Marwan Gebran
Saint Mary's College, Notre Dame, Chemistry/Physics, Faculty Member
- Notre Dame University (Lebanon), Department of Physics & Astronomy, Faculty Memberadd
- I am an observational astrophysicist. My research interests include the chemical composition of stars, synthetic spec... moreI am an observational astrophysicist. My research interests include the chemical composition of stars, synthetic spectra, binary stars, HgMn stars, model atmospheres, and numerical methods for astrophysics. I am currently working on the use of artificial intelligence in stellar astrophysics,.I got my PhD from Universite de Montpellier II in 2007. I did a postdoc at the University of Barcelona (UB): calibration of the photometric instrument of the Gaia space telescope (2008-2010). In Fall 21, I joined the department of chemistry and physics at Saint Mary's College, Notre Dame.edit
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... 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.
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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... 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...
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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... 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.
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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... 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 $\sim 1$ km/s) for cooler and hotter stars as indicated in Smalley (2004).We also present a comparison to preliminary science results of Lobel e...
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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... 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.
Research Interests: Mathematics and Physics
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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... 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...
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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... 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
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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... 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 $\sim 1$ km/s) for cooler and hotter stars as indicated in Smalley (2004).We also present a comparison to preliminary science results of Lobel e...
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Machine learning is an efficient method for analysing and interpreting the increasing amount of astronomical data that are available. In this study, we show a pedagogical approach that should benefit anyone willing to experiment with deep... more
Machine learning is an efficient method for analysing and interpreting the increasing amount of astronomical data that are available. In this study, we show a pedagogical approach that should benefit anyone willing to experiment with deep learning techniques in the context of stellar parameter determination. Using the convolutional neural network architecture, we give a step-by-step overview of how to select the optimal parameters for deriving the most accurate values for the stellar parameters of stars: T eff {T}_{{\rm{eff}}} , log g \log g , [M/H], and v e sin i {v}_{e}\sin i . Synthetic spectra with random noise were used to constrain this method and to mimic the observations. We found that each stellar parameter requires a different combination of network hyperparameters and the maximum accuracy reached depends on this combination as well as the signal-to-noise ratio of the observations, and the architecture of the network. We also show that this technique can be applied to othe...
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We present the results of an abundance analysis of three G and two K dwarfs in the Praesepe open cluster based on high-resolution, moderate signal-to-noise-ratio spectra obtained with the ARC 3.5-m Telescope at Apache Point Observatory.... more
We present the results of an abundance analysis of three G and two K dwarfs in the Praesepe open cluster based on high-resolution, moderate signal-to-noise-ratio spectra obtained with the ARC 3.5-m Telescope at Apache Point Observatory. Using a Principle Component Analysis and the BACCHUS automated spectral analysis code, we determined stellar parameters and abundances of up to 24 elements for each of our targets, which range in temperature from 6000 to 4600 K. The average derived iron abundance for the three G stars is 0.17+/-0.07 dex, consistent with the 0.12+/-0.04 dex derived by Boesgaard et al. (2013) for their sample of 11 solar-type Praesepe members, which included these G stars. To investigate the efficacy of using automated routines to derive the abundances of cooler main-sequence stars, we compared the abundances of the K dwarfs to those of the G dwarfs. Our abundances are consistent to <=0.1~dex for 13 of the 18 elements we report for all five of the stars, providing m...
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We are applying various ML/DL techniques for the purpose of stellar spectroscopy. Having already ran tests with Principal Component Analysis (PCA) and Sliced Inverse Regression (SIR), we now turn our focus to Convolution Neural Network... more
We are applying various ML/DL techniques for the purpose of stellar spectroscopy. Having already ran tests with Principal Component Analysis (PCA) and Sliced Inverse Regression (SIR), we now turn our focus to Convolution Neural Network (CNN), among other techniques, in order to find the most accurate derivations for stellar parameters: effective temperature, surface gravity, projected equatorial rotational velocity, microturbulence velocity and metallicity.
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We present a method for deriving the stellar fundamental parameters. It is based on a regularized sliced inverse regression (RSIR).We first tested it on noisy synthetic spectra of A, F, G, and K-type stars, and inverted simultaneously... more
We present a method for deriving the stellar fundamental parameters. It is based on a regularized sliced inverse regression (RSIR).We first tested it on noisy synthetic spectra of A, F, G, and K-type stars, and inverted simultaneously their atmospheric fundamental parameters: T eff., log g, [M/H] and v sin i. Different learning databases were calculated using a range of sampling in T eff., log g, v sin i, and [M/H]. Combined with a principal component analysis (PCA) nearest neighbors (NN) search, the size of the learning database is reduced. A Tikhonov regularization is applied, given the ill-conditioning of SIR. For all spectral types, decreasing the size of the learning database allowed us to reach internal accuracies better than PCA-based NN-search using larger learning databases. For each analyzed parameter, we have reached internal errors that are smaller than the sampling step of the parameter. We have also applied the technique to a sample of observed FGK and A stars. For a s...
