Rafael Guerço | Universidad Católica del Norte (original) (raw)

Papers by Rafael Guerço

arXiv (Cornell University), Feb 22, 2024

This study probes the chemical abundances of the neutron-capture elements cerium and neodymium in... more This study probes the chemical abundances of the neutron-capture elements cerium and neodymium in the inner Milky Way from an analysis of a sample of ∼2000 stars in the Galactic Bulge/bar spatially

arXiv (Cornell University), Sep 18, 2023

Previous results in the literature have found the young inner-disk open cluster NGC 6705 to be mi... more Previous results in the literature have found the young inner-disk open cluster NGC 6705 to be mildly-enhanced. We examined this possibility via an independent chemical abundance analysis for 11 red-giant members of NGC 6705. The analysis is based on near-infrared APOGEE spectra and relies on LTE calculations using spherical model atmospheres and radiative transfer. We find a mean cluster metallicity of [Fe/H] = +0.13 ± 0.04, indicating that NGC 6705 is metal-rich, as may be expected for a young inner-disk cluster. The mean-element abundance relative to iron is ⟨[ /Fe]⟩ = −0.03 ± 0.05, which is not at odds with expectations from general Galactic abundance trends. NGC 6705 also provides important probes for studying stellar mixing, given its turn-off mass of M∼3.3 M ⊙. Its red giants have low 12 C abundances ([ 12 C/Fe]=−0.16) and enhanced 14 N abundances ([ 14 N/Fe]=+0.51), which are key signatures of the first dredge-up on the red giant branch. An additional signature of dredge-up was found in the Na abundances, which are enhanced by [Na/Fe]=+0.29, with a very small non-LTE correction. The 16 O and Al abundances are found to be near-solar. All of the derived mixing-sensitive abundances are in agreement with stellar models of approximately 3.3 M ⊙ evolving along the red giant branch and onto the red clump. As found in young open clusters with similar metallicities, NGC 6705 exhibits a mild excess in the s-process element cerium, with [Ce/Fe] = +0.13 ± 0.07.

Monthly Notices of the Royal Astronomical Society

The Magellanic Clouds are the most massive and closest satellite galaxies of the Milky Way (MW), ... more The Magellanic Clouds are the most massive and closest satellite galaxies of the Milky Way (MW), with stars covering ages from a few Myr up to 13 Gyr. This makes them important for validating integrated light methods to study stellar populations and star formation processes, which can be applied to more distant galaxies. We characterized a set of stellar clusters in the Small Magellanic Cloud (SMC), using the Southern Photometric Local Universe Survey. This is the first age (metallicity) determination for 11 (65) clusters of this sample. Through its seven narrow bands, centred on important spectral features, and five broad bands, we can retrieve detailed information about stellar populations. We obtained ages and metallicities for all stellar clusters using the Bayesian spectral energy distribution fitting code bagpipes. With a sample of clusters in the colour range −0.20 < r − z < +0.35, for which our determined parameters are most reliable, we modeled the age–metallicity rel...

Monthly Notices of the Royal Astronomical Society

The centre of the Milky Way contains stellar populations spanning a range in age and metallicity,... more The centre of the Milky Way contains stellar populations spanning a range in age and metallicity, with a recent star formation burst producing young and massive stars. Chemical abundances in the most luminous stellar member of the nuclear star cluster (NSC), IRS 7, are presented for 19F, 12C, 13C, 14N, 16O, 17O, and Fe from a local thermodynamic equilibrium analysis based on spherical modelling and radiative transfer with a 25-M⊙ model atmosphere, whose chemistry was tailored to the derived photospheric abundances. We find IRS 7 to be depleted heavily in both 12C (∼–0.8 dex) and 16O (∼–0.4 dex), while exhibiting an extremely enhanced 14N abundance (∼+1.1 dex), which are isotopic signatures of the deep mixing of CNO-cycled material to the stellar surface. The 19F abundance is also heavily depleted by ∼1 dex relative to the baseline fluorine of the NSC, providing evidence that fluorine along with carbon constrain the nature of the deep mixing in this very luminous supergiant. The abun...

