arXiv subjects

Olivier Garde

Publications and source records attributed to Olivier Garde.

15 recordsLinked to original sources

arXiv2026

From Light to Sound: Spectroscopic Evolution & Sonification of the flaring Nova V612 Scuti

We present photometric and spectroscopic observations of the 2017 Galactic nova V612 Sct, whose optical evolution was marked by multiple unusually large maxima. The eruption included two prominent flares lasting around a month each and reaching amplitudes of about 2.5 mag, followed by a series of smaller flares. Extensive spectroscopic monitoring reveals a striking pattern: with each flare, new absorption systems emerge at progressively higher velocities. This behavior, also seen in other flaring novae, provides evidence for repeated episodes of mass ejection or outflow at increasing velocities. V612 Sct also alternated between Fe II and He/N spectral phases during different stages of the eruption, establishing a clear connection between the photometric flares and major spectral transitions. We present two-dimensional dynamic spectra that directly trace the appearance of new absorption features contemporaneous with the light-curve flares. We also introduce a sonification of the spectroscopic sequence, offering an alternative representation of the temporal evolution of the eruption. These results support a picture in which repeated ejection episodes and shock formation play a central role in powering the multiple maxima observed in flaring novae.

astro-ph.SR↗

arXiv2026

Symbiotic binaries in the Gaia data. II. Symbiotic candidates from general variability classification in DR3

We investigate the reliability of the symbiotic star class in the general variability classification of Gaia DR3 and search for new genuine symbiotic systems among the objects assigned to this category. The sample contains 649 sources, including 246 previously known symbiotic stars, 61 literature candidates, and 339 new candidates proposed by the Gaia variability pipeline. Diagnostics based on the Gaia colour-magnitude diagram, near-infrared photometry, and the pseudo-equivalent width of H$α$ indicate that a large fraction of the new candidates are likely contaminants, predominantly pulsating red giants. To quantify the contamination, we constructed a Random Forest classifier trained on confirmed symbiotic stars and on Mira and semi-regular variables, using Gaia photometry, variability parameters, H$α$ measurements from XP spectra, and infrared colours. The classifier reaches a balanced accuracy of $\approx$0.94 and efficiently separates most symbiotic binaries from single evolved stars, leaving only eight strong candidates among the 339 newly proposed objects. Follow-up spectroscopy confirms three new symbiotic stars through the presence of high-excitation emission lines, while several additional objects remain possible symbiotics. Our results show that the Gaia DR3 variability classification efficiently recovers known symbiotic stars but has low purity due to overlap with pulsating red giants. The small number of newly confirmed systems implies that the discrepancy between predicted and observed Galactic symbiotic populations remains unresolved, although Gaia provides a powerful basis for future searches combining variability, spectroscopic indicators, and multi-wavelength data.

astro-ph.SR↗

arXiv2026

Analysis of the Gaia DR3 planetary nebula candidates and the possible symbiotic stars among them

The Gaia DR3, released in June 2022, included low-resolution BP/RP (XP) spectra that have been exploited for the classification of various types of emission-line objects using machine-learning techniques. The Gaia Extended Stellar Parametrizer for Emission-Line Stars (ESP-ELS) algorithm identified 273 sources as potential planetary nebulae (PNe). We aim to analyze the PN sample produced by the ESP-ELS algorithm to investigate the true nature of the objects classified as PNe. We extracted all sources from the catalog classified as PNe by the ESP-ELS algorithm and filtered out 200 objects with secure classifications available in the literature. Of these, $\sim$65% correspond to known Galactic or Magellanic compact PNe, and $\sim$20% to D- or D'-type symbiotic systems. The XP spectra of the remaining sources were visually inspected, leading to a subset of 14 promising candidates showing strong emission features attributable to H$α$ and [O III] $λ$5007. Although typical of PNe, such features are also consistent with D- or D'-type symbiotics, known to mimic compact PNe. We obtained spectroscopic follow-up observations for these objects with the 2SPOT facilities in Chile and France, complemented by an analysis of archival photometric data to further constrain their nature. We report the identification of nine bona-fide or likely D- or D'-type symbiotic systems, one planetary nebula in the LMC, one polar cataclysmic variable, and three possible Be stars in (or in the direction of) the SMC, within our sample of 14 objects.

astro-ph.SR↗

arXiv2026

What determines the $γ$-ray luminosities of classical novae?

