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Alain Coulais

Publications and source records attributed to Alain Coulais.

10 recordsLinked to original sources
arXiv2025

SNR 1987A : Spitzer data from days 6000 to 8000 revisited

An excess emission has been observed by Spitzer in the [3, 5] micron range of the SNR 1987A spectrum. It is generally argued that this excess could be due to the presence of warm amorphous carbon dust in the equatorial ring (ER) around the supernova, but the proposed models all have problems. This prompted us to present an alternative view on the interpretation of the Spectral Energy Distribution (SED) of SNR 1987A from the near-IR wavelengths to the radio frequencies (from 3 micron up to 1.4 GHz), between 6000 and 8000 days after outburst. We argue that the origin of that excess could be attributed instead to a free-free emission. We show that under very specific conditions (the free-free is self-absorbed at a cut-off frequency imposed by the mass of the emitting region), it could be produced by collisional heating of the gas. We then discuss the time evolution of the various components of the SED. We establish a linear relationship between the growth of the warm carbon dust mass and that of the silicates dust during the analyzed period. Finally, we build the Spitzer light curves and we show that our models reproduce the observations pretty well, although our study clearly favors the free-free case. In conclusion, we argue that the free-free model provides a formally very good description of the data, however the model does require some very specific parameter choices, and results in an unusually low temperature for the ionized gas.

astro-ph.SR↗
arXiv2025

SO2, silicate clouds, but no CH4 detected in a warm Neptune

WASP-107b is a warm (~740 K) transiting planet with a Neptune-like mass of ~30.5 Earth masses and Jupiter-like radius of ~0.94 Jupiter radius, whose extended atmosphere is eroding. Previous observations showed evidence for water vapour and a thick high-altitude condensate layer in WASP-107b's atmosphere. Recently, photochemically produced sulphur dioxide (SO2) was detected in the atmosphere of a hot (~1200 K) Saturn-mass planet from transmission spectroscopy near 4.05 microns, but for temperatures below 1000 K sulphur is predicted to preferably form sulphur allotropes instead of SO2. Here we report the 9-sigma detection of two fundamental vibration bands of SO2, at 7.35 microns and 8.69 microns, in the transmission spectrum of WASP-107b using the Mid-Infrared Instrument (MIRI) of the JWST. This discovery establishes WASP-107b as the second irradiated exoplanet with confirmed photochemistry, extending the temperature range of exoplanets exhibiting detected photochemistry from ~1200 K down to ~740 K. Additionally, our spectral analysis reveals the presence of silicate clouds, which are strongly favoured (~7-sigma) over simpler cloud setups. Furthermore, water is detected (~12-sigma), but methane is not. These findings provide evidence of disequilibrium chemistry and indicate a dynamically active atmosphere with a super-solar metallicity.

astro-ph.EP↗
arXiv2025

MIRI-LRS spectrum of a cold exoplanet around a white dwarf: water, ammonia, and methane measurements

The study of the atmosphere of exoplanets orbiting white dwarfs is a largely unexplored field. With WD\,0806-661\,b, we present the first deep dive into the atmospheric physics and chemistry of a cold exoplanet around a white dwarf. We observed WD 0806-661 b using JWST's Mid-InfraRed Instrument Low-Resolution Spectrometer (MIRI-LRS), covering the wavelength range from 5 -- 12~$μ\rm{m}$, and the Imager, providing us with 12.8, 15, 18 and 21\,$μ$m photometric measurements. We carried the data reduction of those datasets, tackling second-order effects to ensure a reliable retrieval analysis. Using the \textsc{TauREx} retrieval code, we inferred the pressure-temperature structure, atmospheric chemistry, mass, and radius of the planet. The spectrum of WD 0806-661 b is shaped by molecular absorption of water, ammonia, and methane, consistent with a cold Jupiter atmosphere, allowing us to retrieve their abundances. From the mixing ratio of water, ammonia and methane we derive $\rm{C/O} = 0.34 \pm 0.06$, $\rm{C/N} = 14.4 ^{+2.5}_{-1.8}$ and $\rm{N/O} = 0.023 \pm 0.004$ and the ratio of detected metals as proxy for metallicity. We also derive upper limits for the abundance of CO and $\rm{CO_2}$ ($1.2\cdot10^{-6} \rm{\,and\,} 1.6\cdot10^{-7}$ respectively), which were not detected by our retrieval models. While our interpretation of WD\,0806-661\,b's atmosphere is mostly consistent with our theoretical understanding, some results -- such as the lack of evidence for water clouds, an apparent increase in the mixing ratio of ammonia at low pressure, or the retrieved mass at odds with the supposed age -- remain surprising and require follow-up observational and theoretical studies to be confirmed.

