationa... more The knowledge of age and spatial distribution of stars in the Galactic bulge require observational constraints to establish whether its stellar population is very old (Larson 1990) or is a younger, disk-like component (Raha et al. 1992), and if its shape is spherical or extended, or perhaps a bar (Blitz & Spergel 1991). Yet other possibilities are a flattened bulge or a disk-like system (Zinn 1985; Armandroff 1989; Ortolani et al. 1993; Minniti 1995).

Research paper thumbnail of B,VandIphotometry of the intermediate metallicity bulge globular clusters NGC 6325 and NGC 6355

Astronomy & Astrophysics, 2003

Colour-Magnitude Diagrams are presented for the globular clusters NGC 6325 and NGC 6355. These cl... more Colour-Magnitude Diagrams are presented for the globular clusters NGC 6325 and NGC 6355. These clusters are embedded in crowded bulge fields. Both clusters are concentrated and have blue Horizontal Branches, and NGC 6325 shows a blue tail. We derive a reddening E(B−V) ≈ 0.95 and a distance from the Sun d ≈ 6.9 kpc for NGC 6325, and E(B−V) ≈ 0.78 and d ≈ 8.8 kpc for NGC 6355. They are both at ∼1 kpc from the Galactic center, above the plane. The metallicity has been estimated based on red giant branch morphology, being consistent with an intermediate metallicity of [Fe/H] ≈ −1.3 for both clusters, at the lower end of the metallicity distribution of bulge field stars. They add to the list of blue Horizontal Branch clusters with intermediate metallicities within the bulge volume.

Research paper thumbnail of Colour Magnitude Diagrams of the moderately metal-rich globular clusters NGC 6569 and Palomar 11

Astronomy & Astrophysics, 2001

For the first time Colour-Magnitude Diagrams are presented for NGC 6569 and Palomar 11. NGC 6569 ... more For the first time Colour-Magnitude Diagrams are presented for NGC 6569 and Palomar 11. NGC 6569 is a rather concentrated cluster whereas Palomar 11 is loose and sparsely populated. Cluster parameters are derived from V and I photometry. These are red Horizontal Branch globular clusters, with metallicities comparable to that of 47 Tucanae. We derive a reddening E(B − V) ≈ 0.53 and a distance from the Sun d ≈ 9.8 kpc for NGC 6569, and E(B − V) ≈ 0.35 and d ≈ 13.2 kpc for Palomar 11. NGC 6569 is located in the bulge, while Palomar 11, despite its rather high metallicity, is in the inner halo, a rare case similar to that of 47 Tucanae.

Research paper thumbnail of SOARBVIphotometry of the metal-poor bulge globular cluster NGC 6642

Astronomy & Astrophysics, 2006

We present BV I photometry of the globular cluster NGC 6642 using the SOI imager at the SOAR Tele... more We present BV I photometry of the globular cluster NGC 6642 using the SOI imager at the SOAR Telescope. The colour magnitude diagrams (CMD) reach ≈1.5 mag in V below the main sequence turn-off. A comparison of the overall sequences, and in particular the Red Giant Branch slope of NGC 6642 with that of M 5, indicates that the two clusters must have a similar metallicity of [Fe/H] ≈-1.3. We also obtained a reddening E(B − V) = 0.42 ± 0.03 for NGC 6642, and a distance from the Sun of d = 7.2 ± 0.5 kpc. Therefore NGC 6642 is a moderately metal-poor globular cluster that is spatially located in the bulge at a galactocentric distance of R GC ≈ 1.7 kpc. The comparison of CMDs of NGC 6642 with those of M 5 shows that there is a very good match of magnitude difference between turn-off and horizontal branch, suggesting comparable ages. The age of M 5 is typical of halo globulars, so NGC 6642 is coeval with the halo. It is a good candidate as one of the few genuine metal-poor and old bulge clusters, and might be one of the oldest fossils in the Galaxy.