Classical novae in the Milky Way have now been well-established as high-energy GeV $γ$-ray sources. In novae with main-sequence companions, this emission is believed to result from shocks internal to the nova ejecta, as a later fast wind collides with an earlier slow outflow. To test this model and constrain the $γ$-ray production mechanism, we present a systematic study of a sample of recent Galactic novae, comparing their $γ$-ray properties ($γ$-ray luminosity and duration) with their outflow velocities, peak $V$-band magnitudes, and the decline times of their optical light curves ($t_2$). We uniformly estimate distances in a luminosity-independent manner, using spectroscopic reddening estimates combined with three-dimensional Galactic dust maps. Across our sample, $γ$-ray luminosities ($>$100 MeV) vary by three orders of magnitude, spanning $10^{34}-10^{37}$ erg s$^{-1}$. Novae with larger velocity of the fast outflow (or larger differential between the fast and slow outflow) have larger $γ$-ray luminosities, but are detectable for a shorter duration. The optical and $γ$-ray fluxes are correlated, consistent with substantial thermal emission in the optical from shock-heated gas. Across six novae with $γ$-ray and infrared light curves, evidence for dust formation appears soon after the end of the detected $γ$-ray emission. Dusty and non-dusty novae appear to have similar $γ$-ray luminosities, though novae that have more material processed by the shocks may be more likely to form dust. We find that the properties of the $γ$-ray emission in novae depend heavily on the ejecta properties, and are consistent with expectations for internal shocks.

astro-ph.HE↗

arXiv2025

Revisiting the Classics: On the Optical Colours of Novae as Standard Crayons

We present a systematic study of the $BVRI$ colours of novae over the course of their eruptions. Where possible, interstellar reddening was measured using the equivalent widths of Diffuse Interstellar Bands (DIBs). Some novae lack spectra with sufficient resolution and signal-to-noise ratios; therefore, we supplement as necessary with 3D and 2D dust maps. Utilising only novae with DIB- or 3D-map-based $E(B-V)$, we find an average intrinsic $(B-V)_0$ colour of novae at $V$-band light curve peak of 0.20 with a standard deviation of 0.31, based on 25 novae. When the light curve has declined by 2 magnitudes ($t_2$), we find an average $(B-V)_0 = -0.03$ with a standard deviation of 0.19. These average colours are consistent with previous findings, although the spreads are larger than previously found due to more accurate reddening estimates. We also examined the intrinsic $(R-I)_0$ and $(V-R)_0$ colours across our sample. These colours behave similarly to $(B-V)_0$, except that the $(V-R)_0$ colour gets redder after peak, likely due to the contributions of emission line flux. We searched for correlations between nova colours and $t_2$, peak $V$-band absolute magnitude, and GeV $γ$-ray luminosity, but find no statistically significant correlations. Nova colours can therefore be used as standard ``crayons" to estimate interstellar reddening from photometry alone, with 0.2--0.3 mag uncertainty. We present a novel Bayesian strategy for estimating distances to Galactic novae based on these $E(B-V)$ measurements, independent of assumptions about luminosity, built using 3D dust maps and a stellar mass model of the Milky Way.

astro-ph.HE↗

arXiv2025

V4141 Sgr: Outflows and repeated outbursts

In this work, we analyze the ongoing brightening of the poorly studied symbiotic star V4141 Sgr and examine its long-term variability. We present new low-resolution spectroscopic observations of the system in its bright state and combine them with multi-color photometric data from our observations, ASAS-SN, ATLAS, and Gaia DR3. To investigate its long-term evolution, we also incorporate historical data, including photographic plates, constructing a light curve spanning more than a century. Our analysis reveals that V4141 Sgr has undergone multiple outbursts, with at least one exhibiting characteristics typical of "slow" symbiotic novae. The current outburst is characterized by the ejection of optically thick material and possibly bipolar jets, a phenomenon observed in only a small fraction of symbiotic stars. These findings establish V4141 Sgr as an intriguing target for continued monitoring.

astro-ph.SR↗

arXiv2025

Spectral evolution of the narrow emission line components in optical during the 2022 nova eruption of U Scorpii

There remains debate over whether the accretion disk survives or is entirely disrupted after the nova eruption. In our previous paper, Muraoka et al. (2024, PASJ, 76, 293) have photometrically demonstrated that the surviving accretion disk was expanded close to the L1 point during the optical plateau stage and then drastically shrank to the tidal truncation radius after the optical plateau stage ended. To approach the clarification of the physical mechanism that drives these structural changes, we have then conducted systematic analyses of the spectral evolution of the narrow emission line components in optical over 22 d following the optical peak during the 2022 nova eruption of U Scorpii (U Sco). Additionally, we present its optical spectrum in quiescence 794 d after the 2022 nova eruption. We find that the single-peaked narrow components of H$α$ and He II 4686 appeared almost simultaneously between roughly days 6 and 8, preceding the onset of the disk eclipses observed after day 11. This finding suggests that the nova wind near the binary system may be the primary origin of these narrow components and even remained active several days after the nova eruption with a velocity of approximately 1000 km s$^{-1}$, likely driving the expansion of the accretion disk until the end of the optical plateau stage. While the contribution of the rotating accretion disk might be dominated by that of the nova wind in the H$α$ line profile, the outward surface flow from the expanded disk might also contribute to these narrow features during the optical plateau stage, making the single-peaked narrow line profiles more pronounced.

