arXiv subjects

R. P. Church

Publications and source records attributed to R. P. Church.

10 recordsLinked to original sources

arXiv2026

From Gaia to GaiaNIR: I. Probing dark matter halos in globular clusters

The proposed GaiaNIR mission would extend Gaia's astrometric capabilities into the near-infrared, improving astrometric precision and enabling observations in heavily dust-obscured regions. In this work, we investigate the impact of GaiaNIR on the detectability of dark matter halos in globular clusters by comparing its performance with that of Gaia. Expected observations from future Gaia data releases and GaiaNIR are modeled for a globular cluster with properties similar to M4. The cluster is simulated with a range of dark matter halo sizes and varying levels of extinction, allowing a direct assessment of each mission's ability to detect and distinguish dark matter halos across different extinction conditions. To support this comparison, the relationship between the Gaia G band and several near-infrared bands is examined. We find that Gaia can resolve extended dark matter halos in low-extinction conditions, but its performance degrades significantly as extinction increases. In contrast, GaiaNIR reduces statistical uncertainties and retains sensitivity to extended halos even with strong extinction. These results indicate that GaiaNIR would both strengthen constraints on kinematic signatures accessible to Gaia and enable the study of clusters in heavily obscured regions that are currently beyond Gaia's reach.

astro-ph.GA↗

arXiv2026

From Gaia to GaiaNIR: II. A new view of the Milky Way bar

The Milky Way (MW) hosts a central bar whose pattern speed, orientation, and length remain uncertain, largely due to observational biases and selection effects, despite the transformative data provided by the Gaia mission. We aim to reassess the MW bar properties using Gaia DR3, explicitly accounting for incompleteness and astrometric uncertainties, and to quantify the expected improvements from future Gaia DR4, DR5, and GaiaNIR data. We combine Gaia DR3 RGB samples with line-of-sight velocities and realistic Gaia and GaiaNIR mock catalogues to characterise observational biases. We then apply standard techniques to infer the bar pattern speed and structural properties, and evaluate their performance for upcoming data releases. Using Gaia DR3 RGB mock catalogues, we find that the bar pattern speed exhibits a systematic offset of $+14.4 \pm 2.3$ km$~$s$^{-1}~$kpc$^{-1}$. Applying this approach to the data yields $\Omega_p = 43.7 \pm 0.1$ km$~$s$^{-1}~$kpc$^{-1}$, which we interpret as a conservative upper limit. Correcting for this bias gives $\Omega_p = 29.3 \pm 2.3$ km$~$s$^{-1}~$kpc$^{-1}$, although this estimate should be treated with caution given the limited number of mock realizations. We also detect bisymmetric perturbations in $v_\phi$ and $\langle |v_R / v_{\rm tot}| \rangle$, with phase angles $\phi_b = 19$-$24^\circ$ in the bar region. Future Gaia data releases, together with GaiaNIR, are expected to reduce systematic offsets in the pattern speed to $\sim +5$ km$~$s$^{-1}~$kpc$^{-1}$. In addition, GaiaNIR will further improve proper motion precision to below $0.001$ mas$~$yr$^{-1}$ for bright sources and extend the spatial coverage. Our results indicate that current measurements of the MW bar pattern speed are significantly affected by systematics, but that forthcoming Gaia and GaiaNIR data will substantially improve both accuracy and robustness.

