Marwan Gebran | Saint Mary's College, Notre Dame (original) (raw)
Papers by Marwan Gebran
The Astronomical Journal, Jul 7, 2023
We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-st... more We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-stars: HD 1279 (B8III), HD 99803 (B9V), HD 123445 (B9V), HD 147550 (B9V), HD 171961 (B8III), and HD 202671 (B5II/III), hitherto reported as normal stars. We compare them to the two classical HgMn stars μ Lep and β Scl, and to the superficially normal star ν Cap. In the spectra of the six stars, the Hg ii line at 3984 Å line is clearly seen and numerous lines of P, Ti, Mn, Fe, Ga, Sr, Y, and Zr appear to be strong absorbers. A comparison of newly acquired and archival spectra of these objects with a grid of synthetic spectra for selected unblended lines reveals large overabundances of P, Ti, Cr, Mn, Sr, Y, Zr, Ba, Pt, and Hg, and underabundances of He, Mg, Sc, and Ni. The effective temperatures, surface gravities, low projected rotational velocities, and the peculiar abundance patterns of the six investigated stars show that they are new chemically peculiar stars, mostly new HgMn stars, and are reclassified as such. The evolutionary status of these stars has been inferred, and their ages and masses estimated. The two most massive objects, HD 1279 and HD 202671, might have evolved away from the main sequence recently, the other stars are main-sequence objects. HD 99803A is a sharp lined HgMn star with grazing eclipses. From TESS and MASCARA photometry, we determine an orbital period of P orb = 26.12022 ± 0.00004 day.
Monthly Notices of the Royal Astronomical Society
WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William He... more WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable ‘mini’ integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366−959 nm at R ∼ 5000, or two shorter ranges at R ∼ 20 000. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy’s origins by completing Gaia’s phase-space information, providing metallicities to its limiting magnitude for ∼3 million stars and detailed abundances for ∼1.5 million brighter field and open-cluster stars; (ii) survey ∼0.4 million Galactic-plane OBA stars, young stellar objects...
Open Astronomy
In this follow-up article, we investigate the use of convolutional neural network for deriving st... more In this follow-up article, we investigate the use of convolutional neural network for deriving stellar parameters from observed spectra. Using hyperparameters determined previously, we have constructed a Neural Network architecture suitable for the derivation of T eff {T}_{{\rm{eff}}} , log g \log g , [ M / H ] \left[M\hspace{0.1em}\text{/}\hspace{0.1em}H] , and v e sin i {v}_{e}\sin i . The network was constrained by applying it to databases of AFGK synthetic spectra at different resolutions. Then, parameters of A stars from Polarbase, SOPHIE, and ELODIE databases are derived, as well as those of FGK stars from the spectroscopic survey of stars in the solar neighbourhood. The network model’s average accuracy on the stellar parameters is found to be as low as 80 K for T eff {T}_{{\rm{eff}}} , 0.06 dex for log g \log g , 0.08 dex for [ M / H ] \left[M\hspace{0.1em}\text{/}\hspace{0.1em}H] , and 3 km/s for v e sin i {v}_{e}\sin i for AFGK stars.
arXiv: Solar and Stellar Astrophysics, 2013
A programme to observe all A dwarfs in open clusters brighter than V=6.5 mag of various ages and ... more A programme to observe all A dwarfs in open clusters brighter than V=6.5 mag of various ages and in the field was initiated several years ago. In this work we present the current status of microturbulent velocity for A and F dwarfs. We have performed high resolution high signal-to-noise spectroscopy of stars well distributed in mass along the Main Sequence. Microturbulent velocities are derived iteratively by fitting grids of synthetic spectra calculated in LTE to observed spectra of 61 A field stars, 55 A and 58 F in open clusters (Pleiades, Coma Berenices, Hyades and the Ursa Major moving group). We compared our results to recent works and found a good agreement with their analytical formulation for the standard microturbulence. Our results show a broad maximum for microturbulent velocities in the range A5V to about A9V and a decrease (to sim1\sim 1sim1 km/s) for cooler and hotter stars as indicated in Smalley (2004).We also present a comparison to preliminary science results of Lobel e...
arXiv: Instrumentation and Methods for Astrophysics, 2017
We aim at finding the value of an explanatory variable, through its expression in a large data-ve... more We aim at finding the value of an explanatory variable, through its expression in a large data-vector, without knowing the link function between the explanatory variable and the data-space. Sliced Inverse Regression (SIR) method allows for the projection of a data-vector onto a subspace consistent with the explanatory variable variation. We suggest a method based on the SIR subspace, that gives the most efficient estimation of an unknown explanatory variable.
