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We present the modeling tool we developed to incorporate multi-technique observations of Cepheids in a single pulsation model: the Spectro-Photo-Interferometry of Pulsating Stars (SPIPS). The combination of angular diameters from optical interferometry, radial velocities and photometry with the coming Gaia DR2 parallaxes of nearby Galactic Cepheids will soon enable us to calibrate the projection factor of the classical Parallax-of-Pulsation method. This will extend its applicability to Cepheids too distant for accurate Gaia parallax measurements, and allow us to precisely calibrate the Leavitt law's zero point. As an example application, we present the SPIPS model of the long-period Cepheid RS Pup that provides a measurement of its projection factor, using the independent distance estimated from its light echoes.

We present our implementation of the parallax of pulsation method which integrates all observables and physical modelling of the photosphere to get the best statistical precision and controlled biases. This method has been validated on well known stars and used to estimate observationally the projection factor of the HST-FGS sample. Our future developments include application to the Gaia Cepheids and modelling of the spectrum.

Forty years ago, it was proposed that gas-phase organic chemistry in the interstellar medium can be initiated by the methyl cation CH 3 + (refs. 1 – 3 ), but so far it has not been observed outside the Solar System 4 , 5 . Alternative routes involving processes on grain surfaces have been invoked 6 , 7 . Here we report James Webb Space Telescope observations of CH 3 + in a protoplanetary disk in the Orion star-forming region. We find that gas-phase organic chemistry is activated by ultraviolet irradiation. JWST observations of CH 3 + in a protoplanetary disk in the Orion star-forming region are reported showing that gas-phase organic chemistry in the interstellar medium is activated by ultraviolet irradiation and the methyl cation.

Through an innovative combination of multiple observing techniques and modeling, we are assembling a comprehensive understanding of the pulsation and close environment of Cepheids. We developed the SPIPS modeling tool that combines all observables (radial velocimetry, photometry, angular diameters from interferometry) to derive the relevant physical parameters of the star (effective temperature, infrared excess, reddening, …) and the ratio of the distance and the projection factor d/p. We present the application of SPIPS to the long-period Cepheid RS Pup, for which we derive p = 1.25±0.06. The addition of this massive Cepheid consolidates the existing sample of p-factor measurements towards long-period pulsators. This allows us to conclude that p is constant or mildly variable around p = 1.29±0.04 (±3%) as a function of the pulsation period. The forthcoming Gaia DR2 will provide a considerable improvement in quantity and accuracy of the trigonometric parallaxes of Cepheids. From this sample, the SPIPS modeling tool will enable a robust calibration of the Cepheid distance scale.

Water is a key ingredient for the emergence of life as we know it. Yet, its destruction and reformation in space remain unprobed in warm gas ( T  > 300 K). Here we detect with the James Webb Space Telescope the emission of the hydroxyl radical (OH) from d203-506, a planet-forming disk exposed to external far-ultraviolet (FUV) radiation. These observations were made as part of the Early Release Science programme PDRs4All, which is focused on the Orion bar. The observed OH spectrum is compared with the results of quantum dynamical calculations to reveal two essential molecular processes. The highly excited rotational lines of OH in the mid-infrared are telltale signs of H 2 O destruction by FUV radiation. The OH rovibrational lines in the near-infrared are attributed to chemical excitation by the key reaction O + H 2  → OH + H, which seeds the formation of water in the gas phase. These results show that under warm and irradiated conditions, water is destroyed and efficiently reformed through gas-phase reactions. We infer that, in this source, the equivalent of Earth oceans’ worth of water is destroyed per month and replenished. This warm-water cycle could reprocess some water inherited from cold interstellar clouds and explain the lower deuterium fraction of water in Earth’s oceans compared with that found around protostars. The hydroxyl radical OH has been detected in a planet-forming disk exposed to ultraviolet radiation and in a rovibrationally excited state. These JWST observations, when coupled with quantum calculations, reveal the ongoing photodissociation of water and its reformation in the gas phase.

Massive stars disrupt their natal molecular cloud material through radiative and mechanical feedback processes. These processes have profound effects on the evolution of interstellar matter in our Galaxy and throughout the universe, from the era of vigorous star formation at redshifts of 1–3 to the present day. The dominant feedback processes can be probed by observations of the Photo-Dissociation Regions (PDRs) where the far-ultraviolet photons of massive stars create warm regions of gas and dust in the neutral atomic and molecular gas. PDR emission provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter- and circumstellar media including diffuse clouds, proto-planetary disks, and molecular cloud surfaces, globules, planetary nebulae, and star-forming regions. PDR emission dominates the infrared (IR) spectra of star-forming galaxies. Most of the Galactic and extragalactic observations obtained with the JamesWebb Space Telescope (JWST) will therefore arise in PDR emission. In this paper we present an Early Release Science program using the MIRI, NIRSpec, and NIRCam instruments dedicated to the observations of an emblematic and nearby PDR: the Orion Bar. These early JWST observations will provide template data sets designed to identify key PDR characteristics in JWST observations. These data will serve to benchmark PDR models and extend them into the JWST era. We also present the Science-Enabling products that we will provide to the community. These template data sets and Science-Enabling products will guide the preparation of future proposals on star-forming regions in our Galaxy and beyond and will facilitate data analysis and interpretation of forthcoming JWST observations.