astro-ph.GA↗

arXiv2024

Gaia dr3 reveals the complex dynamical evolution within star clusters

Context: Star clusters, composed of stars born from the same molecular cloud, serve as invaluable natural laboratories for understanding the fundamental processes governing stellar formation and evolution. Aims: This study aims to investigate correlations between the Mean Interdistance ($\bar{D_{i}}$), Mean Closest Interdistance ($\bar{D_{c}}$) and Median Weighted Central Interdistance ($\bar{D_{cc}}$) with the age of star clusters, examining their evolutionary trends and assessing the robustness of these quantities as possible age indicators. Methods: We selected a sample of open clusters in the solar region and with a representative number of members (e.g. well populated and without outliers). The interdistances are derived from the spatial distribution of member stars within a cluster. Their evolution over time allows us to use them as an age indicators for star clusters. Results: Our investigation reveals a high-significant correlation between the interdistances and cluster age. Considering the full sample of clusters between 7 and 9 kpc, the relationship is very broad. This is due to uncertainties in parallax, which increase with increasing distance. In particular, we must limit the sample to a maximum distance from the Sun of about 200 pc to avoid artificial effects on cluster shape and on the spatial distribution of their stars along the line of sight. Conclusions: By conservatively restraining the distance to a maximum of $\sim$200 pc, we have established a relationship between the interdistances and the age of the clusters. In our sample, the relationship is mainly driven by the internal expansion of the clusters, and is marginally affected by external perturbative effects. Such relation might enhance our comprehension of cluster dynamics and might be used to derive cluster dynamical ages.

astro-ph.GA↗

arXiv2019

Stellar escapers from M67 can reach solar-like Galactic orbits

We investigate the possibility that the Sun could have been born in M67 by carrying out $N$-body simulations of M67-like clusters in a time-varying Galactic environment, and following the galactic orbits of stars that escape from them. We find that model clusters that occupy similar orbits to M67 today can be divided up into three groups. Hot clusters are born with a high initial $z$-velocity, depleted clusters are born on cold orbits but are destroyed by GMC encounters in the Galactic disc, and scattered clusters are born on cold orbits and survive with more than 1000 stars at an age of 4.6 Gyr. We find that all cluster models in all three cluster groups have stellar escapers that are kinematicaly similar to the Sun. Hot clusters having the lowest such fraction $f_{\odot} = 0.06$ %, whilst depleted clusters have the highest fraction, $f_{\odot} = 6.61$ %. We calculate that clusters that are destroyed in the Galactic disc have a specific frequency of escapers that end up on solar-like orbits that is $\sim$ 2 times that of escapers from clusters that survive their journey.

astro-ph.GA↗

arXiv2016

Hubble Space Telescope observations of the host galaxies and environments of calcium-rich supernovae

Calcium-rich supernovae represent a significant challenge for our understanding of the fates of stellar systems. They are less luminous than other supernova (SN) types and they evolve more rapidly to reveal nebular spectra dominated by strong calcium lines with weak or absent signatures of other intermediate- and iron-group elements, which are seen in other SNe. Strikingly, their explosion sites also mark them out as distinct from other SN types. Their galactocentric offset distribution is strongly skewed to very large offsets (around one third are offset greater than 20 kpc), meaning they do not trace the stellar light of their hosts. Many of the suggestions to explain this extreme offset distribution have invoked the necessity for unusual formation sites such as globular clusters or dwarf satellite galaxies, which are therefore difficult to detect. Building on previous work attempting to detect host systems of nearby Ca-rich SNe, we here present Hubble Space Telescope imaging of 5 members of the class - 3 exhibiting large offsets and 2 coincident with the disk of their hosts. We find no underlying sources at the explosion sites of any of our sample. Combining with previous work, the lack of a host system now appears to be a ubiquitous feature amongst Ca-rich SNe. In this case the offset distribution is most readily explained as a signature of high-velocity progenitor systems that have travelled significant distances before exploding.