The Astrophysical Journal, 2018
The Astronomical Journal, 2019
Astronomy & Astrophysics, 2018
Context. Determination of high-precision abundances of late-type stars has been and always will b... more Context. Determination of high-precision abundances of late-type stars has been and always will be an important goal of spectroscopic studies, which requires accurate modeling of their stellar spectra with non-local thermodynamic equilibrium (NLTE) radiative transfer methods. This entails using up-to-date atomic data of the elements under study, which are still subject to large uncertainties.Aims. We investigate the role of hydrogen collisions in NLTE spectral line synthesis, and introduce a new general empirical recipe to determine inelastic charge transfer (CT) and bound-bound hydrogen collisional rates. This recipe is based on fitting the energy functional dependence of published quantum collisional rate coefficients of several neutral elements (BeI, Na I, Mg I, Al I, Si Iand Ca I) using simple polynomial equations.Methods. We perform thorough NLTE abundance calculation tests using our method for four different atoms, Na, Mg, Al and Si, for a broad range of stellar parameters. We...
Monthly Notices of the Royal Astronomical Society, 2018
Journal of Physics: Conference Series, 2017
Astronomy & Astrophysics, 2017
The Astrophysical Journal, 2017
The Astrophysical Journal, 2016
Astronomy & Astrophysics, 2016
Astrophysics and Space Science, 2016
Astronomy & Astrophysics, 2016
Abundances of 18 chemical elements have been derived for 27 A/Am and 16 F stars members of the Pl... more Abundances of 18 chemical elements have been derived for 27 A/Am and 16 F stars members of the Pleiades and Coma Berenices open clusters. We have specifically computed, with the Montrèal code, a series of evolutionary models for two A star members of these two clusters. None of the models reproduces entirely the overall shape of the abundance patterns. The
Astronomy & Astrophysics, 2015
The Astronomical Journal, Jul 7, 2023
We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-st... more We report on a detailed abundance study of six bright, mostly southern, slowly rotating late B-stars: HD 1279 (B8III), HD 99803 (B9V), HD 123445 (B9V), HD 147550 (B9V), HD 171961 (B8III), and HD 202671 (B5II/III), hitherto reported as normal stars. We compare them to the two classical HgMn stars μ Lep and β Scl, and to the superficially normal star ν Cap. In the spectra of the six stars, the Hg ii line at 3984 Å line is clearly seen and numerous lines of P, Ti, Mn, Fe, Ga, Sr, Y, and Zr appear to be strong absorbers. A comparison of newly acquired and archival spectra of these objects with a grid of synthetic spectra for selected unblended lines reveals large overabundances of P, Ti, Cr, Mn, Sr, Y, Zr, Ba, Pt, and Hg, and underabundances of He, Mg, Sc, and Ni. The effective temperatures, surface gravities, low projected rotational velocities, and the peculiar abundance patterns of the six investigated stars show that they are new chemically peculiar stars, mostly new HgMn stars, and are reclassified as such. The evolutionary status of these stars has been inferred, and their ages and masses estimated. The two most massive objects, HD 1279 and HD 202671, might have evolved away from the main sequence recently, the other stars are main-sequence objects. HD 99803A is a sharp lined HgMn star with grazing eclipses. From TESS and MASCARA photometry, we determine an orbital period of P orb = 26.12022 ± 0.00004 day.
Monthly Notices of the Royal Astronomical Society
WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William He... more WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, will see first light in late 2022. WEAVE comprises a new 2-degree field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable ‘mini’ integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366−959 nm at R ∼ 5000, or two shorter ranges at R ∼ 20 000. After summarising the design and implementation of WEAVE and its data systems, we present the organisation, science drivers and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy’s origins by completing Gaia’s phase-space information, providing metallicities to its limiting magnitude for ∼3 million stars and detailed abundances for ∼1.5 million brighter field and open-cluster stars; (ii) survey ∼0.4 million Galactic-plane OBA stars, young stellar objects...
