arXiv:2410.00136v3 Announce Type: replace
Abstract: We present JWST/MIRI observations of T~Cha, a highly variable ($\Delta V \sim$3-5\,mag) accreting Sun-like star surrounded by a disk with a large ($\sim 15$\,au) dust gap. We find that the JWST mid-infrared spectrum is signiticantly different from the {\it Spitzer} spectrum obtained 17 years before, where the emission at short wavelengths ($5-10 \mu m$) has decreased by $\sim 2/3$ while at longer wavelengths ($15-25 \mu m$) it increased by up to a factor of $\sim 3$. This 'seesaw' behavior is contemporary with a fairly constant higher optical emission captured by the All Sky Automated Survey. By analyzing and modelling both SEDs, we propose that JWST caught the star during an outburst that destructed the asymmetric inner disk wall responsible for the high optical variability and lower $15-25$\,micron\ emission during the {\it Spitzer} time. The dust mass lost during this outburst is estimated to be comparable ($\sim 1/5$) to the upper limit of the total micron-sized dust mass in the inner disk of T~Cha now. Monitoring this system during possible future outbursts and more observations of its quiescent state will reveal if the inner disk can be replenished or will continue to be depleted and vanish.
arXiv:2410.00136v3 Announce Type: replace
Abstract: We present JWST/MIRI observations of T~Cha, a highly variable ($\Delta V \sim$3-5\,mag) accreting Sun-like star surrounded by a disk with a large ($\sim 15$\,au) dust gap. We find that the JWST mid-infrared spectrum is signiticantly different from the {\it Spitzer} spectrum obtained 17 years before, where the emission at short wavelengths ($5-10 \mu m$) has decreased by $\sim 2/3$ while at longer wavelengths ($15-25 \mu m$) it increased by up to a factor of $\sim 3$. This 'seesaw' behavior is contemporary with a fairly constant higher optical emission captured by the All Sky Automated Survey. By analyzing and modelling both SEDs, we propose that JWST caught the star during an outburst that destructed the asymmetric inner disk wall responsible for the high optical variability and lower $15-25$\,micron\ emission during the {\it Spitzer} time. The dust mass lost during this outburst is estimated to be comparable ($\sim 1/5$) to the upper limit of the total micron-sized dust mass in the inner disk of T~Cha now. Monitoring this system during possible future outbursts and more observations of its quiescent state will reveal if the inner disk can be replenished or will continue to be depleted and vanish.
arXiv:2412.08809v1 Announce Type: new
Abstract: We use hierarchical Bayesian modelling to calibrate a network of 32 all-sky faint DA white dwarf (DA WD) spectrophotometric standards ($16.5 < V < 19.5$) alongside the three CALSPEC standards, from 912 \r{A} to 32 $\mu$m. The framework is the first of its kind to jointly infer photometric zeropoints and WD parameters ($\log g$, $T_{\text{eff}}$, $A_V$, $R_V$) by simultaneously modelling both photometric and spectroscopic data. We model panchromatic HST/WFC3 UVIS and IR fluxes, HST/STIS UV spectroscopy and ground-based optical spectroscopy to sub-percent precision. Photometric residuals for the sample are the lowest yet yielding $<0.004$ mag RMS on average from the UV to the NIR, achieved by jointly inferring time-dependent changes in system sensitivity and WFC3/IR count-rate nonlinearity. Our GPU-accelerated implementation enables efficient sampling via Hamiltonian Monte Carlo, critical for exploring the high-dimensional posterior space. The hierarchical nature of the model enables population analysis of intrinsic WD and dust parameters. Inferred SEDs from this model will be essential for calibrating the James Webb Space Telescope as well as next-generation surveys, including Vera Rubin Observatory's Legacy Survey of Space and Time, and the Nancy Grace Roman Space Telescope.
arXiv:2412.07970v1 Announce Type: new
Abstract: The visibility of the Lyman-$\alpha$ (Ly$\alpha$) emission from reionization-epoch galaxies depends sensitively on the extent of the intrinsic \lya emission redwards of 1215.67~\AA. The prominent red peak resulting from resonant radiative transfer in the interstellar medium is often modelled as a single Gaussian. We use the \textsc{Azahar} simulation suite of a massive-reionization epoch galaxy to show that a significantly larger fraction of the \lya emission extends to $400$-$800$~km~s$^{-1}$, and thus significantly further to the red than predicted by a Gaussian line profile. A cycle of frequent galaxy mergers strongly modulates the \lya luminosity, the red peak velocity and its extended red wing emerging from the galaxy, which all also strongly vary with viewing angle. The \lya emission also depends sensitively on the implemented feedback, dust and star formation physics. Our simulations including cosmic rays reproduce the observed spectral properties of reionization epoch \lya emitters (LAEs) well if we assume that the \lya emission is affected by very little dust. The visibility of LAEs can be strongly underestimated if the extended red wings of the intrinsic \lya emission are not accounted for. We discuss implications for using the visibility of LAEs to constrain the evolution of the volume-averaged neutral fraction during reionization.