Research paper thumbnail of Detection of Ks-excess stars in the 14 Myr open cluster NGC 4755

Astronomy & Astrophysics, 2006

Aims. We derive the structure, distribution of MS and PMS stars and dynamical state of the young ... more Aims. We derive the structure, distribution of MS and PMS stars and dynamical state of the young open cluster NGC 4755. We explore the possibility that, at the cluster age, some MS and PMS stars still present infrared excesses related to dust envelopes and proto-planetary discs. Methods. J, H and K s 2MASS photometry is used to build CMD and colour-colour diagrams, radial density profiles, luminosity and mass functions. Field-star decontamination is applied to uncover the cluster's intrinsic CMD morphology and detect candidate PMS stars. Proper motions from UCAC2 are used to determine cluster membership. Results. The radial density profile follows King's law with a core radius R core = 0.7 ± 0.1 pc and a limiting radius R lim = 6.9 ± 0.1 pc. The cluster age derived from Padova isochrones is 14 ± 2 Myr. Field-star decontamination reveals a low-MS limit at ≈1.4 M. The core MF (χ = 0.94 ± 0.16) is flatter than the halo's (χ = 1.58 ± 0.11). NGC 4755 contains ∼285 candidate PMS stars of age ∼1−15 Myr, and a few evolved stars. The mass locked up in PMS, MS and evolved stars amounts to ∼1150 M. Proper motions show that K s-excess MS and PMS stars are cluster members. K s-excess fractions in PMS and MS stars are 5.4 ± 2.1% and 3.9 ± 1.5% respectively, consistent with the cluster age. The core is deficient in PMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halo but they are scarce in the core. Conclusions. Compared to open clusters in different dynamical states studied with similar methods, NGC 4755 fits relations involving structural and dynamical parameters in the expected locus for its age and mass. On the other hand, the flatter core MF probably originates from primordial processes related to parent molecular cloud fragmentation and mass segregation over ∼14 Myr. Star formation in NGC 4755 began ≈14 Myr ago and proceeded for about the same length of time. Detection of K s-excess emission in member MS stars suggests that some circumstellar dust discs survived for ∼10 7 yr, occurring both in some MS and PMS stars for the age and spread observed in NGC 4755.

Research paper thumbnail of Gemini-Phoenix infrared high-resolution abundance analysis of five giants in the bulge globular cluster NGC 6553

Astronomy & Astrophysics, 2003

A detailed abundance analysis of 5 giants of the metal-rich bulge globular cluster NGC 6553 was c... more A detailed abundance analysis of 5 giants of the metal-rich bulge globular cluster NGC 6553 was carried out using high resolution infrared spectra in the H band, obtained at the Gemini-South 8m telescope. JK photometry collected at ESO and VI photometry from the Hubble Space Telescope are used to derive effective temperatures. The present analysis provides a metallicity [Fe/H] = −0.20 ± 0.10. An overabundance of oxygen of [O/Fe] = +0.20 is found from IR OH lines.

Research paper thumbnail of The Initial Mass Function of the Galactic Bulge down to ∼0.15 M ⊙

The Astrophysical Journal, 2000

We present a luminosity function (LF) for lower main-sequence stars in the Galactic bulge near (l... more We present a luminosity function (LF) for lower main-sequence stars in the Galactic bulge near (l, b) \ (0¡, [6¡) to J \ 24, corresponding to This LF is derived from Hubble Space T elescope M J D 9.3. (HST) near infrared camera and multiobject spectrometer (NICMOS) observations of a region of 22A .5 with the F110W and F160W Ðlters. The main-sequence locus in the infrared shows a strong ] 22A .5, change in slope at J D 20.5 that is well Ðtted by new low-mass models that include water (M J D 5.75) and molecular hydrogen opacity. Our derived mass function (which is not corrected for binary companions) is the deepest measured to date in the bulge and extends to 0.15 with a power-law M _ , slope of a \ [1.33^0.07 ; a Salpeter mass function would have a \ [2.35. We also combine our J-band LF with previously published data for the evolved stars to produce a bulge LF spanning D15 mag. We show that this mass function has negligible dependence on the adopted bulge metallicity and distance modulus. Although shallower than the Salpeter slope, the slope of the bulge initial mass function (IMF) is steeper than that recently found for the Galactic disk (a \ [0.8 and a \ [0.54 from the data of Reid & Gizis and Gould et al., respectively, in the same mass interval) but is virtually identical to the disk IMF derived by Kroupa and coworkers. The bulge IMF is also quite similar to the mass functions derived for those globular clusters that are believed to have experienced little or no dynamical evolution. Finally, we derive the ratio of the bulge to be D0.9^0.1 and brieÑy discuss the impli-M/L J cations of this bulge IMF for the interpretation of the microlensing events observed in the direction of the Galactic bulge.

