arXiv subjects

Emily L. Hunt

Publications and source records attributed to Emily L. Hunt.

16 recordsLinked to original sources

arXiv2026

The fading hierarchy of Galactic open clusters

Galactic open clusters provide a record of both hierarchical star formation and the subsequent dynamical evolution of the Milky Way disk. We use the two-point correlation function to characterize the spatial and kinematic clustering of open clusters in the solar neighborhood, considering their three-dimensional (3D) distributions and their projections onto the Galactic plane, as well as subsamples divided by age, mass, and survey volume. We quantify the clustering through fractal dimensions and characteristic correlation scales. We find that the youngest clusters have fractal dimensions comparable to those measured in the interstellar medium, suggesting that they retain part of the hierarchical structure of their natal molecular clouds. The clustering strength then decreases systematically with age and becomes weak on a characteristic timescale of order 100 Myr. This evolution is scale dependent: the excess correlation at tens of parsecs fades more rapidly than the weaker correlation at hundreds of parsecs. By contrast, we find only a weak dependence of the clustering properties on cluster mass and volume. Using the same cluster sample, we derive an empirical mapping between projected and 3D fractal dimensions, providing an observational calibration for comparing the Milky Way with studies of external galaxies, where only projected measurements are available. The open-cluster population exhibits characteristic spatial and kinematic clustering scales of approximately 370 pc and 11 km/s, respectively. These results support a picture in which young open clusters inherit a spatial hierarchy from star formation and progressively lose their spatial correlations through subsequent evolution.

astro-ph.GA↗

arXiv2026

The superclumps of the local Milky Way. Supercloud fragmentation and the sites of star formation

Using a Gaia-based 3D dust map of the solar neighborhood, we analyze the internal structure of the seven local superclouds. We identify quasi-periodic density enhancements along their spines, which we term "superclumps" and show that 73% of the known star-forming regions in the dust map volume can be associated with them. Across the six superclouds with more than one recovered superclump, the spacings are characteristic per cloud: $\sim$150-250 pc for the Split, Malpolon Cloud and Vela Ridge Cloud, and $\sim$250-380 pc for the Radcliffe Wave, Natrix Cloud and Sagittarius Spur Extension. The observed separations are two to three times smaller than the $\sim$560 pc predicted for an isolated self-gravitating cylinder of the same effective diameter, indicating fragmentation under external pressure rather than in isolation. The recurring spacing suggests that giant molecular cloud assembly is not a local, stochastic process, but is instead influenced by the large-scale gravitational fragmentation of the parent superclouds. Regardless of the precise formation mechanism, the superclumps occupy a critical intermediate scale in the hierarchical organization of the interstellar medium, bridging the gap between the kiloparsec-scale gas lanes and the $\sim$10-100 pc scale of individual giant molecular clouds.

astro-ph.GA↗

arXiv2026

The Nearby Star Formation and Supernova Histories Reconstructed from Young Star Clusters

We reconstruct the recent star formation and core-collapse supernova (ccSN) histories of the Solar Neighborhood from the past trajectories of young star clusters. Using a \textit{Gaia}-based cluster sample with newly derived ages, masses, and bulk 3D velocities, we integrate orbits backward in an assumed axisymmetric Galactic potential and combine the trajectories with IMF sampling and stellar lifetimes to infer ccSN times and locations over the past 50 Myr. The result is an all-sky, 3D, time-resolved map of nearby ccSN activity for comparison with high-resolution 3D views of the local interstellar medium. The 0--15 Myr map shows strong enhancements toward Orion, Vela, Sco--Cen, and Cepheus, many within present-day cavities and shells. At earlier times, the dominant enhancements trace the Collinder 135, Messier 6, and Alpha Persei cluster families, showing how the remnants of massive star-forming complexes have shaped the recent local feedback history. We recover a bursty star formation history followed by a delayed, smoother ccSN history. Over the last 40 Myr, the mean star formation and ccSN rates are $823~M_\odot~\mathrm{Myr}^{-1}$ and $7.7~\mathrm{Myr}^{-1}$, respectively, corresponding to a Milky Way rate of $0.55\pm0.03~\mathrm{century}^{-1}$. Present-day OB-star catalogs yield rates ranging from agreement with the cluster reconstruction to several times higher. Because the catalogs overlap weakly and require different corrections, we do not rescale the ccSN map. Our reconstruction provides an empirical framework for connecting the recent history of massive-star feedback to the 3D structure and life cycle of gas in the nearby Milky Way.

astro-ph.GA↗

arXiv2026

Unveiling the Milky Way with a Gaia DR3 census of OB-type stars within 2 kpc. I. Tracing local Galactic structure, massive star-forming regions and core-collapse supernova progenitors