arXiv:2412.08557v1 Announce Type: new
Abstract: Aims: We aim to observe the transits and occultations of WASP-33b, which orbits a rapidly-rotating $\delta$ Scuti pulsator, with the goal of measuring the orbital obliquity via the gravity-darkening effect, and constraining the geometric albedo via the occultation depth. Methods: We observed four transits and four occultations with CHEOPS, and employ a variety of techniques to remove the effects of the stellar pulsations from the light curves, as well as the usual CHEOPS systematic effects. We also performed a comprehensive analysis of low-resolution spectral and Gaia data to re-determine the stellar properties of WASP-33. Results: We measure an orbital obliquity 111.3 +0.2 -0.7 degrees, which is consistent with previous measurements made via Doppler tomography. We also measure the planetary impact parameter, and confirm that this parameter is undergoing rapid secular evolution as a result of nodal precession of the planetary orbit. This precession allows us to determine the second-order fluid Love number of the star, which we find agrees well with the predictions of theoretical stellar models. We are unable to robustly measure a unique value of the occultation depth, and emphasise the need for long-baseline observations to better measure the pulsation periods.
arXiv:2412.08557v1 Announce Type: new
Abstract: Aims: We aim to observe the transits and occultations of WASP-33b, which orbits a rapidly-rotating $\delta$ Scuti pulsator, with the goal of measuring the orbital obliquity via the gravity-darkening effect, and constraining the geometric albedo via the occultation depth. Methods: We observed four transits and four occultations with CHEOPS, and employ a variety of techniques to remove the effects of the stellar pulsations from the light curves, as well as the usual CHEOPS systematic effects. We also performed a comprehensive analysis of low-resolution spectral and Gaia data to re-determine the stellar properties of WASP-33. Results: We measure an orbital obliquity 111.3 +0.2 -0.7 degrees, which is consistent with previous measurements made via Doppler tomography. We also measure the planetary impact parameter, and confirm that this parameter is undergoing rapid secular evolution as a result of nodal precession of the planetary orbit. This precession allows us to determine the second-order fluid Love number of the star, which we find agrees well with the predictions of theoretical stellar models. We are unable to robustly measure a unique value of the occultation depth, and emphasise the need for long-baseline observations to better measure the pulsation periods.
arXiv:2412.06982v1 Announce Type: new
Abstract: Precision radial velocity (RV) spectrographs that use adaptive optics (AO) show promise to advance telescope observing capabilities beyond those of seeing-limited designs. We are building a spectrograph for the Large Binocular Telescope (LBT) named iLocater that uses AO to inject starlight directly into single mode fibers (SMF). iLocater's first acquisition camera system (the `SX' camera), which receives light from one of the 8.4m diameter primary mirrors of the LBT, was initially installed in summer 2019 and has since been used for several commissioning runs. We present results from first-light observations that include on-sky measurements as part of commissioning activities. Imaging measurements of the bright B3IV star 2 Cygni ($V=4.98$) resulted in the direct detection of a candidate companion star at an angular separation of only $\theta = 70$ mas. Follow-up AO measurements using Keck/NIRC2 recover the candidate companion in multiple filters. An $R\approx1500$ miniature spectrograph recently installed at the LBT named ``Lili'' provides spatially resolved spectra of each binary component, indicating similar spectral types and strengthening the case for companionship. Studying the multiplicity of young runaway star systems like 2 Cygni ($36.6 \pm 0.5$ Myr) can help to understand formation mechanisms for stars that exhibit anomalous velocities through the galaxy. This on-sky demonstration illustrates the spatial resolution of the iLocater SX acquisition camera working in tandem with the LBT AO system; it further derisks a number of technical hurdles involved in combining AO with Doppler spectroscopy.
