arXiv subjects

P. Le Dû

Publications and source records attributed to P. Le Dû.

6 recordsLinked to original sources

arXiv2026

The changing optical and X-ray emission of the dormant $γ$ Cas star HD 45314

$γ$ Cas stars are Oe/Be stars that exhibit bright and hard X-ray emission. HD45314 belonged to this category, but lost its $γ$ Cas characteristics as its circumstellar disc started to fade away. The star was monitored in optical spectroscopy, optical photometry, and X-ray spectroscopy to follow its variability on timescales of hours to years. Time series of magnitudes, colours, line equivalent widths and violet over red peak intensity ratios were analysed with Fourier methods to uncover possible periodicities. For five years, HD45314 exhibited oscillations of its magnitude and line strengths on a timescale of 230 d. It then declined towards a nearly disc-free stage and is now slowly rebuilding its disc. During these phases, the violet over red peak intensity ratio exhibited a modulation on a timescale of about 1190 d. Over the whole campaign, prominent short-term photometric variations, notably at a frequency of 3.369 d$^{-1}$, were probably due to $β$ Cep-type pulsations. The amplitude of these pulsations was stronger during phases of overall brightness changes. In parallel, the X-ray spectrum switched to a low state at the onset of the oscillation phase and has remained in this low state since then. Some residual hard emission is nonetheless observed. HD45314's long-term optical variations bear resemblance with the predictions of smooth particle hydrodynamics calculations, although a close look does reveal some inconsistencies. Unlike some other $γ$ Cas stars, where the X-ray emission remained nearly constant despite major changes of the circumstellar disc, HD45314's X-ray properties changed dramatically as the disc begun to dissipate. These differences might arise from a wider orbital separation between the Oe star and its putative white dwarf companion or from the ablation of the disc by the radiation field of the Oe star.

astro-ph.SR↗

arXiv2025

The birth of Be star disks I. From localized ejection to circularization

Classical Be stars are well known to eject mass, but the details governing the initial distribution and evolution of this matter into a disk are poorly constrained by observations. By combining high-cadence spectroscopy with contemporaneous space photometry from TESS, we have sampled about 30 mass ejection events in 13 Be stars. Our goal is to constrain the geometrical and kinematic properties of the ejecta, facilitating the investigation into the initial conditions and evolution, and understanding its interactions with preexisting material. The photometric variability is analyzed together with measurements of the rapidly changing emission features to identify the onset of outburst events and obtain information about the geometry of the ejecta and its evolution. All Be stars observed with sufficiently high cadence exhibit rapid oscillations of line asymmetry with a single frequency in the days following the start of the event. The emission asymmetry cycles break down after roughly 5 - 10 cycles, with the emission line profile converging toward approximate symmetry. In photometry, several frequencies typically emerge at relatively high amplitude at some point during the mass ejection process. In all observed cases, freshly ejected material was initially within a narrow azimuthal range, indicating it was launched from a localized region on the star. The material orbits the star with a frequency consistent with the near-surface Keplerian orbital frequency. This material circularizes into a disk configuration after several orbital timescales. This is true whether or not there was a preexisting disk. We find no evidence for precursor phases prior to the ejection of mass in our sample. The several photometric frequencies that emerge during outburst are at least partially stellar in origin. (Abstract abridged)

astro-ph.SR↗

arXiv2023

V618 Sgr: Galactic eclipsing symbiotic nova detected in repeated outbursts

V618 Sgr was previously classified as an R CrB-type variable and later as a possible symbiotic star. Our study aims to analyse the nature of this target, which is currently undergoing significant brightening in properties similar to those of known symbiotic novae. We analyse literature information, photometric observations, and 35 new optical spectra. Our findings strongly suggest that V618 Sgr is an eclipsing symbiotic nova currently in outburst. Additionally, since the star has demonstrated at least two similar brightenings in the past, we propose that V618 Sgr could be the first known galactic symbiotic nova observed in repeated outbursts of this type and may host a relatively massive white dwarf.