The Astrophysical Journal

Abundances of fluorine (19F), as well as isotopic ratios of 16O/17O, are derived in a sample of l... more Abundances of fluorine (19F), as well as isotopic ratios of 16O/17O, are derived in a sample of luminous young (∼107–108 yr) red giants in the Galactic center (with galactocentric distances ranging from 0.6–30 pc), using high-resolution infrared spectra and vibration-rotation lines of H19F near λ2.3 μm. Five of the six red giants are members of the Nuclear star cluster that orbits the central supermassive black hole. Previous investigations of the chemical evolution of 19F in Galactic thin and thick-disk stars have revealed that the nucleosynthetic origins of 19F may be rather complex, resulting from two, or more, astrophysical sites; fluorine abundances behave as a primary element with respect to Fe abundances for thick-disk stars and as a secondary element in thin-disk stars. The Galactic center red giants analyzed fall within the thin-disk relation of F with Fe, having near-solar, to slightly larger, abundances of Fe (〈[Fe/H]〉 = +0.08 ± 0.04), with a slight enhancement of the F/F...

The origin of the element fluorine is not well known, but it is known that this element can be pr... more The origin of the element fluorine is not well known, but it is known that this element can be produced in three sites: spallation induced by neutrinos after the core collapse phase of a massive supernova; synthesis during thermal pulses in He - burning in AGB stars; fluorine production in cores of Wolf-Rayet stars in early phases of helium burning. In this work we present abundance results for fluorine in 42 stars of the spectral types K, M, MS, S, N and J of the Galactic disk, 3 stars of the Galactic bulge and 7 stars of the globular cluster M4. For stars in the Galactic disk the fluorine abundances were calculated using the MOOG code in LTE and from published equivalent widths values for R9, R13, R14 and R16 lines of the HF molecule at 2.3 micron. The spectra of the disk stars were observed with the 4m Telescope of the Kitt Peak Observatory and Fourier Transform Spectrometer (FTS) spectrometer. For the stars in the Galactic bulge and the globular cluster M4 fluorine abundances were also calculated using the MOOG code, however via spectral synthesis of the rotational vibrational R9 line of the HF molecule. The stars in the Galactic bulge and the M4 globular cluster were observed in high resolution with the 8m Telescope of Gemini South and using the Phoenix spectrograph of NOAO (National Optical Astronomy Observatory). Our results indicate a significant reduction in the fluorine abundances of disk stars, the bulge stars and stars in the globular cluster M4 when compared to previous results from the literature. This is understood because of the change in the excitation potentials of the HF lines analyzed in this study in comparison with those used in the previous works. Our fluorine results for the disk stars confirm directly that carbon-stars have high fluorine abundances as they are producers of this element. In the Galactic bulge the majority of our results suggest production in SNe II, although some stars have low values of [F/O] more in agreement with production in AGB stars. We confirm the anti-correlation previously found between fluorine and sodium in the globular cluster M4.