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This work describes a procedure to derive several fundamental parameters such as the effective temperature, surface gravity, equatorial rotational velocity and microturbulent velocity. In this work, we have written a numerical procedure... more
This work describes a procedure to derive several fundamental parameters such as the effective temperature, surface gravity, equatorial rotational velocity and microturbulent velocity. In this work, we have written a numerical procedure in Python which finds the best fit between a grid of synthetic spectra and the observed spectra by minimizing a standard chi-square. LTE model atmospheres were calculated using the ATLAS9 code and were used as inputs to the spectrum synthesis code SYNSPEC48 in order to compute a large grid of synthetic Balmer line profiles. This new procedure has been applied to a large number of new observations (GIRAFFE spectra) of B and A stars members of the young open cluster NGC3293. These observations are part of the GAIA ESO Survey. Takeda's procedure was also used to derive rotational velocities and microturbulent velocities. The results have been compared to previous determinations by other authors and are found to agree with them. As a first result, we...
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We compare the averaged photospheric abundances of A and F stars in open clusters of different ages: M6 (∼ 80 Myr), Pleiades (∼ 100 Myr), Coma Berenices (∼ 450 Myr), and the Hyades (∼ 800 Myr). The variation in the averaged abundances... more
We compare the averaged photospheric abundances of A and F stars in open clusters of different ages: M6 (∼ 80 Myr), Pleiades (∼ 100 Myr), Coma Berenices (∼ 450 Myr), and the Hyades (∼ 800 Myr). The variation in the averaged abundances among F stars generally reflects the differences between the initial compositions of the clusters in their various birthplaces. The differences of the averaged chemical composition of A stars may also reveal the effects of radiative difussion for the stars of different ages. We also discuss the methods, resolutions and wavelength coverages of spectra and discrepancies in the derived microturbulent velocities among the various studies to check if these studies are comparable. We also present the pattern of mean abundances and metallicity for the M6 cluster determined by spectral analysis of GIRAFFE spectra acquired with the VLT, Paranal Observatory.
Research Interests: Pleiades and Open cluster
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Using high dispersion high quality spectra of HD 30085 obtained with the echelle spectrograph SOPHIE at Observatoire de Haute Provence, we show that this star, hitherto classified as a A0IV superficially normal star, is actually a... more
Using high dispersion high quality spectra of HD 30085 obtained with the echelle spectrograph SOPHIE at Observatoire de Haute Provence, we show that this star, hitherto classified as a A0IV superficially normal star, is actually a Chemically Peculiar star of the HgMn type. Spectrum synthesis reveals large overabundances of Mn, Sr, Y, Zr , Pt and Hg which are characteristic of HgMn stars. We therefore propose that this interesting object be reclassified as a B8 HgMn star.
Research Interests: Physics and Astrophysics
In table 5 and 6, we present the abundances of 15 chemical elements in 16 A and 26 F stars members of the Hyades open cluster. These abundances are relative to hydrogen and to the sun. The abundances derived for procyon are also included.... more
In table 5 and 6, we present the abundances of 15 chemical elements in 16 A and 26 F stars members of the Hyades open cluster. These abundances are relative to hydrogen and to the sun. The abundances derived for procyon are also included. The absolute parameters for the observed stars as the luminosity, effective temperature, mass and fractional age (fraction of time spent on the Main Sequence) are displayed in table 7. Table 8 collects the abundances derived for each transition for each studied element in all A and F stars including Procyon (F5V) which served as control star for the spectral synthesis. In this table, the absolute values are represented (log(X/H)+12) and the wavelengths are in Angstrom. (4 data files).
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A description of the data (high resolution synthetic spectra and spectral energy distributions) in the POLLUX database is presented in the form of an ascii table providing parameters that can be queried (Teff, logg, code, metallicity) and... more
A description of the data (high resolution synthetic spectra and spectral energy distributions) in the POLLUX database is presented in the form of an ascii table providing parameters that can be queried (Teff, logg, code, metallicity) and giving the filename and URL where a fits file can be retrieved. (1 data file).