Research paper thumbnail of 1.9. Globular clusters within 5° of the Galactic center

Symposium - International Astronomical Union, 1998

The knowledge of age and spatial distribution of stars in the Galactic bulge require observationa... more The knowledge of age and spatial distribution of stars in the Galactic bulge require observational constraints to establish whether its stellar population is very old (Larson 1990) or is a younger, disk-like component (Raha et al. 1992), and if its shape is spherical or extended, or perhaps a bar (Blitz & Spergel 1991). Yet other possibilities are a flattened bulge or a disk-like system (Zinn 1985; Armandroff 1989; Ortolani et al. 1993; Minniti 1995).

Research paper thumbnail of B,VandIphotometry of the intermediate metallicity bulge globular clusters NGC 6325 and NGC 6355

Astronomy & Astrophysics, 2003

Colour-Magnitude Diagrams are presented for the globular clusters NGC 6325 and NGC 6355. These cl... more Colour-Magnitude Diagrams are presented for the globular clusters NGC 6325 and NGC 6355. These clusters are embedded in crowded bulge fields. Both clusters are concentrated and have blue Horizontal Branches, and NGC 6325 shows a blue tail. We derive a reddening E(B−V) ≈ 0.95 and a distance from the Sun d ≈ 6.9 kpc for NGC 6325, and E(B−V) ≈ 0.78 and d ≈ 8.8 kpc for NGC 6355. They are both at ∼1 kpc from the Galactic center, above the plane. The metallicity has been estimated based on red giant branch morphology, being consistent with an intermediate metallicity of [Fe/H] ≈ −1.3 for both clusters, at the lower end of the metallicity distribution of bulge field stars. They add to the list of blue Horizontal Branch clusters with intermediate metallicities within the bulge volume.

Research paper thumbnail of Colour Magnitude Diagrams of the moderately metal-rich globular clusters NGC 6569 and Palomar 11

Astronomy & Astrophysics, 2001

For the first time Colour-Magnitude Diagrams are presented for NGC 6569 and Palomar 11. NGC 6569 ... more For the first time Colour-Magnitude Diagrams are presented for NGC 6569 and Palomar 11. NGC 6569 is a rather concentrated cluster whereas Palomar 11 is loose and sparsely populated. Cluster parameters are derived from V and I photometry. These are red Horizontal Branch globular clusters, with metallicities comparable to that of 47 Tucanae. We derive a reddening E(B − V) ≈ 0.53 and a distance from the Sun d ≈ 9.8 kpc for NGC 6569, and E(B − V) ≈ 0.35 and d ≈ 13.2 kpc for Palomar 11. NGC 6569 is located in the bulge, while Palomar 11, despite its rather high metallicity, is in the inner halo, a rare case similar to that of 47 Tucanae.

Research paper thumbnail of SOARBVIphotometry of the metal-poor bulge globular cluster NGC 6642

Astronomy & Astrophysics, 2006

We present BV I photometry of the globular cluster NGC 6642 using the SOI imager at the SOAR Tele... more We present BV I photometry of the globular cluster NGC 6642 using the SOI imager at the SOAR Telescope. The colour magnitude diagrams (CMD) reach ≈1.5 mag in V below the main sequence turn-off. A comparison of the overall sequences, and in particular the Red Giant Branch slope of NGC 6642 with that of M 5, indicates that the two clusters must have a similar metallicity of [Fe/H] ≈-1.3. We also obtained a reddening E(B − V) = 0.42 ± 0.03 for NGC 6642, and a distance from the Sun of d = 7.2 ± 0.5 kpc. Therefore NGC 6642 is a moderately metal-poor globular cluster that is spatially located in the bulge at a galactocentric distance of R GC ≈ 1.7 kpc. The comparison of CMDs of NGC 6642 with those of M 5 shows that there is a very good match of magnitude difference between turn-off and horizontal branch, suggesting comparable ages. The age of M 5 is typical of halo globulars, so NGC 6642 is coeval with the halo. It is a good candidate as one of the few genuine metal-poor and old bulge clusters, and might be one of the oldest fossils in the Galaxy.

