arXiv subjects

Lou Baya Ould Rouis

Publications and source records attributed to Lou Baya Ould Rouis.

9 recordsLinked to original sources

arXiv2026

Searching for Massive Remnant Planetary System Hosts with White Dwarf Gaia XP Spectra

Remnant planetary systems around massive white dwarfs ($M_{\rm WD} > 0.8\,{\rm M}_\odot$) are intrinsically rare and remain poorly characterized. As the evolutionary endpoints of intermediate-mass B- and A-type stars, these white dwarfs provide valuable constraints on the formation and survival of planetary systems around higher-mass progenitors. In the absence of large, dedicated spectroscopic surveys, extremely-low-resolution Gaia XP spectra provide a powerful means of identifying candidate metal-polluted white dwarfs. We obtained follow-up observations with low-resolution optical spectroscopy (R=500-4000) to search primarily for Ca II H & K absorption in massive, metal-polluted white dwarf candidates selected from Gaia XP spectra to increase the sample of massive remnant planetary system hosts among objects with effective temperatures below $\sim15{,}000$ K. We find that Gaia XP spectra recover metal-polluted white dwarfs efficiently, confirming 82$\pm$5% recovery of photospheric metals (59 of 72 targets) in high-confidence candidates of our sample (with classifier probability $P_{\rm DZ} > 0.65$). However, Gaia photometric masses derived from $G$, $G_{BP}$, and $G_{RP}$ broad-band photometry are not sufficient to identify the most massive systems, as reliable mass determinations require detailed metal-enriched atmospheric fits. We present a selection of cool white dwarfs confirmed to be both massive and metal polluted, demonstrating that Gaia XP spectra can substantially expand the sample of white dwarf exoplanet hosts. Our search led to the discovery of a new transiting debris system, SDSS J1624+5404, the 21st published system to date and eighth to show periodic transits.

astro-ph.SR↗

arXiv2026

White dwarf planetary systems in the ultraviolet

Almost every known planet host will evolve into a white dwarf, and the surviving planetary material will continue to orbit this stellar remnant. Asteroids perturbed onto star-grazing orbits will become disrupted, forming an accretion disk which causes "enrichment" of the otherwise pure hydrogen or helium atmosphere. Measurements of these photospheric abundances give detailed insights into the interior compositions of exo-planetesimals with an accuracy not possible for intact exoplanets around main sequence stars. This method has revealed the diversity of rocky material in our solar neighborhood, including primitive, chondritic planetesimals, fragments of planetary cores, and even analogues of Kuiper belt objects. The planetesimal abundances can be used as an input to interior structure models. The far-ultraviolet is a key wavelength range for this field because it contains strong transitions for almost every element of interest, many of which are undetectable using ground-based optical spectroscopy. Without the FUV, we will no longer have access to the C, N, P, S content of exoplanetary bodies and thus will no longer be able to probe how volatiles interact with refractories, which is crucial to understanding planet formation-and even the origin of life. The medium resolution and high sensitivity of COS on HST has been indispensable in determining the compositions of dozens of exo-planetesimals. However, the only two medium resolution FUV-capable spectrographs are currently onboard HST, with no plans for replacements until the 2040s. An extension to the HST mission is critical for the field of white dwarf planetary systems, because the loss of FUV capability would leave us blind to volatiles. Boosting the orbit of HST would allow us to measure volatile abundances, determine the rocky planetary occurrence rate, investigate differentiation, and probe for photospheric abundance variability.

astro-ph.IM↗

arXiv2026

From Hubble to HWO: Bridging the Frontier of White Dwarf Exoplanet Science

White dwarf stars, the endpoint of stellar evolution for 97% of stars in our Milky Way, offer a unique and powerful window into the bulk elemental composition of rocky exoplanetary bodies. Up to 50% of single white dwarfs are observed with photospheric metal lines from accreted exoplanetary bodies (called 'polluted' white dwarfs), and spectroscopic observations reveal the bulk composition of this material. High-resolution (R>15,000) UV spectra are essential for detecting many elements present in the material, such as the volatile elements imperative for habitability studies (C, N, O, P, S) and key rock-forming elements required to constrain interior structure (e.g. Fe, Si, Mg, Al, Ni). HST, through its COS and STIS spectrographs, remains the only facility capable of performing this science in the near future. Looking to the next decade, the scientific case for continued HST UV observations of polluted white dwarfs is compelling on three fronts (i) as a standalone to enable the bulk composition of exoplanetary material to be measured in a statistically significant sample, (ii) as essential groundwork for the Habitable Worlds Observatory (HWO), and (iii) in a powerful synergy with JWST, to enable characterization of the bulk mineralogy and bulk elemental composition of exoplanetary material. This white paper argues that continued UV spectroscopic capabilities with HST is a high-return investment for white dwarf and exoplanet science, and preserving and prioritizing HST's UV capabilities through at least 2035 is crucial to maximize the scientific return from HST, JWST, and HWO.