The Astrophysical Journal, Apr 11, 2022

Abundances of fluorine (¹⁹F), as well as isotopic ratios of ¹⁶6O/¹⁷O, are derived in a sample of ... more Abundances of fluorine (¹⁹F), as well as isotopic ratios of ¹⁶6O/¹⁷O, are derived in a sample of luminous young (∼10⁷--10⁸ yrs) red giants in the Galactic center (with galactocentric distances ranging from 0.6--30 pc), using high-resolution infrared spectra and vibration-rotation lines of H¹⁹F near λ2.3μm. Five of the six red giants are members of the Nuclear star cluster that orbits the central supermassive black hole. Previous investigations of the chemical evolution of ¹⁹F in Galactic thin and thick disk stars have revealed that the nucleosynthetic origins of ¹⁹F may be rather complex, resulting from two, or more, astrophysical sites; fluorine abundances behave as a primary element with respect to Fe abundances for thick disk stars and as a secondary element in thin disk stars. The Galactic center red giants analyzed fall within the thin disk relation of F with Fe, having near-solar, to slightly larger, abundances of Fe (<[Fe/H]>=+0.08±0.04), with a slight enhancement of the F/Fe abundance ratio (<[F/Fe]>=+0.28±0.17). In terms of their F and Fe abundances, the Galactic center stars follow the thin disk population, which requires an efficient source of ¹⁹F that could be the winds from core-He burning Wolf Rayet stars, or thermally-pulsing AGB stars, or a combination of both. The observed increase of [F/Fe] with increasing [Fe/H] found in thin disk and Galactic center stars is not predicted by any published chemical evolution models that are discussed, thus a quantitative understanding of yields from the various possible sources of ¹⁹F remains unknown.

The Astrophysical Journal, 2019

We present chemical abundances for the elements carbon, sodium, and fluorine in 15 red giants of ... more We present chemical abundances for the elements carbon, sodium, and fluorine in 15 red giants of the globular cluster M4, as well as six red giants of the globular cluster ω Centauri. The chemical abundances were calculated in LTE via spectral synthesis. The spectra analyzed are high-resolution spectra obtained in the near-infrared region around 2.3 μm with the Phoenix spectrograph on the 8.1 m Gemini South Telescope, the IGRINS spectrograph on the McDonald Observatory 2.7 m Telescope, and the CRIRES spectrograph on the ESO 8.2 m Very Large Telescope. The results indicate a significant reduction in the fluorine abundances when compared to previous values from the literature for M4 and ω Centauri, due to a downward revision in the excitation potentials of the HF (1-0) R9 line used in the analysis. The fluorine abundances obtained for the M4 red giants are found to be anticorrelated with those of Na, following the typical pattern of abundance variations seen in globular clusters between distinct stellar populations. In M4, as the Na abundance increases by ∼+0.4 dex, the F abundance decreases by ∼-0.2 dex. A comparison with abundance predictions from two sets of stellar evolution models finds that the models predict somewhat less F depletion (∼-0.1 dex) for the same increase of +0.4 dex in Na.

The Astrophysical Journal, 2019

The chemical evolution of fluorine is investigated in a sample of Milky Way red giant stars that ... more The chemical evolution of fluorine is investigated in a sample of Milky Way red giant stars that span a significant range in metallicity from [Fe/H] ∼ -1.3 to 0.0 dex. Fluorine abundances are derived from vibration-rotation lines of HF in high-resolution infrared spectra near 2.335 μm. The red giants are members of the thin and thick disk/halo, with two stars being likely members of the outer disk Monoceros overdensity. At lower metallicities, with [Fe/H] < -0.4 to -0.5, the abundance of F varies as a primary element with respect to the Fe abundance, with a constant subsolar value of [F/Fe] ∼ -0.3 to -0.4 dex. At larger metallicities, however, [F/Fe] increases rapidly with [Fe/H] and displays a near-secondary behavior with respect to Fe. Comparisons with various models of chemical evolution suggest that in the low-metallicity regime (dominated here by thick-disk stars), a primary evolution of 19F with Fe, with a subsolar [F/Fe] value that roughly matches the observed plateau, can be reproduced by a model incorporating neutrino nucleosynthesis in the aftermath of the core collapse in Type II supernovae. A primary behavior for [F/Fe] at low metallicity is also observed for a model including rapidly rotating low-metallicity massive stars, but this overproduces [F/Fe] at low metallicity. The thick-disk red giants in our sample span a large range of galactocentric distance (R g ∼ 6-13.7 kpc) yet display a roughly constant value of [F/Fe], indicating a very flat gradient (with a slope of 0.02 ± 0.03 dex kpc-1) of this elemental ratio over a significant portion of the Galaxy having | Z| > 300 pc away from the Galaxy midplane.