arXiv:2412.06896v1 Announce Type: new
Abstract: The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is a satellite orbiting the Milky Way that has experienced multiple stripping events due to tidal interactions with our Galaxy. Its accretion history has led to a distinct stellar overdensity, which is the remnant of the core of the progenitor. We present a complete chemical analysis of 111 giant stars in the core of Sgr dSph to investigate the chemical evolution and enrichment history of this satellite. Employing the metallicity-sensitive Ca H&K photometry from the Pristine Inner Galaxy Survey, we selected stars spanning a wide metallicity range and obtained high-resolution spectra with the ESO FLAMES/GIRAFFE multi-object spectrograph. For the stellar sample covering $-2.13 < \rm{[Fe/H] < -0.35}$, we derived abundances for up to 14 chemical elements with average uncertainties of $\sim 0.09$ dex and a set of stellar ages which allowed us to build an age-metallicity relation (AMR) for the entire sample. With the most comprehensive set of chemical species measured for the core of Sgr, we studied several [X/Fe] ratios. Most trends align closely with Galactic chemical trends, but notable differences emerge in the heavy $n$-capture elements, which offer independent insights into the star formation history of a stellar population. The deficiency in the $\alpha$-elements with respect the Milky Way suggests a slower, less efficient early star formation history, similar to other massive satellites. $S$-process element patterns indicate significant enrichment from AGB stars over time. The AMR and chemical ratios point to an extended star formation history, with a rapid early phase in the first Gyr, followed by declining activity and later star-forming episodes. These findings are consistent with Sgr hosting multiple stellar populations, from young ($\sim 4$ Gyr) to old, metal-poor stars ($\sim 10$ Gyr)
arXiv:2408.13314v2 Announce Type: replace
Abstract: Enhanced emission in the months to years preceding explosion has been detected for several core-collapse supernovae (SNe). Though the physical mechanisms driving the emission remain hotly debated, the light curves of detected events show long-lived ($\geq$50 days), plateau-like behavior, suggesting hydrogen recombination may significantly contribute to the total energy budget. The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) will provide a decade-long photometric baseline to search for this emission, both in binned pre-explosion observations after an SN is detected and in single-visit observations prior to the SN explosion. In anticipation of these searches, we simulate a range of eruptive precursor models to core-collapse SNe and forecast the discovery rates of these phenomena in LSST data. We find a detection rate of ~40-130 yr$^{-1}$ for SN IIP/IIL precursors and ~110 yr$^{-1}$ for SN IIn precursors in single-epoch photometry. Considering the first three years of observations with the effects of rolling and observing triplets included, this number grows to a total of 150-400 in binned photometry, with the highest number recovered when binning in 100-day bins for 2020tlf-like precursors and in 20-day bins for other recombination-driven models from the literature. We quantify the impact of using templates contaminated by residual light (from either long-lived or separate precursor emission) on these detection rates, and explore strategies for estimating baseline flux to mitigate these issues. Spectroscopic follow-up of the eruptions preceding core-collapse SNe and detected with LSST will offer important clues to the underlying drivers of terminal-stage mass loss in massive stars.
A team of astronomers has found that Venus has never been habitable, despite decades of speculation that our closest planetary neighbour was once much more like Earth than it is today. The researchers, from the University of Cambridge, studied the chemical composition of the Venusian atmosphere and inferred that its...
arXiv:2412.01797v1 Announce Type: new
Abstract: A century or less separates the thermonuclear-powered eruptions of recurrent novae in the hydrogen-rich envelopes of massive white dwarfs. The colliding ejecta of successive recurrent nova events are predicted to always generate very large (tens of parsecs) super-remnants; only two examples are currently known. T CrB offers an excellent opportunity to test this prediction. As it will almost certainly undergo its next, once-in ~80-year recurrent nova event between 2024 and 2026, we carried out very deep narrowband and continuum imaging to search for the predicted, piled-up ejecta of the past millenia. While nothing is detected in continuum or narrowband [OIII] images, a ~30-parsec-diameter, faint nebulosity surrounding T CrB is clearly present in deep Halpha, [NII] and [SII] narrowband Condor Array Telescope imagery. We predict that these newly detected nebulosities, as well as the recent ejecta that have not yet reached the super-remnant, are far too optically-thin to capture all but a tiny fraction of the photons emitted by RN flashes. We thus predict that fluorescent light echoes will NOT be detectable following the imminent nova flash of T CrB. Dust may be released by the T CrB red giant wind in pre-eruption outbursts, but we have no reliable estimates of its quantity or geometrical distribution. While we cannot predict the morphology or intensity of dust-induced continuum light echoes following the coming flash, we encourage multi-epoch Hubble Space Telescope optical imaging as well as James Webb Space Telescope infrared imaging of T CrB during the year after it erupts.