astro-ph.SR↗

arXiv2025

Search for new Galactic Wolf-Rayet stars using Gaia DR3. I. Candidate selection and the follow-up of the bright sample

Gaia DR3, released in June 2022, included low-resolution XP spectra that have been used for the classification of various types of emission-line objects through machine-learning techniques. The Gaia Extended Stellar Parametrizer for Emission-Line Stars (ESP-ELS) algorithm identified 565 sources as potential Wolf-Rayet (WR) stars. Over half of them were already known as WR stars in the Milky Way and Magellanic Clouds. This study aimed to utilize Gaia DR3 data to identify new Galactic WR stars. We extracted all sources classified as WC or WN type stars by the ESP-ELS algorithm from the Gaia catalog. By applying judicious 2MASS color selection criteria, leveraging Gaia H$α$ measurements, and filtering out objects already cataloged in various databases, we selected 37 bright candidates ($G \leq $ 16 mag) and 22 faint candidates ($G > $ 16 mag). Spectroscopic follow-up observations of these candidates were conducted using the 2SPOT facilities in Chile and France, as well as 1-m C2PU's Epsilon telescope at the Calern Observatory. This paper focuses on the brighter sample. Among the 37 targets, we confirmed 17 and 16 new Galactic WC and WN type WR stars, respectively. Three of them were recently reported as new WR stars in an independent study. The Gaia mission provides a valuable resource for uncovering WR stars missed in earlier surveys. While this work concentrated on a relatively small starting sample provided by the ESP-ELS algorithm, our findings highlight the potential for refining selection criteria to identify additional candidates not included in the outputs of the algorithm. Furthermore, the observation program underscores the utility of small telescopes in acquiring initial spectral data for sources with magnitudes up to $G \sim 16$ mag.

astro-ph.SR↗

arXiv2024

Gaia23ckh: Symbiotic outburst of the assumed Mira variable V390 Sco

The poorly studied variable star V390 Sco, previously classified as a Mira pulsator, was detected in a brightening event by the ESA Gaia satellite in September 2023. This work presents an analysis of available archival multifrequency photometric data of this target, along with our spectroscopic observations. Our findings lead to the conclusion that V390 Sco is a new symbiotic star identified by Gaia, currently undergoing a classical symbiotic outburst. Additionally, we uncovered three prior outbursts of this system through archival photometry. The outbursts recur approximately every 2330 - 2400 days, and we hypothesize the periastron passage in an eccentric orbit may trigger them, similarly to the case of BX Mon, DD Mic, or MWC 560. A detailed investigation into the nature of the donor star suggested that V390 Sco is an S-type symbiotic star, likely hosting a less evolved, semiregularly pulsating giant donor, but not a Mira variable.

astro-ph.SR↗

arXiv2023

A planetary collision afterglow and transit of the resultant debris cloud

Planets grow in rotating disks of dust and gas around forming stars, some of which can subsequently collide in giant impacts after the gas component is removed from the disk. Monitoring programs with the warm Spitzer mission have recorded significant and rapid changes in mid-infrared output for several stars, interpreted as variations in the surface area of warm dusty material ejected by planetary-scale collisions and heated by the central star: e.g., NGC 2354-ID8, HD 166191 and V844 Persei. Here we report combined observations of the young (about 300 Myr), solar-like star ASASSN-21qj: an infrared brightening consistent with a blackbody temperature of 1000 K and a luminosity of 4 percent of that of the star lasting for about 1000 days, partially overlapping in time with a complex and deep wavelength-dependent optical eclipse that lasted for about 500 days. The optical eclipse started 2.5 years after the infrared brightening, implying an orbital period of at least that duration. These observations are consistent with a collision between two exoplanets of several to tens of Earth masses at 2 to 16 au from the central star. Such an impact produces a hot, highly-extended post-impact remnant with sufficient luminosity to explain the infrared observations. Transit of the impact debris, sheared by orbital motion into a long cloud, causes the subsequent complex eclipse of the host star.

astro-ph.EP↗

arXiv2023

Intensity Interferometry observations of the H$α$ envelope of $γ$ Cas with MéO and a portable telescope

We report on observations of the extended environment of the bright Be star $γ$-Cas performed using intensity interferometry measurements within its H$α$ emission line. These observations were performed using a modified version of the I2C intensity interferometry instrument installed onto the 1.54 meter MéO optical metrology telescope and a portable 1-meter telescope (T1M). In order to better constrain the extent of the H$α$ envelope, observations were performed for two different positions of the T1M telescope, corresponding to an intermediate and long baselines in which the extended region was partially and fully resolved. We find that the observed data are consistent with past interferometric observations of $γ$-Cas. These observations demonstrate the capability to equip optical telescopes of different optical designs with intensity interferometry capabilities and illustrate the potential to scale a similar system onto many additional telescopes.