arXiv (Cornell University), Feb 22, 2024

This study probes the chemical abundances of the neutron-capture elements cerium and neodymium in... more This study probes the chemical abundances of the neutron-capture elements cerium and neodymium in the inner Milky Way from an analysis of a sample of ∼2000 stars in the Galactic Bulge/bar spatially

arXiv (Cornell University), Sep 18, 2023

Previous results in the literature have found the young inner-disk open cluster NGC 6705 to be mi... more Previous results in the literature have found the young inner-disk open cluster NGC 6705 to be mildly-enhanced. We examined this possibility via an independent chemical abundance analysis for 11 red-giant members of NGC 6705. The analysis is based on near-infrared APOGEE spectra and relies on LTE calculations using spherical model atmospheres and radiative transfer. We find a mean cluster metallicity of [Fe/H] = +0.13 ± 0.04, indicating that NGC 6705 is metal-rich, as may be expected for a young inner-disk cluster. The mean-element abundance relative to iron is ⟨[ /Fe]⟩ = −0.03 ± 0.05, which is not at odds with expectations from general Galactic abundance trends. NGC 6705 also provides important probes for studying stellar mixing, given its turn-off mass of M∼3.3 M ⊙. Its red giants have low 12 C abundances ([ 12 C/Fe]=−0.16) and enhanced 14 N abundances ([ 14 N/Fe]=+0.51), which are key signatures of the first dredge-up on the red giant branch. An additional signature of dredge-up was found in the Na abundances, which are enhanced by [Na/Fe]=+0.29, with a very small non-LTE correction. The 16 O and Al abundances are found to be near-solar. All of the derived mixing-sensitive abundances are in agreement with stellar models of approximately 3.3 M ⊙ evolving along the red giant branch and onto the red clump. As found in young open clusters with similar metallicities, NGC 6705 exhibits a mild excess in the s-process element cerium, with [Ce/Fe] = +0.13 ± 0.07.

Monthly Notices of the Royal Astronomical Society

The Magellanic Clouds are the most massive and closest satellite galaxies of the Milky Way (MW), ... more The Magellanic Clouds are the most massive and closest satellite galaxies of the Milky Way (MW), with stars covering ages from a few Myr up to 13 Gyr. This makes them important for validating integrated light methods to study stellar populations and star formation processes, which can be applied to more distant galaxies. We characterized a set of stellar clusters in the Small Magellanic Cloud (SMC), using the Southern Photometric Local Universe Survey. This is the first age (metallicity) determination for 11 (65) clusters of this sample. Through its seven narrow bands, centred on important spectral features, and five broad bands, we can retrieve detailed information about stellar populations. We obtained ages and metallicities for all stellar clusters using the Bayesian spectral energy distribution fitting code bagpipes. With a sample of clusters in the colour range −0.20 < r − z < +0.35, for which our determined parameters are most reliable, we modeled the age–metallicity rel...

Monthly Notices of the Royal Astronomical Society

The centre of the Milky Way contains stellar populations spanning a range in age and metallicity,... more The centre of the Milky Way contains stellar populations spanning a range in age and metallicity, with a recent star formation burst producing young and massive stars. Chemical abundances in the most luminous stellar member of the nuclear star cluster (NSC), IRS 7, are presented for 19F, 12C, 13C, 14N, 16O, 17O, and Fe from a local thermodynamic equilibrium analysis based on spherical modelling and radiative transfer with a 25-M⊙ model atmosphere, whose chemistry was tailored to the derived photospheric abundances. We find IRS 7 to be depleted heavily in both 12C (∼–0.8 dex) and 16O (∼–0.4 dex), while exhibiting an extremely enhanced 14N abundance (∼+1.1 dex), which are isotopic signatures of the deep mixing of CNO-cycled material to the stellar surface. The 19F abundance is also heavily depleted by ∼1 dex relative to the baseline fluorine of the NSC, providing evidence that fluorine along with carbon constrain the nature of the deep mixing in this very luminous supergiant. The abun...