arXiv:2411.18692v1 Announce Type: new
Abstract: Over the last few decades, there has been considerable interest in the violation of the sacred "Chandrasekhar" mass limit of white dwarfs (WDs). Peculiar over-luminous type Ia supernovae (such as SNLS-03D3bb) lend observational support to the idea that these super-Chandrasekhar WDs exist. Our group, for more than a decade, has been actively working on the theoretical possibility of these objects through the presence of the star's magnetic field. The magnetic field greatly contributes to the existence of these massive WDs, both through classical and quantum effects. In this work, we explore super-Chandrasekhar WDs, formed via evolution from a main sequence star, as a result of the classical effects of the star's magnetic field. We obtain super-Chandrasekhar WDs and new mass limit(s), depending on the magnetic field geometry. We explore the full evolution and stability of these objects from the main sequence stage through the one-dimensional stellar evolution code STARS. In order to do so, we have appropriately modified the given codes by introducing magnetic effect and cooling. Our simulation confirms that massive WDs are possible in the presence of a magnetic field satisfying underlying stability.
Abiotic Ozone in the Observable Atmospheres of Venus-like Planets
Ozone is a potential exoplanet biosignature due to its association with photosynthetically produced oxygen and strong absorption in the mid-infrared. However, the existence of ozone in Venus’s observable atmosphere, a planet with no known life, raises the possibility of ozone biosignature false-positives on Venus-like exoplanets. We use a photochemical model of Venus’s atmosphere to investigate the origin of its mesospheric ozone layer, and how similar ozone layers would manifest for Venus-like exoplanets. Our model shows that the hypothesis that Venus’s ozone forms on the nightside due to a flux of O radicals from the dayside, cannot generate enough ozone to match observed levels. Furthermore, we show that sufficient ozone cannot be produced by varying the surface chemistry, atmospheric thermal structure, or stellar flux in our model of Venus’s atmosphere, implying that a presently unknown chemical pathway is responsible for the ozone in Venus’s mesosphere. Until the origin of Venus’s ozone is understood, we cannot rule out that ozone production will be common on abiotic Venus-like worlds, a possibility that limits the usefulness of ozone as a biosignature.
Add to your calendar or Include in your list
Linking the low surface brightness Universe and galactic growth
In hierarchical models of galaxy formation and evolution, galaxies grow through gas accretion and merger events. The tidal debris produced during these mergers serves as a fossil record of a galaxy’s assembly history, with characteristics that depend on the nature of the merger. Exploring these faint Low Surface Brightness (LSB) structures around galaxies of varying masses and environments is essential to fully understanding galactic evolution. Advances in dedicated instruments and data reduction pipelines from past, ongoing, and future surveys have significantly expanded our ability to explore the LSB Universe.
In this talk, I will discuss how LSB features can be leveraged to investigate galactic growth, using deep imaging data from the Canada-France-Hawaii Telescope (CFHT) and ESA ’s Euclid space telescope, combined with an innovative online annotation tool.
Add to your calendar or Include in your list
arXiv:2411.17049v1 Announce Type: new
Abstract: We present an analysis of the spatial distribution of globular cluster (GC) systems of 118 nearby early-type galaxies in the Next Generation Virgo Cluster Survey (NGVS) and Mass Assembly of early-Type GaLAxies with their fine Structures (MATLAS) survey programs, which both used MegaCam on the Canada-France-Hawaii Telescope. We describe the procedure used to select GC candidates and fit the spatial distributions of GCs to a two-dimensional S\'ersic function, which provides effective radii (half number radii) and S\'ersic indices, and estimate background contamination by adding a constant term to the S'ersic function. In cases where a neighboring galaxy affects the estimation of the GC spatial distribution in the target galaxy, we fit two 2D S\'ersic functions, simultaneously. We also investigate the color distributions of GCs in our sample by using Gaussian Mixture Modeling. For GC systems with bimodal color distributions, we divide the GCs into blue and red subgroups and fit their respective spatial distributions with S\'ersic functions. Finally, we measure the total number of GCs based on our fitted S\'ersic function, and calculate the GC specific frequency.
