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Azimuthal metallicity variations, spiral structure, and the failure of radial actions based on assuming axisymmetry
No full textInternational audienceABSTRACT We study azimuthal variations in the mean stellar metallicity, , in a self-consistent, isolated simulation in which all stars form out of gas. We find variations comparable to those observed in the Milky Way and which are coincident with the spiral density waves. The azimuthal variations are present in young and old stars and therefore are not a result of recently formed stars. Similar variations are present in the mean age and -abundance. We measure the pattern speeds of the -variations and find that they match those of the spirals, indicating that spirals are the cause of the metallicity patterns. Because younger stellar populations are not just more -rich and -poor but also dynamically cooler, we expect them to more strongly support spirals, which is indeed the case in the simulation. However, if we measure the radial action, , using the Stäckel axisymmetric approximation, we find that the spiral ridges are traced by regions of high , contrary to expectations. Assuming that the passage of stars through the spirals leads to unphysical variations in the measured , we obtain an improved estimate of by averaging over a time interval. This time-averaged is a much better tracer of the spiral structure, with minima at the spiral ridges. We conclude that the errors incurred by the axisymmetric approximation introduce correlated deviations large enough to render the instantaneous radial actions inadequate for tracing spirals
Transmission of the AtmosPhere for AStronomical data: TAPAS upgrade
No full textInternational audienceContext: Current molecular databases and realistic global atmospheric models allow us to predict accurate atmospheric transmittance spectra. Observers with ground-based spectrographs may use this information to identify the telluric absorption lines, to correct their astronomical spectra for these lines fully or partially, or take them into account in forward models.Aims: The TAPAS online service provides atmospheric transmittance spectra of the most important species, as well as Rayleigh extinction, adapted to any observing location, date, and direction. We describe recent updates, improvements, and additional tools.Methods: TAPAS interpolates the location in the atmospheric profiles of temperature, pressure, H2O, O2 and O3 that are extracted from the meteorological field of the European Centre for Medium Term Weather Forecast (ECMWF) for the date and time of the observation. The composite profiles are produced by a Data Terra/AERIS/ESPRI product called Arletty, and they are supplemented by auxiliary climatological models for additional species. The transmittance spectra are computed with the code LBLRTM. The default width of the spectral pixels is chosen to ensure that the shapes of all the absorption lines are reproduced for each species. Major improvements with respect to the previous TAPAS are the extension of the wavelength range in the near-UV down to 300 nm and the extension in the near-IR up to 3500 nm; the use of the recent version of the HITRAN database (HITRAN2020); the addition of NO2 transmittance to complement H2O, O2, O3, N2O, CO2, and CH4; an increased accessibility and a reduced time to obtain the results; and the possibility to force the total H2O column to match the column measured at the observatory at the time of record.Results: We show O3 absorption in the near-UV and near-IR and NO2 absorption in the visible. We illustrate the quality of TAPAS by means of comparisons between models and ESO/VLT/CRIRES recorded spectra of a hot star with a spectral resolution of ~130 000 in two intervals in the near-IR with strong H2O, N2O, CO2, and CH4 absorption. We describe the measurement of an instrumental line spread function based on TAPAS O2 lines and a method using the singular value decomposition technique that can be made entirely automated.Conclusions: The new TAPAS tool provides realistic simulations of the telluric lines. It gives access to the weakest H2O or O2 lines, and to the very weak highly irregular NO2 lines. It can be used to improve the wavelength assignment when calibration lamps provide only a few emission lines, and to accurately measure the line spread function in most regions in which telluric features are present. The extended wavelength range will be particularly useful for future or recent spectrographs in the near-UV and in the near-IR
GW250114: Testing Hawking’s Area Law and the Kerr Nature of Black Holes
No full textInternational audienceThe gravitational-wave signal GW250114 was observed by the two LIGO detectors with a network matched-filter signal-to-noise ratio of 80. The signal was emitted by the coalescence of two black holes with near-equal masses m_{1}=33.6_{-0.8}^{+1.2}M_{⊙} and m_{2}=32.2_{-1.3}^{+0.8}M_{⊙}, and small spins χ_{1,2}≤0.26 (90% credibility) and negligible eccentricity e≤0.03. Postmerger data excluding the peak region are consistent with the dominant quadrupolar (ℓ=|m|=2) mode of a Kerr black hole and its first overtone. We constrain the modes' frequencies to ±30% of the Kerr spectrum, providing a test of the remnant's Kerr nature. We also examine Hawking's area law, also known as the second law of black hole mechanics, which states that the total area of the black hole event horizons cannot decrease with time. A range of analyses that exclude up to five of the strongest merger cycles confirm that the remnant area is larger than the sum of the initial areas to high credibility
Sub-Kelvin Spectral Hole Burning in Eu:YSO for Laser Frequency Stabilisation