astro-ph.EP↗
arXiv2025

HCN and C2H2 in the atmosphere of a T8.5+T9 brown dwarf binary

T-type brown dwarfs present an opportunity to explore atmospheres teeming with molecules such as H2O, CH4 and NH3, which exhibit a wealth of absorption features in the mid-infrared. With JWST, we can finally explore this chemistry in detail, including for the coldest brown dwarfs that were not yet discovered in the Spitzer era. This allows precise derivations of the molecular abundances, which in turn informs our understanding of vertical transport in these atmospheres and can provide clues about the formation of cold brown dwarfs and exoplanets. This study presents the first JWST/MRS mid-IR spectrum (R ~ 1500-3000) of a T-dwarf: the T8.5+T9 brown dwarf binary WISE J045853.90+643451.9. We fit the spectrum using a parameterized P-T profile and free molecular abundances (i.e., a retrieval analysis), treating the binary as unresolved. We find a good fit with a cloud-free atmosphere and identify H2O, CH4 and NH3 features. Moreover, we make the first detections of HCN and C2H2 (at 13.4$σ$ and 9.5$σ$ respectively) in any brown dwarf atmosphere. The detection of HCN suggests intense vertical mixing ($K_{zz}\sim10^{11}$cm$^2$s$^{-1}$), challenging previous literature derivations of $K_{zz}$ values for T-type brown dwarfs. Even more surprising is the C2H2 detection, which cannot be explained with existing atmospheric models for isolated objects. This result challenges model assumptions about vertical mixing, and/or our understanding of the C2H2 chemical network, or might hint towards a more complex atmospheric processes such as magnetic fields driving aurorae, or lightning driving ionization. These findings open a new frontier in studying carbon chemistry within brown dwarf atmospheres.

astro-ph.EP↗
arXiv2024

Combined analysis of the 12.8 and 15 $μm$ JWST/MIRI eclipse observations of TRAPPIST-1 b

The first JWST/MIRI photometric observations of TRAPPIST-1 b allowed for the detection of the thermal emission of the planet at 15 $μm$, suggesting that the planet could be a bare rock with a zero albedo and no redistribution of heat. These observations at 15 $μm$ were acquired as part of GTO time that included a twin program at 12.8 $μm$ in order to have a measurement in and outside the CO$_2$ absorption band. Here we present five new occultations of TRAPPIST-1 b observed with MIRI in an additional photometric band at 12.8 $μm$. We perform a global fit of the 10 eclipses and derive a planet-to-star flux ratio and 1-$σ$ error of 452 $\pm$ 86 ppm and 775 $\pm$ 90 ppm at 12.8 $μm$ and 15 $μm$, respectively. We find that two main scenarios emerge. An airless planet model with an unweathered (fresh) ultramafic surface, that could be indicative of relatively recent geological processes fits well the data. Alternatively, a thick, pure-CO2 atmosphere with photochemical hazes that create a temperature inversion and result in the CO2 feature being seen in emission also works, although with some caveats. Our results highlight the challenges in accurately determining a planet's atmospheric or surface nature solely from broadband filter measurements of its emission, but also point towards two very interesting scenarios that will be further investigated with the forthcoming phase curve of TRAPPIST-1 b.

astro-ph.EP↗
arXiv2024

JWST MIRI Flight Performance: Imaging

The Mid-Infrared Instrument (MIRI) aboard the James Webb Space Telescope (JWST) provides the observatory with a huge advance in mid-infrared imaging and spectroscopy covering the wavelength range of 5 to 28 microns. This paper describes the performance and characteristics of the MIRI imager as understood during observatory commissioning activities, and through its first year of science operations. We discuss the measurements and results of the imager's point spread function, flux calibration, background, distortion and flat fields as well as results pertaining to best observing practices for MIRI imaging, and discuss known imaging artefacts that may be seen during or after data processing. Overall, we show that the MIRI imager has met or exceeded all its pre-flight requirements, and we expect it to make a significant contribution to mid-infrared science for the astronomy community for years to come.