As primary anchors of the distance scale, Cepheid stars play a crucial role in our understanding of the distance scale of the Universe because of their period-luminosity relation. Determining precise and consistent parameters (radius, temperature, color excess, and projection factor) of Cepheid pulsating stars is therefore very important. With the high-precision parallaxes delivered by the early third Gaia data release, we aim to derive various parameters of Cepheid stars in order to calibrate the period-luminosity and period-radius relations and to investigate the relation of period to p-factor. We applied an implementation of the parallax-of-pulsation method through the algorithm called Spectro-Photo-Interferometry of Pulsating Stars, which combines all types of available data for a variable star in a global modeling of its pulsation. We present the SPIPS modeling of a sample of 63 Galactic Cepheids. Adopting Gaia EDR3 parallaxes as an input associated with the best available dataset, we derive consistent values of parameters for these stars such as the radius, multiband apparent magnitudes, effective temperatures, color excesses, period changes, Fourier parameters, and the projection factor. We then derive new calibrations of the period-luminosity and period-radius relations. After investigating the dependences of the p-factor on the parameters of the stars, we find a high dispersion of its values and no evidence of its correlation with the period or with any other parameters. Statistically, the p-factor has an average value of p=1.26\\(\\pm\\)0.07, but with an unsatisfactory agreement. In absence of any clear correlation between the p-factor and other quantities, the best agreement is obtained under the assumption that the p-factor can take any value in a band with a width of 0.15. This result highlights the need for a further examination of the physics behind the p-factor.

We present a low-dispersion period-luminosity relation (PL) based on 154 Cepheids in Messier 33 (M33) with Hubble Space Telescope (HST) photometry from the PHATTER survey. Using high-quality ground-based light curves, we recover Cepheid phases and amplitudes for multi-epoch HST data and we perform template fitting to derive intensity-averaged mean magnitudes. HST observations in the SH0ES near-infrared Wesenheit system significantly reduce the effect of crowding relative to ground-based data, as seen in the final PL scatter of \\(\\sigma\\) = 0.11 mag. We adopt the absolute calibration of the PL based on HST observations in the Large Magellanic Cloud (LMC) and a distance derived using late-type detached eclipsing binaries to obtain a distance modulus for M33 of \\(\\mu\\) = 24.622 \\(\\pm\\) 0.030 mag (d = 840 \\(\\pm\\) 11 kpc), a best-to-date precision of 1.3%. We find very good agreement with past Cepheid-based measurements. Several TRGB estimates bracket our result while disagreeing with each other. Finally, we show that the flux contribution from star clusters hosting Cepheids in M33 does not impact the distance measurement and we find only 3.7% of the sample is located in (or nearby) young clusters. M33 offers one of the best sites for the cross-calibration of many primary distance indicators. Thus, a precise independent geometric determination of its distance would provide a valuable new anchor to measure the Hubble constant.

The Cepheid Period-Luminosity (PL) relation is the key tool for measuring astronomical distances and for establishing the extragalactic distance scale. In particular, the local value of the Hubble constant (\\(H_0\\)) strongly depends on Cepheid distance measurements. The recent Gaia Data Releases and other parallax measurements from the Hubble Space Telescope (HST) already enabled to improve the accuracy of the slope (\\(\\alpha\\)) and intercept (\\(\\beta\\)) of the PL relation. However, the dependence of this law on metallicity is still largely debated. In this paper, we combine three samples of Cepheids in the Milky Way (MW), the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) in order to derive the metallicity term (hereafter \\(\\gamma\\)) of the PL relation. The recent publication of extremely precise LMC and SMC distances based on late-type detached eclipsing binary systems (DEBs) provides a solid anchor for the Magellanic Clouds. In the MW, we adopt Cepheid parallaxes from the early third Gaia Data Release. We derive the metallicity effect in \\(V\\), \\(I\\), \\(J\\), \\(H\\), \\(K_S\\), \\(W_{VI}\\) and \\(W_{JK}\\). In the \\(K_S\\) band we report a metallicity effect of \\(-0.221 \\pm 0.051\\) mag/dex, the negative sign meaning that more metal-rich Cepheids are intrinsically brighter than their more metal-poor counterparts of the same pulsation period.