No full textInternational audienceAs basic building blocks for quantum devices, ultra-stable lasers have a profound impact on various fields. Longer laser coherence times enhance the performance of quantum sensors, computers, communications, and optical clocks. Although optical lattice clocks have accomplished significant high-precision measurements, they often fall short of their quantum projection noise limits due to dead times during atom preparation: the instability in the laser frequency introduces stochastic fluctuations during the interrogation of atomic transitions (Dick effect). High-resolution optical clock comparisons are crucial for evaluating their accuracy and could lead to a redefinition of the SI second based on optical transitions [1].However, the most common technique for achieving ultra-stable lasers, the Pound-Drever-Hall method with Fabry-Perot cavities, faces thermal noise limitations of the cavities. To overcome these constraints, alternative methods such as Spectral Hole Burning (SHB) in rare-earth-doped crystals (REICs) have been explored. Indeed, REICs experience inhomogeneous broadening due to local lattice distortions caused by the size difference between dopant and host ions. An SHB protocol, which creates narrow spectral features through selective depletion of specific dopant ion populations using narrow-linewidth lasers, allows to overcome the inhomogeneous broadening and fully exploit the coherence of rare-earth ions. At cryogenic temperatures, Eu:YSO crystal exhibit persistent Spectral Holes a few kHz narrow and with a lifetime up to 10 h, at 4 K [2], making it a high-precision frequency reference, ideal for intermediate-timescale sensing.We report recent experimental progress on SHB in Eu:YSO crystals at specific sub-K dilution temperature setpoints that cancel the thermal sensitivity of the hole frequency [3]. The latest recorded performances on our experiment reach a fractional frequency stability of 4.10-16 at 1 s [4], which is close to the perfomance of our reference cavity at LTE. Therefore, a second SHB setup is needed for our future stability comparisons. On one front, we rationalize our SHB protocol and investigate new optimal burning parameters as well as test various doping concentrations to achieve the narrowest possible spectral holes on both setups. On another, we continue to further improving the laser frequency lock onto spectral holes, by cancelling as many systematic noise sources as possible. We provide the performances of our Michelson interferometer to reject frequency noise induced on light propagation to the crystal and by the cryostat acoustic vibrations. In addition, we explore a group delay measurement technique via phase modulation to enhance the precision on the pointing of the center of spectral holes. This should help removing slow frequency drifts due to asymmetric burning of the hole while it is probed by the laser
Horizon-scale variability of M87* from 2017–2021 EHT observations
No full textInternational audienceWe report three epochs of polarized images of M87* at 230 GHz using data from the Event Horizon Telescope (EHT) taken in 2017, 2018, and 2021. The baseline coverage of the 2021 observations is significantly improved through the addition of two new EHT stations: the 12 m Kitt Peak Telescope and the Northern Extended Millimetre Array (NOEMA). All observations result in images dominated by a bright, asymmetric ring with a persistent diameter of 43.9 ± 0.6 μas, consistent with expectations for lensed synchrotron emission encircling the apparent shadow of a supermassive black hole. We find that the total intensity and linear polarization of M87* vary significantly across the three epochs. Specifically, the azimuthal brightness distribution of the total intensity images varies from year to year, as expected for a stochastic accretion flow. However, despite a gamma-ray flare erupting in M87 quasi-contemporaneously to the 2018 observations, the 2018 and 2021 images look remarkably similar. The resolved linear polarization fractions in 2018 and 2021 peak at ∼5%, compared to ∼15% in 2017. The spiral polarization pattern on the ring also varies from year to year, including a change in the electric vector position angle helicity in 2021 that could reflect changes in the magnetized accretion flow or an external Faraday screen. The improved 2021 coverage also provides the first EHT constraints on jet emission outside the ring, on scales of ≲1 mas. Overall, these observations provide strong proof of the reliability of the EHT images and probe the dynamic properties of the horizon-scale accretion flow surrounding M87*.Key words: accretion / accretion disks / black hole physics / gravitation / galaxies: active / galaxies: individual: M87* / galaxies: jets★ NASA Hubble Fellowship Program, Einstein Fellow.† Deceased
Jupiter's UV Auroral Response to a Magnetospheric Compression Event
No full textInternational audienceThe highly elliptical polar orbit of the Juno mission provides a unique opportunity to simultaneously measure the compression state of Jupiter's magnetosphere and the total power emitted by the planet's ultraviolet aurora, using a single spacecraft. This allows us to study how Jupiter's aurora respond to a compression event. In this paper, we present a case study of an extreme compression event that occurred on December 6–7 2022 when Juno was a distance of 70 R J from Jupiter. This extreme compression was accompanied by a very large increase in the ultraviolet auroral emissions to 12 TW, a factor of six higher than the baseline level. This event coincided with the predicted arrival of a powerful interplanetary shock, which was expected to cause the largest increase in the solar wind dynamic pressure seen thus far during the Juno mission. The simultaneous occurrence of the interplanetary shock, the extreme compression and the bright ultraviolet aurora suggests that in this case, the auroral brightening was caused by the solar wind shock compressing the magnetosphere
Origin, evolution, and fate of Titan’s polar clouds
No full textInternational audienceSaturn’s moon Titan exhibits a rich interplay of atmospheric chemistry and meteorology, including the seasonal formation of polar stratospheric clouds, first observed by Voyager and later by the Cassini mission. Using a global climate model with haze and cloud microphysics, we investigate their origin, evolution, and fate. We find that cloud formation begins in early autumn, triggered by rapid radiative cooling and chemical enrichment within the stratospheric polar vortex. Initially extending to 336 km altitude and composed of benzene and hydrogen cyanide ices, these clouds descend to lower altitudes during autumn and winter, evolving in composition as additional species condense, before dissipating in spring. This process reflects a unified, hemisphere-spanning seasonal mechanism shaping Titan’s climate. Our model explains observed seasonal patterns and predicts the onset of the next northern polar cloud in 2027. These results provide a predictive framework for future Titan observations, missions, and long-term climate evolution studies
HARPS-N, TESS, and CHEOPS discover a transiting sub-Neptune and two outer companions around the bright solar analogue HD 85426
No full textInternational audience27 pages, 20 figures, accepted for publication in MNRA