Abiotic Ozone in the Observable Atmospheres of Venus-like Planets
Ozone is a potential exoplanet biosignature due to its association with photosynthetically produced oxygen and strong absorption in the mid-infrared. However, the existence of ozone in Venus’s observable atmosphere, a planet with no known life, raises the possibility of ozone biosignature false-positives on Venus-like exoplanets. We use a photochemical model of Venus’s atmosphere to investigate the origin of its mesospheric ozone layer, and how similar ozone layers would manifest for Venus-like exoplanets. Our model shows that the hypothesis that Venus’s ozone forms on the nightside due to a flux of O radicals from the dayside, cannot generate enough ozone to match observed levels. Furthermore, we show that sufficient ozone cannot be produced by varying the surface chemistry, atmospheric thermal structure, or stellar flux in our model of Venus’s atmosphere, implying that a presently unknown chemical pathway is responsible for the ozone in Venus’s mesosphere. Until the origin of Venus’s ozone is understood, we cannot rule out that ozone production will be common on abiotic Venus-like worlds, a possibility that limits the usefulness of ozone as a biosignature.
Add to your calendar or Include in your list
Tracing velocity substructures in the planet-forming disks of exoALMA
The measurement of rotation curves led to big discoveries in astronomy, like the proposition of dark matter halos around galaxies. With the emergence of ALMA , it is now similarly possible to measure the rotation of gas in protoplanetary disks, which are several orders of magnitude smaller than galaxies. While the overall motion of the gas around newborn stars is Keplerian, with high spectral resolution molecular line observations, we can trace small-scale velocity perturbations caused by local pressure variations in the disk, possibly due to embedded planets. In the talk, I will discuss how we can observe gas rotation in planet-forming disks and what we can learn from studying the deviations from Keplerian rotation. In particular, I will present results from the rotation curve study for the disks of the exoALMA Large Program. We find that substructures in the deviation from Keplerian rotation are ubiquitous in our sample, on both small and large scales, and can reach up to 15 percent in the most extreme cases. Interestingly, the majority of the dust continuum rings and gaps are co-located with pressure maxima and minima, respectively. Finally, I will compare the presented results with the predictions from the theory and put them into the bigger picture of planet formation.
Add to your calendar or Include in your list
Toward a Self-Consistent Evaluation of Gas Dwarf Scenarios for Temperate Sub-Neptunes
Sub-Neptunes—planets intermediate in size between Earth and Neptune—are a major focus in exoplanet science. Despite having no Solar System counterparts, they are the largest known class of exoplanets. They exhibit an unexpected bimodal size distribution, the larger portion of which has a density inconsistent with an Earth-like rocky composition. This has led to hypotheses about their internal composition, including “gas dwarfs” (rocky planets with hydrogen-rich envelopes) and “volatile-rich” scenarios (water-rich planets with thin hydrogen atmospheres). Interest has grown with suggestions that some volatile-rich planets could be potentially habitable “hycean” worlds. Notably, JWST observations of the temperate sub-Neptune K2-18 b have been variously interpreted: as confirming its hycean candidate status, supporting a gas-dwarf scenario with a magma ocean, or classifying it as a mini-Neptune.
In this seminar, I will present an integrated framework we have developed to model gas dwarf scenarios—with and without magma oceans—on temperate sub-Neptunes and to predict their unique observable features. I will then discuss how we applied this framework to K2-18 b, and compare our predictions with available observations to draw conclusions about its likely internal structure.
Add to your calendar or Include in your list
Tracing velocity substructures in the planet-forming disks of exoALMA
The measurement of rotation curves led to big discoveries in astronomy, like the proposition of dark matter halos around galaxies. With the emergence of ALMA , it is now similarly possible to measure the rotation of gas in protoplanetary disks, which are several orders of magnitude smaller than galaxies. While the overall motion of the gas around newborn stars is Keplerian, with high spectral resolution molecular line observations, we can trace small-scale velocity perturbations caused by local pressure variations in the disk, possibly due to embedded planets. In the talk, I will discuss how we can observe gas rotation in planet-forming disks and what we can learn from studying the deviations from Keplerian rotation. In particular, I will present results from the rotation curve study for the disks of the exoALMA Large Program. We find that substructures in the deviation from Keplerian rotation are ubiquitous in our sample, on both small and large scales, and can reach up to 15 percent in the most extreme cases. Interestingly, the majority of the dust continuum rings and gaps are co-located with pressure maxima and minima, respectively. Finally, I will compare the presented results with the predictions from the theory and put them into the bigger picture of planet formation.
Add to your calendar or Include in your list