Research paper thumbnail of Detection of Ks-excess stars in the 14 Myr open cluster NGC 4755

Astronomy & Astrophysics, 2006

Aims. We derive the structure, distribution of MS and PMS stars and dynamical state of the young ... more Aims. We derive the structure, distribution of MS and PMS stars and dynamical state of the young open cluster NGC 4755. We explore the possibility that, at the cluster age, some MS and PMS stars still present infrared excesses related to dust envelopes and proto-planetary discs. Methods. J, H and K s 2MASS photometry is used to build CMD and colour-colour diagrams, radial density profiles, luminosity and mass functions. Field-star decontamination is applied to uncover the cluster's intrinsic CMD morphology and detect candidate PMS stars. Proper motions from UCAC2 are used to determine cluster membership. Results. The radial density profile follows King's law with a core radius R core = 0.7 ± 0.1 pc and a limiting radius R lim = 6.9 ± 0.1 pc. The cluster age derived from Padova isochrones is 14 ± 2 Myr. Field-star decontamination reveals a low-MS limit at ≈1.4 M. The core MF (χ = 0.94 ± 0.16) is flatter than the halo's (χ = 1.58 ± 0.11). NGC 4755 contains ∼285 candidate PMS stars of age ∼1−15 Myr, and a few evolved stars. The mass locked up in PMS, MS and evolved stars amounts to ∼1150 M. Proper motions show that K s-excess MS and PMS stars are cluster members. K s-excess fractions in PMS and MS stars are 5.4 ± 2.1% and 3.9 ± 1.5% respectively, consistent with the cluster age. The core is deficient in PMS stars, as compared with MS ones. NGC 4755 hosts binaries in the halo but they are scarce in the core. Conclusions. Compared to open clusters in different dynamical states studied with similar methods, NGC 4755 fits relations involving structural and dynamical parameters in the expected locus for its age and mass. On the other hand, the flatter core MF probably originates from primordial processes related to parent molecular cloud fragmentation and mass segregation over ∼14 Myr. Star formation in NGC 4755 began ≈14 Myr ago and proceeded for about the same length of time. Detection of K s-excess emission in member MS stars suggests that some circumstellar dust discs survived for ∼10 7 yr, occurring both in some MS and PMS stars for the age and spread observed in NGC 4755.

Research paper thumbnail of Gemini-Phoenix infrared high-resolution abundance analysis of five giants in the bulge globular cluster NGC 6553

Astronomy & Astrophysics, 2003

A detailed abundance analysis of 5 giants of the metal-rich bulge globular cluster NGC 6553 was c... more A detailed abundance analysis of 5 giants of the metal-rich bulge globular cluster NGC 6553 was carried out using high resolution infrared spectra in the H band, obtained at the Gemini-South 8m telescope. JK photometry collected at ESO and VI photometry from the Hubble Space Telescope are used to derive effective temperatures. The present analysis provides a metallicity [Fe/H] = −0.20 ± 0.10. An overabundance of oxygen of [O/Fe] = +0.20 is found from IR OH lines.

Research paper thumbnail of The Initial Mass Function of the Galactic Bulge down to ∼0.15 M ⊙

The Astrophysical Journal, 2000

We present a luminosity function (LF) for lower main-sequence stars in the Galactic bulge near (l... more We present a luminosity function (LF) for lower main-sequence stars in the Galactic bulge near (l, b) \ (0¡, [6¡) to J \ 24, corresponding to This LF is derived from Hubble Space T elescope M J D 9.3. (HST) near infrared camera and multiobject spectrometer (NICMOS) observations of a region of 22A .5 with the F110W and F160W Ðlters. The main-sequence locus in the infrared shows a strong ] 22A .5, change in slope at J D 20.5 that is well Ðtted by new low-mass models that include water (M J D 5.75) and molecular hydrogen opacity. Our derived mass function (which is not corrected for binary companions) is the deepest measured to date in the bulge and extends to 0.15 with a power-law M _ , slope of a \ [1.33^0.07 ; a Salpeter mass function would have a \ [2.35. We also combine our J-band LF with previously published data for the evolved stars to produce a bulge LF spanning D15 mag. We show that this mass function has negligible dependence on the adopted bulge metallicity and distance modulus. Although shallower than the Salpeter slope, the slope of the bulge initial mass function (IMF) is steeper than that recently found for the Galactic disk (a \ [0.8 and a \ [0.54 from the data of Reid & Gizis and Gould et al., respectively, in the same mass interval) but is virtually identical to the disk IMF derived by Kroupa and coworkers. The bulge IMF is also quite similar to the mass functions derived for those globular clusters that are believed to have experienced little or no dynamical evolution. Finally, we derive the ratio of the bulge to be D0.9^0.1 and brieÑy discuss the impli-M/L J cations of this bulge IMF for the interpretation of the microlensing events observed in the direction of the Galactic bulge.