O- and B-type stars are young and hot, thereby serving as vital tracers of the star formation and spiral arm structure of the Milky Way. At the dusk of the \textit{Gaia} DR3 era, a high-confidence and accurate catalogue appears timely. Here we have characterized a population of 105,971 OB-type stars (T$_{\rm eff} >$ 10,000 K; hereafter OB stars) within 2 kpc from the Sun, using an astro-photometric Bayesian inference tool. Our resulting map unveils a complex view of the young stellar populations across the thin disk, with prominent large-scale features such as the Cepheus Spur, the Giant Oval Cavity, and a segment of the Sagittarius-Carina spiral arm all visible. Their inhomogeneous spatial distribution implies that massive star formation has taken place clustered across a few highly concentrated regions. We find a correlation between the overdensities of OB stars and young open clusters ($<$20 Myr), although OB stars can be better detected in high-extinction regions. We identify over 4200 OB stars as core-collapse supernova (ccSN) or direct-collapse black hole (BH) progenitor candidates, and therefore targets of interest for spectroscopic follow-up. Furthermore, we find no OB-type star ccSN progenitor to explode within the next 1 Myr within 100 pc, at which such an event could be harmful to Earth's biosphere. Finally, we identify more BH progenitors to collapse within the next 1 Myr than ccSN to explode, despite the former's much scarcer number - which could be indicative of a recent massive star formation burst in the local Milky Way.

astro-ph.GA↗

arXiv2026

The Astrosky Ecosystem: An independent online platform for science communication and social networking

While almost everything that astronomers study occurs in the vacuum of space, astronomy itself does not `happen in a vacuum'. Interactions between scientists, as well as outreach to members of the public, improve extensively from access to good communication tools. Social media has become a key tool for communication in astronomy, being widely used by individuals and organizations alike for networking, outreach, and more. However, traditional social media is reliant on benevolent corporations providing a free service without compromising on quality, and the recent takeover and decline of Twitter has shown how vulnerable these platforms can be. In this proceeding, we present The Astrosky Ecosystem, which is an initiative to develop open-source tools and integrations for social media, principally the Bluesky social network. We explain how our project enables the astronomy community to operate its own social media infrastructure, independent of for-profit corporations. We also discuss some of the project's technical aspects, including its use of the AT Protocol for social networking, before concluding with ideas for the future.

astro-ph.IM↗

arXiv2026

The Secret Lives of Open Clusters: a Multiwavelength Examination of Three Open Clusters

Star clusters are well known for their dynamical interactions, an outcome of their high stellar densities; in this paper we use multiwavelength observations to search for the unique outcomes of these interactions in three nearby Galactic open clusters: IC 2602 (30 Myr), NGC 2632 (750 Myr) and M67 (4 Gyr). We compared X-ray observations from all-sky surveys like eROSITA, plus archival observations from Chandra X-ray Observatory, survey radio observations from ASKAP's Evolutionary Map of the Universe survey plus archival VLA observations, in conjunction with new cluster catalogs with Gaia. From X-ray, we found 77 X-ray sources likely associated with IC 2602, 31 X-ray sources in NGC 2632, and 31 near M67's central regions. We were further able to classify these X-ray sources based on their optical variability and any radio emission. Three IC 2602 X-ray sources had radio counterparts, which are likely all chromospherically active binary stars. We also identified luminous radio and X-ray variability from a spectroscopic triple system in M67, WOCS 3012/S1077, which is either consistent with a quiescent black hole binary, or due to an active binary stellar system. A recent population study of optical variables by Anderson & Hunt 2025 shows that the population of optical variables in open clusters clearly changes over cluster age; this pilot study gives evidence that the X-ray population also changes with time, and demonstrates the need for a broader multiwavelength study of Galactic open clusters.

astro-ph.GA↗

arXiv2025

The selection function of the Gaia DR3 open cluster census

Open clusters are among the most useful and widespread tracers of Galactic structure. The completeness of the Galactic open cluster census, however, remains poorly understood. For the first time ever, we establish the selection function of an entire open cluster census, publishing our results as an open-source Python package for use by the community. Our work is valid for the Hunt & Reffert catalogue of clusters in Gaia DR3. We developed and open-sourced our cluster simulator from our first work. Then, we performed 80,590 injection and retrievals of simulated open clusters to test the Hunt & Reffert catalogue's sensitivity. We fit a logistic model of cluster detectability that depends only on a cluster's number of stars, median parallax error, Gaia data density, and a user-specified significance threshold. We find that our simple model accurately predicts cluster detectability, with a 94.53\% accuracy on our training data that is comparable to a machine-learning based model with orders of magnitude more parameters. Our model itself offers numerous insights on why certain clusters are detected. We briefly use our model to show that cluster detectability depends on non-intuitive parameters, such as a cluster's proper motion, and we show that even a modest 25 km/s boost to a cluster's orbital speed can result in an almost 3$\times$ higher detection probability, depending on its position. In addition, we publish our raw cluster injection and retrievals and cluster memberships, which could be used for a number of other science cases -- such as estimating cluster membership incompleteness. Using our results, selection effect-corrected studies are now possible with the open cluster census. Our work will enable a number of brand new types of study, such as detailed comparisons between the Milky Way's cluster census and recent extragalactic cluster samples.