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Abundance anomalies have been determined at the surface of many field and open cluster A and F dwarfs. These abundance anomalies are most likely caused by microscopic diffusion at work within the stable envelopes of A stars. However... more
Abundance anomalies have been determined at the surface of many field and open cluster A and F dwarfs. These abundance anomalies are most likely caused by microscopic diffusion at work within the stable envelopes of A stars. However diffusion can be counteracted by several other mixing processes such as convection, rotational mixing and mass loss. We present a short review of the surface abundance patterns of A/F stars in the Pleiades (100 Myr), Coma Berenices (450 Myr) and Hyades (650 Myr) open clusters. Real star-to-star variations of the abundances were found for several chemical elements in the A dwarfs in these clusters. The derived abundances are then compared to evolutionary models from the Montreal group. These comparisons strongly suggest the occurence of hydrodynamical processes at play within the radiative zones of these stars and hindering the effects of microscopic diffusion (mixing processes/mass loss). In the frame of Gaia mission, simulations are presented that predi...
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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 Montr\'eal code, a series of evolutionary models for two... 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 Montr\'eal code, a series of evolutionary models for two A stars members of these two clusters. None of the models reproduces entirely the overall shape of the abundances patterns. The inclusion of competing processes such as rotational mixing in the radiative zones of these stars seems necessary to improve the agreement between observed and predicted abundances patterns.
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ABSTRACT Royer et al. [7] studied a sample of A0–A1 stars that did not contain any previously known binary or chemically peculiar (CP) star. They showed that the distribution of the equatorial rotational velocities of the sample is... more
ABSTRACT Royer et al. [7] studied a sample of A0–A1 stars that did not contain any previously known binary or chemically peculiar (CP) star. They showed that the distribution of the equatorial rotational velocities of the sample is bimodal. We report here the preliminary results of the abundance analysis of 47 of those &amp;quot;normal&amp;quot; A0– A1 main-sequence stars. The selection criterion was v sin i &amp;lt; 65 km s −1 . An hierarchical classification based on the abundances has supported the identification of new CP stars, while normal composition was confirmed the for ∼40% of the sample. That clean sample of normal A0–A1 stars with low v sin i was then examined for intrinsically slow rotators.
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In table 11, we present the magnitudes values for Gaia, Johnson-Cousins, Hipparcos, Tycho and Sloan bands. These magnitude are derived for the BaSeL3.1 sample stars with four absorption values (0,1,3,5). Table 12 displays the bolometric... more
In table 11, we present the magnitudes values for Gaia, Johnson-Cousins, Hipparcos, Tycho and Sloan bands. These magnitude are derived for the BaSeL3.1 sample stars with four absorption values (0,1,3,5). Table 12 displays the bolometric correction un G, BP and RP for the BaSeL3.1 stars. Table 13 represents the computed ratios for interstellar absorption in different bands. (3 data files).
The current status of microturbulent velocity and abundance determinations for A and F dwarfs in open clusters is reviewed. A programme to observe several tens of A and F dwarfs in open clusters of various ages was initiated several years... more
The current status of microturbulent velocity and abundance determinations for A and F dwarfs in open clusters is reviewed. A programme to observe several tens of A and F dwarfs in open clusters of various ages was initiated several years ago. We have performed high resolution high signal-to-noise spectroscopy of stars well distributed in mass along the Main Sequence. Microturbulent velocities and abundances of several chemical elements have been derived iteratively by fitting grids of synthetic spectra calculated in LTE to the observed spectra. Curve of growths were used in a few instances as well. The results obtained are reviewed for Coma Berenices, the Pleiades, Alpha Persei and the Ursa Major moving group.The microturbulent velocities exhibit a broad maximum in the range A5V to about F0V as indicated in Smalley (2004).
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Abundances of Fe, Ti, Cr, Mg and He have been determined for 10 stars (5 B, 3 A, 1 F dwarfs and 1 chemically peculiar giant B8IIIp with apparent velocities ranging from 15 to 130 km.s-1) of the Alpha Persei open cluster (log age=7.40... more
Abundances of Fe, Ti, Cr, Mg and He have been determined for 10 stars (5 B, 3 A, 1 F dwarfs and 1 chemically peculiar giant B8IIIp with apparent velocities ranging from 15 to 130 km.s-1) of the Alpha Persei open cluster (log age=7.40 yrs). The method consists in adjusting grids of synthetic spectra calculated with SYNSPEC48 code (Hubeny & Lanz 1992) in the region of 4450-4600 Å and Kurucz's ATLAS9 model atmospheres. A correlation may exist between [Fe/H] and the effective temperature. Further observations of a much larger sample of B, A and F stars in this cluster are foreseen in the near future.