astro-ph.IM↗

arXiv2026

White Dwarf Merger Remnants with Cooling Delays on the Q Branch Lack Strong Magnetism

A population of anomalous ultra-massive white dwarfs discovered with Gaia, often referred to as the Q branch, show high (multi-Gyr) cooling delays produced by exotic physical mechanisms. They are believed to be the products of stellar mergers, but the exact origin and formation channel remain unclear. We obtained a spectroscopically complete, volume-limited sample of the Q branch region within 100 pc, and found significant differences in atmospheric composition and rotation rates as a function of tangential velocity. In particular, we discover that stellar remnants with the longest cooling delays do not show strong magnetism nor detectable short-period rotational variability, as opposed to what is generally believed for double-degenerate mergers. This indicates that either these white dwarfs arise from a formation channel with no strong magnetism induced, or that the magnetism produced from the merger dissipates over the cooling delay timescales. Our follow-up photometry has also discovered pulsations in the second and third hydrogen-dominated DAQ white dwarfs, one hotter than 15,500 K, possibly extending the boundaries of the DAV instability strip for white dwarfs with thin hydrogen layers.

astro-ph.SR↗

arXiv2025

Picture an Astronomer: Best Practices for Retaining Talent in Astrophysics

Women are consistently underrepresented in astrophysics yet are simultaneously subject to disproportionate attrition at every career stage. This disparity between demonstrated efficacy in job performance and ultimate career outcome was the primary motivation for the Picture an Astronomer series, which included both targeted public outreach to increase representation of women in astrophysics and high-level, solution-oriented discussions among professional astronomers. In March 2025, more than 200 astronomers came together in a hybrid-format symposium focused on the state of the field for female scientists, combining scientific exchange with discussions of policies and practices to strengthen retention of talent in the field. This white paper is the result of those discussions, offering a wide range of recommendations developed in the context of gendered attrition in astrophysics but which ultimately support a healthier climate for all scientists alike.

astro-ph.IM↗

arXiv2025

Transiting Planetary Debris near the Roche Limit of a White Dwarf on a 4.97$\,$hr Orbit -- and its Vanishing

We present the discovery of deep, irregular, periodic transits towards the white dwarf ZTF$\,$J1944$+$4557 using follow-up time-series photometry and spectroscopy from Palomar, Keck, McDonald, Perkins, and Lowell observatories. We find a predominant period of 4.9704$\,$hr, consistent with an orbit near the Roche limit of the white dwarf, with individual dips over 30$\%$ deep and lasting between 15 and 40 minutes. Similar to the first known white dwarf with transiting debris, WD$\,$1145$+$017, the transit events are well-defined with prominent out-of-transit phases where the white dwarf appears unobscured. Spectroscopy concurrent with transit photometry reveals the average Ca$\,$K equivalent width remains constant in and out of transit. The broadening observed in several absorption features cannot be reproduced by synthetic photospheric models, suggesting the presence of circumstellar gas. Simultaneous $g+r$- and $g+i$-band light curves from the CHIMERA instrument reveal no color dependence to the transit depths, requiring transiting dust grains to have sizes $s \gtrsim0.2\,μ$m. The transit morphologies appear to be constantly changing at a rate faster than the orbital period. Overall transit activity varies in the system, with transit features completely disappearing during the seven months between our 2023 and 2024 observing seasons and then reappearing in 2025$~$March, still repeating at 4.9704$\,$hr. Our observations of the complete cessation and resumption of transit activity provide a novel laboratory for constraining the evolution of disrupted debris and processes like disk exhaustion and replenishment timescales at white dwarfs.

astro-ph.SR↗

arXiv2024

Constraints on Remnant Planetary Systems as a Function of Main-Sequence Mass with HST/COS