Open Astronomy
In this follow-up article, we investigate the use of convolutional neural network for deriving st... more In this follow-up article, we investigate the use of convolutional neural network for deriving stellar parameters from observed spectra. Using hyperparameters determined previously, we have constructed a Neural Network architecture suitable for the derivation of T eff {T}_{{\rm{eff}}} , log g \log g , [ M / H ] \left[M\hspace{0.1em}\text{/}\hspace{0.1em}H] , and v e sin i {v}_{e}\sin i . The network was constrained by applying it to databases of AFGK synthetic spectra at different resolutions. Then, parameters of A stars from Polarbase, SOPHIE, and ELODIE databases are derived, as well as those of FGK stars from the spectroscopic survey of stars in the solar neighbourhood. The network model’s average accuracy on the stellar parameters is found to be as low as 80 K for T eff {T}_{{\rm{eff}}} , 0.06 dex for log g \log g , 0.08 dex for [ M / H ] \left[M\hspace{0.1em}\text{/}\hspace{0.1em}H] , and 3 km/s for v e sin i {v}_{e}\sin i for AFGK stars.
arXiv: Solar and Stellar Astrophysics, 2013
A programme to observe all A dwarfs in open clusters brighter than V=6.5 mag of various ages and ... more A programme to observe all A dwarfs in open clusters brighter than V=6.5 mag of various ages and in the field was initiated several years ago. In this work we present the current status of microturbulent velocity for A and F dwarfs. We have performed high resolution high signal-to-noise spectroscopy of stars well distributed in mass along the Main Sequence. Microturbulent velocities are derived iteratively by fitting grids of synthetic spectra calculated in LTE to observed spectra of 61 A field stars, 55 A and 58 F in open clusters (Pleiades, Coma Berenices, Hyades and the Ursa Major moving group). We compared our results to recent works and found a good agreement with their analytical formulation for the standard microturbulence. Our results show a broad maximum for microturbulent velocities in the range A5V to about A9V and a decrease (to sim1\sim 1sim1 km/s) for cooler and hotter stars as indicated in Smalley (2004).We also present a comparison to preliminary science results of Lobel e...
arXiv: Instrumentation and Methods for Astrophysics, 2017
We aim at finding the value of an explanatory variable, through its expression in a large data-ve... more We aim at finding the value of an explanatory variable, through its expression in a large data-vector, without knowing the link function between the explanatory variable and the data-space. Sliced Inverse Regression (SIR) method allows for the projection of a data-vector onto a subspace consistent with the explanatory variable variation. We suggest a method based on the SIR subspace, that gives the most efficient estimation of an unknown explanatory variable.
The Astrophysical Journal, 2018
The Astronomical Journal, 2019
Astronomy & Astrophysics, 2018
Context. Determination of high-precision abundances of late-type stars has been and always will b... more Context. Determination of high-precision abundances of late-type stars has been and always will be an important goal of spectroscopic studies, which requires accurate modeling of their stellar spectra with non-local thermodynamic equilibrium (NLTE) radiative transfer methods. This entails using up-to-date atomic data of the elements under study, which are still subject to large uncertainties.Aims. We investigate the role of hydrogen collisions in NLTE spectral line synthesis, and introduce a new general empirical recipe to determine inelastic charge transfer (CT) and bound-bound hydrogen collisional rates. This recipe is based on fitting the energy functional dependence of published quantum collisional rate coefficients of several neutral elements (BeI, Na I, Mg I, Al I, Si Iand Ca I) using simple polynomial equations.Methods. We perform thorough NLTE abundance calculation tests using our method for four different atoms, Na, Mg, Al and Si, for a broad range of stellar parameters. We...
Monthly Notices of the Royal Astronomical Society, 2018
Journal of Physics: Conference Series, 2017
Astronomy & Astrophysics, 2017
The Astrophysical Journal, 2017
The Astrophysical Journal, 2016
Astronomy & Astrophysics, 2016
Astrophysics and Space Science, 2016
Astronomy & Astrophysics, 2016
Abundances of 18 chemical elements have been derived for 27 A/Am and 16 F stars members of the Pl... more Abundances of 18 chemical elements have been derived for 27 A/Am and 16 F stars members of the Pleiades and Coma Berenices open clusters. We have specifically computed, with the Montrèal code, a series of evolutionary models for two A star members of these two clusters. None of the models reproduces entirely the overall shape of the abundance patterns. The
Astronomy & Astrophysics, 2015