astro-ph.IM↗

arXiv2022

WHTZ 1: A high excitation Planetary Nebula not a gaseous cocoon from runaway star HD 185806

We present evidence that the nebular cocoon and bow-shock emission nebula putatively and recently reported as deriving from the 9th magnitude "runaway" star HD 185806 is the previously discovered but obscure planetary nebula WHTZ 1 (Ra 7). It has a Gaia DR3 G~16 blue ionizing star at its geometric centre. We present imagery, spectroscopy, other data and arguments to support that this emission source is a high excitation Planetary Nebula not a stellar wind bow shock.

astro-ph.SR↗

arXiv2022

DeGaPe 35: Amateur discovery of a new southern symbiotic star

In this work, we present the discovery and characterization of a new southern S-type symbiotic star, DeGaPe 35. We have obtained the low-resolution spectroscopic observations and supplemented them with photometry from Gaia DR3 and other surveys. The optical spectra of this target show prominent emission lines, including highly ionized [Fe VII] and O VI lines. The cool component of this symbiotic binary is an M4-5 giant with effective temperature ~ 3 380 - 3 470 K and luminosity ~ 3 000 L$_\odot$ (for the adopted distance of 3 kpc). The hot component is a shell-burning white dwarf. The photometric observations of the Gaia satellite, published recently in the Gaia DR3 suggested the variability with the period of about 700 - 800 days that we tentatively attributed to the orbital motion of the binary.

astro-ph.SR↗

arXiv2022

Combined spectroscopy and intensity interferometry to determine the distances of the blue supergiants P Cygni and Rigel

In this paper we report on spatial intensity interferometry measurements within the H$α$ line on two stars: the Luminous Blue Variable supergiant \PCygni\,and the late-type B supergiant Rigel. The experimental setup was upgraded to allow simultaneous measurement of two polarization channels, instead of one in our previous setup, and the zero baseline correlation function on-sky to validate independent estimates obtained from the stellar spectrum and the instrumental spectral throughput. Combined with simultaneous spectra measurements and based on radiative transfer models calculated with the code CMFGEN, we were able to fit our measured visibility curves to extract the stellar distances. Our distance determinations for both \PCygni\ (1.61 $\pm$ 0.18 kpc) and Rigel (0.26 $\pm$ 0.02 kpc) agree very well with the values provided by astrometry with the Gaia and Hipparcos missions, respectively. This result for Rigel was obtained by adopting a stellar luminosity of $L_{\star}$ = 123000 $L_{\odot}$, which is reported in the literature as being consistent with the Hipparcos distance to Rigel. However, due to the lack of consensus on Rigel's luminosity, we also explore how the adoption of the stellar luminosity in our models affects our distance determination for Rigel. In conclusion, we support, in an independent way, the distance to Rigel as the one provided by the Hipparcos mission, when taking the luminosity of 123000 $L_{\odot}$ at face value. This study is the first successful step towards extending the application of the Wind Momentum Luminosity Relation method for distance calibration from an LBV supergiant to a more normal late-type B supergiant.

astro-ph.SR↗

arXiv2021

Outbursts and stellar properties of the classical Be star HD 6226

The bright and understudied classical Be star HD 6226 has exhibited multiple outbursts in the last several years during which the star grew a viscous decretion disk. We analyze 659 optical spectra of the system collected from 2017-2020, along with a UV spectrum from the Hubble Space Telescope and high cadence photometry from both TESS and the KELT survey. We find that the star has a spectral type of B2.5IIIe, with a rotation rate of 74% of critical. The star is nearly pole-on with an inclination of $13.4$ degree. We confirm the spectroscopic pulsational properties previously reported, and report on three photometric oscillations from KELT photometry. The outbursting behavior is studied with equivalent width measurements of H$α$ and H$β$, and the variations in both of these can be quantitatively explained with two frequencies through a Fourier analysis. One of the frequencies for the emission outbursts is equal to the difference between two photometric oscillations, linking these pulsation modes to the mass ejection mechanism for some outbursts. During the TESS observation time period of 2019 October 7 to 2019 November 2, the star was building a disk. With a large dataset of H$α$ and H$β$ spectroscopy, we are able to determine the timescales of dissipation in both of these lines, similar to past work on Be stars that has been done with optical photometry. HD 6226 is an ideal target with which to study the Be disk-evolution given its apparent periodic nature, allowing for targeted observations with other facilities in the future.

astro-ph.SR↗