astro-ph.SR↗

arXiv2022

Grantecan spectroscopic observations and confirmations of Planetary Nebulae candidates in the Northern Galactic Plane

We present Grantecan 10 m telescope (GTC) spectroscopic confirmations of 55 faint Planetary Nebulae (PNe) candidates discovered largely in the INT Photometric H$_α$ Survey of the Northern Galactic Plane (IPHAS) by our pro-am collaboration. We confirm 46 of them as 'True' (T), 4 as 'Likely' (L) and 5 as 'Possible' (P) PNe and including 5 new PNe central star (CSPN) discoveries. This was from observations of 62 new candidates yielding a maximum PN discovery success rate of 89%. The sensitivity and longer wavelength coverage of IPHAS allows PNe to be found in regions of greater extinction and at these lower Galactic latitudes, including PNe in a more advanced evolutionary state and at larger distances compared to previously known Galactic PNe. We use an holistic set of observed characteristics and optical emission-line diagnostics to confirm candidates. Plasma properties have been determined in a self-consistent way using PyNeb. This work is facilitated by the functionality of our powerful, multi-wavelength database 'HASH' (Hong Kong, Australian Astronomical Observatory, Strasbourg Observatory H-alpha Planetary Nebula catalogue) that federates known imaging, spectroscopy and other pertinent data for all Galactic T, L, P PNe and the significant numbers of mimics. Reddenings, corrected radial velocities and PNe electron density and temperature estimates are provided for these new PNe where possible.

astro-ph.SR↗

arXiv2019

Four newly discovered HII galaxies

We present the results of spectroscopy campaigns for planetary nebula candidates, where we have identified four objects as Seyfert galaxies. All observations have been carried out by a group of French amateur astronomers. During the campaigns at the Cote d'Azur observatory at Calern (France), four HII galaxies could be identified. Using the naming convention of our campaign, these objects are (1) App 1 (RA: 22h 49m 20.23s, DEC: +46°07{\arcmin}37.17{\arcsec}), (2) Pre 21 (RA: 18h 04m 19.62s, DEC: +00°08{\arcmin}04.96{\arcsec}), (3) Pre 24 (RA: 04h 25m 53.63s, DEC: +39°49{\arcmin}19.69{\arcsec}), and (4) Ra 69 (RA: 19h 30m 23.64s, DEC: +37°37{\arcmin}06.58{\arcsec}).

astro-ph.GA↗

arXiv2006

Contribution of high energy physics techniques to the medical imaging field

The purpose of this study was to show how advanced concepts of compact, lossless and "Time Of Flight" (TOF) capable electronics similar to those foreseen for the LHC and ILC experiments could be fairly and easily transferred to the medical imaging field through Positron Emission Tomography (PET). As a wish of explanation, the two overriding weaknesses of PET camera readout electronics, namely dead-time and timing resolution, were investigated analytically and with a Monte-Carlo simulator presently dedicated to this task. Results have shown there was room left for count rate enhancement through a huge decrease of the timing resolution well below the nanosecond. The novel electronics scheme suggested for PET in this paper has been partly inspired by the long experience led in High Energy Physics where the latter requirement is compulsory. Its structure entirely pipelined combined to a pixelation of the whole detector should allow dead-times to be suppressed, while the absence of devoted timing channel would remove the preponderant contributions to the timing resolution. To the common solution for timing would substitute an optimal filtering method witch clearly appears as a good candidate as timing resolution of a few tens of picoseconds may be achieved provided the shape of the signal is known and sufficient samples are available with enough accuracy. First investigations have yield encouraging results as a sampling frequency of 50 MHz with a 7 bits precision appears sufficient to ensure the 500ps coincidence timing resolution planed. At this point, there will be a baby step ahead to draw benefice from a TOF implementation to the design and the enormous noise variance enhancement that would come with.

physics.ins-det↗