As the descendants of stars with masses less than 8 M$_{\odot}$ on the main sequence, white dwarfs provide a unique way to constrain planetary occurrence around intermediate-mass stars (spectral types BAF) that are otherwise difficult to measure with radial-velocity or transit surveys. We update the analysis of more than 250 ultraviolet spectra of hot ($13{,}000$ K $< T_{\mathrm{eff}} <$ $30{,}000$ K), young (less than $800$ Myr) white dwarfs collected by the Hubble Space Telescope, which reveals that more than 40% of all white dwarfs show photospheric silicon and sometimes carbon, signpost for the presence of remnant planetary systems. However, the fraction of white dwarfs with metals significantly decreases for massive white dwarfs (M$_{\rm WD}~>$ 0.8 M$_{\odot}$), descendants of stars with masses greater than 3.5 M$_{\odot}$ on the main sequence, as just $11^{+6}_{-4}$% exhibit metal pollution. In contrast, $44\pm6$% of a subset of white dwarfs (M$\rm _{WD}~<$ 0.7 M$_{\odot}$) unbiased by the effects of radiative levitation are actively accreting planetary debris. While the population of massive white dwarfs is expected to be influenced by the outcome of binary evolution, we do not find merger remnants to broadly affect our sample. We connect our measured occurrence rates of metal pollution on massive white dwarfs to empirical constraints into planetary formation and survival around stars with masses greater than 3.5 M$_{\odot}$ on the main sequence.

astro-ph.SR↗

arXiv2024

Using 3.4-$μ$m Variability towards White Dwarfs as a Signpost of Remnant Planetary Systems

Roughly 2% of white dwarfs harbor planetary debris disks detectable via infrared excesses, but only a few percent of these disks show a gaseous component, distinguished by their double-peaked emission at the near-infrared calcium triplet. Previous studies found most debris disks around white dwarfs are variable at 3.4 and 4.5 $μ$m, but they analyzed only a few of the now 21 published disks showing calcium emission. To test if most published calcium emission disks exhibit large-amplitude stochastic variability in the near-infrared, we use light curves generated from the unWISE images at 3.4 $μ$m that are corrected for proper motion to characterize the near-infrared variability of these disks against samples of disks without calcium emission, highly variable cataclysmic variables, and 3215 isolated white dwarfs. We find most calcium emission disks are extremely variable: 6/11 with sufficient signal-to-noise show high-amplitude variability in their 3.4-$μ$m light curves. These results lend further credence to the notion that disks showing gaseous debris in emission are the most collisionally active. Under the assumption that 3.4-$μ$m variability is characteristic of white dwarfs with dusty debris disks, we generate a catalog of 104 high-confidence near-infrared variable white dwarfs, 84 of which are published as variable for the first time. We do near-infrared spectroscopic follow-up of seven new candidate 3.4-$μ$m variables, confirming at least one new remnant planetary system, and posit that empirical near-infrared variability can be a discovery engine for debris disks showing gaseous emission.

astro-ph.SR↗

arXiv2024

A Test of Spectroscopic Age Estimates of White Dwarfs using Wide WD+WD Binaries

White dwarf stars have been used for decades as precise and accurate age indicators. This work presents a test of the reliability of white dwarf total ages when spectroscopic observations are available. We conduct follow-up spectroscopy of 148 individual white dwarfs in widely separated double-white-dwarf (WD+WD) binaries. We supplement the sample with 264 previously published white dwarf spectra, as well as 1292 high-confidence white dwarf spectral types inferred from their Gaia XP spectra. We find that spectroscopic fits to optical spectra do not provide noticeable improvement to the age agreement among white dwarfs in wide WD+WD binaries. The median age agreement is $\approx$$1.5σ$ for both photometrically and spectroscopically determined total ages, for pairs of white dwarfs with each having a total age uncertaintiy $<$ 20\%. For DA white dwarfs, we further find that photometrically determined atmospheric parameters from spectral energy distribution fitting give better total age agreement ($1.0σ$, 0.2 Gyr, or 14\% of the binary's average total age) compared to spectroscopically determined parameters from Balmer-line fits (agreement of $1.5σ$, 0.3 Gyr, or 28\% of binary's average total age). We find further evidence of a significant merger fraction among wide WD+WD binaries: across multiple spectroscopically identified samples, roughly 20\% are inconsistent with a monotonically increasing initial-final mass relation. We recommend the acquisition of an identification spectrum to ensure the correct atmospheric models are used in photometric fits in order to determine the most accurate total age of a white dwarf star.

astro-ph.SR↗