The Astrophysical Journal

Abundances of fluorine (19F), as well as isotopic ratios of 16O/17O, are derived in a sample of l... more Abundances of fluorine (19F), as well as isotopic ratios of 16O/17O, are derived in a sample of luminous young (∼107–108 yr) red giants in the Galactic center (with galactocentric distances ranging from 0.6–30 pc), using high-resolution infrared spectra and vibration-rotation lines of H19F near λ2.3 μm. Five of the six red giants are members of the Nuclear star cluster that orbits the central supermassive black hole. Previous investigations of the chemical evolution of 19F in Galactic thin and thick-disk stars have revealed that the nucleosynthetic origins of 19F may be rather complex, resulting from two, or more, astrophysical sites; fluorine abundances behave as a primary element with respect to Fe abundances for thick-disk stars and as a secondary element in thin-disk stars. The Galactic center red giants analyzed fall within the thin-disk relation of F with Fe, having near-solar, to slightly larger, abundances of Fe (〈[Fe/H]〉 = +0.08 ± 0.04), with a slight enhancement of the F/F...

The origin of the element fluorine is not well known, but it is known that this element can be pr... more The origin of the element fluorine is not well known, but it is known that this element can be produced in three sites: spallation induced by neutrinos after the core collapse phase of a massive supernova; synthesis during thermal pulses in He - burning in AGB stars; fluorine production in cores of Wolf-Rayet stars in early phases of helium burning. In this work we present abundance results for fluorine in 42 stars of the spectral types K, M, MS, S, N and J of the Galactic disk, 3 stars of the Galactic bulge and 7 stars of the globular cluster M4. For stars in the Galactic disk the fluorine abundances were calculated using the MOOG code in LTE and from published equivalent widths values for R9, R13, R14 and R16 lines of the HF molecule at 2.3 micron. The spectra of the disk stars were observed with the 4m Telescope of the Kitt Peak Observatory and Fourier Transform Spectrometer (FTS) spectrometer. For the stars in the Galactic bulge and the globular cluster M4 fluorine abundances were also calculated using the MOOG code, however via spectral synthesis of the rotational vibrational R9 line of the HF molecule. The stars in the Galactic bulge and the M4 globular cluster were observed in high resolution with the 8m Telescope of Gemini South and using the Phoenix spectrograph of NOAO (National Optical Astronomy Observatory). Our results indicate a significant reduction in the fluorine abundances of disk stars, the bulge stars and stars in the globular cluster M4 when compared to previous results from the literature. This is understood because of the change in the excitation potentials of the HF lines analyzed in this study in comparison with those used in the previous works. Our fluorine results for the disk stars confirm directly that carbon-stars have high fluorine abundances as they are producers of this element. In the Galactic bulge the majority of our results suggest production in SNe II, although some stars have low values of [F/O] more in agreement with production in AGB stars. We confirm the anti-correlation previously found between fluorine and sodium in the globular cluster M4.