astro-ph.HE↗

arXiv2014

The PLATO 2.0 Mission

PLATO 2.0 has recently been selected for ESA's M3 launch opportunity (2022/24). Providing accurate key planet parameters (radius, mass, density and age) in statistical numbers, it addresses fundamental questions such as: How do planetary systems form and evolve? Are there other systems with planets like ours, including potentially habitable planets? The PLATO 2.0 instrument consists of 34 small aperture telescopes (32 with 25 sec readout cadence and 2 with 2.5 sec candence) providing a wide field-of-view (2232 deg2) and a large photometric magnitude range (4-16 mag). It focusses on bright (4-11 mag) stars in wide fields to detect and characterize planets down to Earth-size by photometric transits, whose masses can then be determined by ground-based radial-velocity follow-up measurements. Asteroseismology will be performed for these bright stars to obtain highly accurate stellar parameters, including masses and ages. The combination of bright targets and asteroseismology results in high accuracy for the bulk planet parameters: 2%, 4-10% and 10% for planet radii, masses and ages, respectively. The planned baseline observing strategy includes two long pointings (2-3 years) to detect and bulk characterize planets reaching into the habitable zone (HZ) of solar-like stars and an additional step-and-stare phase to cover in total about 50% of the sky. PLATO 2.0 will observe up to 1,000,000 stars and detect and characterize hundreds of small planets, and thousands of planets in the Neptune to gas giant regime out to the HZ. It will therefore provide the first large-scale catalogue of bulk characterized planets with accurate radii, masses, mean densities and ages. This catalogue will include terrestrial planets at intermediate orbital distances, where surface temperatures are moderate. Coverage of this parameter range with statistical numbers of bulk characterized planets is unique to PLATO 2.0.

astro-ph.EP↗

arXiv2010

Observational Constraints for Thermohaline Mixing

We provide a brief review of thermohaline physics and why it is a candidate extra mixing mechanism during the red giant branch (RGB). We discuss how thermohaline mixing (also called $δ$ $μ$ mixing) during the RGB due to helium-3 burning, is more complicated than the operation of thermohaline mixing in other stellar contexts (such as following accretion from a binary companion). We try to use observations of carbon depletion in globular clusters to help constrain the formalism and the diffusion coefficient or mixing velocity that should be used in stellar models. We are able to match the spread of carbon depletion for metal poor field giants but are unable to do so for cluster giants, which may show evidence of mixing prior to even the first dredge-up event.

astro-ph.SR↗

arXiv2010

The Impact of Stellar Collisions in the Galactic Center

We consider whether stellar collisions can explain the observed depletion of red giants in the Galactic center. We model the stellar population with two different IMFs: 1) the Miller-Scalo and 2) a much flatter IMF. In the former case, low-mass main-sequence stars dominate the population, and collisions are unable to remove red giants out to 0.4 pc although brighter red giants much closer in may be depleted via collisions with stellar-mass black holes. For a much flatter IMF, the stellar population is dominated by compact remnants (i.e. black holes, white dwarfs and neutron stars). The most common collisions are then those between main-sequence stars and compact remnants. Such encounters are likely to destroy the main-sequence stars and thus prevent their evolution into red giants. In this way, the red-giant population could be depleted out to 0.4 pc matching observations. If this is the case, it implies the Galactic center contains a much larger population of stellar-mass black holes than would be expected from a regular IMF. This may in turn have implications for the formation and growth of the central supermassive black hole.

astro-ph.GA↗

arXiv2009

MONTAGE: AGB nucleosynthesis with full s-process calculations

We present MONTAGE, a post-processing nucleosynthesis code that combines a traditional network for isotopes lighter than calcium with a rapid algorithm for calculating the s-process nucleosynthesis of the heavier isotopes. The separation of those parts of the network where only neutron-capture and beta-decay reactions are significant provides a substantial advantage in computational efficiency. We present the yields for a complete set of s-process isotopes for a 3 Mo, Z = 0.02 stellar model, as a demonstration of the utility of the approach. Future work will include a large grid of models suitable for use in calculations of Galactic chemical evolution.

astro-ph.SR↗

arXiv2007

Is our Sun a Singleton?

Most stars are formed in a cluster or association, where the number density of stars can be high. This means that a large fraction of initially-single stars will undergo close encounters with other stars and/or exchange into binaries. We describe how such close encounters and exchange encounters can affect the properties of a planetary system around a single star. We define a singleton as a single star which has never suffered close encounters with other stars or spent time within a binary system. It may be that planetary systems similar to our own solar system can only survive around singletons. Close encounters or the presence of a stellar companion will perturb the planetary system, often leaving planets on tighter and more eccentric orbits. Thus planetary systems which initially resembled our own solar system may later more closely resemble some of the observed exoplanet systems.

astro-ph↗