astro-ph.IM↗
arXiv2023

15NH3 in the atmosphere of a cool brown dwarf

Brown dwarfs serve as ideal laboratories for studying the atmospheres of giant exoplanets on wide orbits as the governing physical and chemical processes in them are nearly identical. Understanding the formation of gas giant planets is challenging, often involving the endeavour to link atmospheric abundance ratios, such as the carbon-to-oxygen (C/O) ratio, to formation scenarios. However, the complexity of planet formation requires additional tracers, as the unambiguous interpretation of the measured C/O ratio is fraught with complexity. Isotope ratios, such as deuterium-to-hydrogen and 14N/15N, offer a promising avenue to gain further insight into this formation process, mirroring their utility within the solar system. For exoplanets only a handful of constraints on 12C/13C exist, pointing to the accretion of 13C-rich ice from beyond the disks' CO iceline. Here we report on the mid-infrared detection of the 14NH3 and 15NH3 isotopologues in the atmosphere of a cool brown dwarf with an effective temperature of 380 K in a spectrum taken with the Mid-InfraRed Instrument of the James Webb Space Telescope. As expected, our results reveal a 14N/15N value consistent with star-like formation by gravitational collapse, demonstrating that this ratio can be accurately constrained. Since young stars and their planets should be more strongly enriched in the 15N isotope, we expect that 15NH3 will be detectable in a number of cold, wide-separation exoplanets.

astro-ph.EP↗
arXiv2023

Spectroscopic time series performance of the Mid-Infrared Instrument on the JWST

We present here the first ever mid-infrared spectroscopic time series observation of the transiting exoplanet \object{L 168-9 b} with the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope. The data were obtained as part of the MIRI commissioning activities, to characterize the performance of the Low Resolution Spectroscopy (LRS) mode for these challenging observations. To assess the MIRI LRS performance, we performed two independent analyses of the data. We find that with a single transit observation we reached a spectro-photometric precision of $\sim$50 ppm in the 7-8 \micron range at R=50, consistent with $\sim$25 ppm systematic noise. The derived band averaged transit depth is 524 $\pm$ 15 ppm and 547 $\pm$ 13 ppm for the two applied analysis methods, respectively, recovering the known transit depth to within 1 $σ$. The measured noise in the planet's transmission spectrum is approximately 15-20 \% higher than random noise simulations over wavelengths $6.8 \lesssim λ\lesssim 11$ $μ$m. \added{We observed an larger excess noise at the shortest wavelengths of up to a factor of two, for which possible causes are discussed.} This performance was achieved with limited in-flight calibration data, demonstrating the future potential of MIRI for the characterization of exoplanet atmospheres.

astro-ph.IM↗
arXiv2014

Analytical approach to optimizing alternating current biasing of bolometers

Bolometers are most often biased by alternating current (AC) in order to get rid of low-frequency noises that plague direct current (DC) amplification systems. When stray capacitance is present, the respon- sivity of the bolometer differs significantly from the expectations of the classical theories. We develop an analytical model that facilitates the optimization of the AC readout electronics design and tuning. This model is applied to cases similar to the bolometers in the Planck space mission. We study how the re- sponsivity and the noise equivalent power (NEP) of an AC biased bolometer depend on these essential parameters: bias current, heat sink temperature and background power, modulation frequency of the bias, and stray capacitance. We show that the optimal AC bias current in the bolometer is significantly different from that of the DC case as soon as a stray capacitance is present due to the difference in the electrothermal feedback. We also compare the performance of square and sine bias currents and show a slight theoretical advantage for the latter. This work resulted from the need to be able to predict the real behavior of AC biased bolometers in an extended range of working parameters. It proved to be applicable to optimize the tuning of the Planck High-Frequency Instrument bolometers.

astro-ph.IM↗