The Astrophysical Journal, Apr 11, 2022

Abundances of fluorine (¹⁹F), as well as isotopic ratios of ¹⁶6O/¹⁷O, are derived in a sample of ... more Abundances of fluorine (¹⁹F), as well as isotopic ratios of ¹⁶6O/¹⁷O, are derived in a sample of luminous young (∼10⁷--10⁸ yrs) red giants in the Galactic center (with galactocentric distances ranging from 0.6--30 pc), using high-resolution infrared spectra and vibration-rotation lines of H¹⁹F near λ2.3μm. Five of the six red giants are members of the Nuclear star cluster that orbits the central supermassive black hole. Previous investigations of the chemical evolution of ¹⁹F in Galactic thin and thick disk stars have revealed that the nucleosynthetic origins of ¹⁹F may be rather complex, resulting from two, or more, astrophysical sites; fluorine abundances behave as a primary element with respect to Fe abundances for thick disk stars and as a secondary element in thin disk stars. The Galactic center red giants analyzed fall within the thin disk relation of F with Fe, having near-solar, to slightly larger, abundances of Fe (<[Fe/H]>=+0.08±0.04), with a slight enhancement of the F/Fe abundance ratio (<[F/Fe]>=+0.28±0.17). In terms of their F and Fe abundances, the Galactic center stars follow the thin disk population, which requires an efficient source of ¹⁹F that could be the winds from core-He burning Wolf Rayet stars, or thermally-pulsing AGB stars, or a combination of both. The observed increase of [F/Fe] with increasing [Fe/H] found in thin disk and Galactic center stars is not predicted by any published chemical evolution models that are discussed, thus a quantitative understanding of yields from the various possible sources of ¹⁹F remains unknown.

The Astrophysical Journal, 2019

We present chemical abundances for the elements carbon, sodium, and fluorine in 15 red giants of ... more We present chemical abundances for the elements carbon, sodium, and fluorine in 15 red giants of the globular cluster M4, as well as six red giants of the globular cluster ω Centauri. The chemical abundances were calculated in LTE via spectral synthesis. The spectra analyzed are high-resolution spectra obtained in the near-infrared region around 2.3 μm with the Phoenix spectrograph on the 8.1 m Gemini South Telescope, the IGRINS spectrograph on the McDonald Observatory 2.7 m Telescope, and the CRIRES spectrograph on the ESO 8.2 m Very Large Telescope. The results indicate a significant reduction in the fluorine abundances when compared to previous values from the literature for M4 and ω Centauri, due to a downward revision in the excitation potentials of the HF (1-0) R9 line used in the analysis. The fluorine abundances obtained for the M4 red giants are found to be anticorrelated with those of Na, following the typical pattern of abundance variations seen in globular clusters between distinct stellar populations. In M4, as the Na abundance increases by ∼+0.4 dex, the F abundance decreases by ∼-0.2 dex. A comparison with abundance predictions from two sets of stellar evolution models finds that the models predict somewhat less F depletion (∼-0.1 dex) for the same increase of +0.4 dex in Na.

The Astrophysical Journal, 2019

The chemical evolution of fluorine is investigated in a sample of Milky Way red giant stars that ... more The chemical evolution of fluorine is investigated in a sample of Milky Way red giant stars that span a significant range in metallicity from [Fe/H] ∼ -1.3 to 0.0 dex. Fluorine abundances are derived from vibration-rotation lines of HF in high-resolution infrared spectra near 2.335 μm. The red giants are members of the thin and thick disk/halo, with two stars being likely members of the outer disk Monoceros overdensity. At lower metallicities, with [Fe/H] < -0.4 to -0.5, the abundance of F varies as a primary element with respect to the Fe abundance, with a constant subsolar value of [F/Fe] ∼ -0.3 to -0.4 dex. At larger metallicities, however, [F/Fe] increases rapidly with [Fe/H] and displays a near-secondary behavior with respect to Fe. Comparisons with various models of chemical evolution suggest that in the low-metallicity regime (dominated here by thick-disk stars), a primary evolution of 19F with Fe, with a subsolar [F/Fe] value that roughly matches the observed plateau, can be reproduced by a model incorporating neutrino nucleosynthesis in the aftermath of the core collapse in Type II supernovae. A primary behavior for [F/Fe] at low metallicity is also observed for a model including rapidly rotating low-metallicity massive stars, but this overproduces [F/Fe] at low metallicity. The thick-disk red giants in our sample span a large range of galactocentric distance (R g ∼ 6-13.7 kpc) yet display a roughly constant value of [F/Fe], indicating a very flat gradient (with a slope of 0.02 ± 0.03 dex kpc-1) of this elemental ratio over a significant portion of the Galaxy having | Z| > 300 pc away from the Galaxy midplane.