astro-ph.GA↗

arXiv2025

Toward the fabric of the Milky Way I. The density of disk streams from a local $250^3$ pc$^3$ volume

We studied 12 disk streams found in a 250$^3$ pc$^3$ volume in the solar neighborhood, which we define as coeval and comoving stellar structures with aspect ratios greater than 3:1. Using Gaia Data Release 3 data and the advanced clustering algorithms SigMA and Uncover, we identified and characterized these streams beyond the search volume, doubling, on average, their known populations. We estimate the number density of disk streams to be $\approx 820$ objects / kpc$^3$ (for $|Z| < 100$ pc), or surface densities of $\approx 160$ objects / kpc$^2$. These estimates surpass N-body estimates by one to two orders of magnitude and challenge the prevailing understanding of their destruction mechanisms. Our analysis reveals that these 12 disk streams are dynamically cold with 3D velocity dispersions between 2 and 5 km s$^{-1}$, exhibit narrow sequences in the Hertzsprung-Russell diagram, and are highly elongated with average aspect ratios of 7:1, extending up to several hundred parsecs. We find evidence suggesting that one of the disk streams, currently embedded in the Scorpius-Centaurus association, is experiencing disruption, likely due to the primordial gas mass of the association.

astro-ph.GA↗

arXiv2025

How many stars form in compact clusters in the local Milky Way?

Two main models coexist for the environment in which stars form. The clustered model stipulates that the bulk of star formation occurs within dense embedded clusters, but only a minority of them survive the residual gas expulsion phase caused by massive stellar feedback unbinding the clusters. On the other hand, the hierarchical model predicts that star formation happens at a range of scales and densities, where open clusters (OCs) only emerge from the densest regions. We aim to exploit a recent catalog of compact OCs, corrected for completeness, to obtain an updated estimation of the surface density star formation rate within OCs ($\sum_{\rm SFR,OC}$), which we compare with recent estimates of $\sum_{\rm SFR}$ to determine which model is more likely. We have applied two methods. The first one consisted of integrating over the power law that was fit for the mass function of the youngest OCs using a MC sampling. The second one consisted of counting the total compact mass within these youngest OCs within 1 kpc, so that the result could be directly compared with local values of $\sum_{\rm SFR}$. We estimated new $\sum_{\rm SFR,OC}$ values between $736^{+159}_{-176}$ and $875^{+34}_{-35}$ M$_{\odot}$ Myr$^{-1}$ kpc$^{-2}$, depending on the methodology. These results are significantly higher than previous $\sum_{\rm SFR,OC}$ estimates, which we attribute to the incompleteness of past catalogs, and are consistent with the majority ($\geq$ 50 \%) or even the vast majority ($\geq$ 80 \%) of the star formation occurring in initially compact clusters, through comparisons with $\sum_{\rm SFR}$ from the recent literature. Our new $\sum_{\rm SFR,OC}$ values are consistent with clustered formation being the most dominant mode of star formation.

astro-ph.GA↗

arXiv2025

A bird's eye view of stellar evolution through populations of variable stars in Galactic open clusters

Both star clusters and variable stars are sensitive laboratories of stellar astrophysics and evolution: cluster member stars provide context for interpreting cluster populations, whereas variability reveals the nature of individual stellar systems. The European Space Agency's Gaia mission has revolutionized the census of star clusters in the Milky Way, while simultaneously providing an unprecedented homogeneous all-sky catalog of variable stars. Here, we leverage the third Gaia data release to obtain an empirical bird's eye view of stellar evolution based on 34760 variable stars residing in 1192 Galactic open clusters (OCs) containing 173294 members (variable member fraction 20.0%). Using precise OC distances, dereddened magnitudes, and consistently determined ages, we a) pinpointed regions of pulsational instability across the color-absolute magnitude diagram (CaMD); b) traced the occurrence rate of variables as a function of age, and c) considered the evolution of rotation periods and photometric activity (gyrochronology). The occurrence of pulsating stars can serve as a model- and reddening-independent age estimator. Our results underline that jointly considering stellar variability and OC membership enables a plethora of further applications, such as age dating or dereddening OCs based on expected CaMD locations of variable stars. Upcoming Gaia data releases and the Vera C. Rubin Observatory will vastly increase the extent to which the details of variable stars in OCs can empirically unravel the astrophysics and evolution of stellar populations.

astro-ph.SR↗

arXiv2025

The completeness of the open cluster census towards the Galactic anticentre

Open clusters have long been used as tracers of Galactic structure. However, without a selection function to describe the completeness of the cluster census, it is difficult to quantitatively interpret their distribution. We create a method to empirically determine the selection function of a Galactic cluster catalogue. We test it by investigating the completeness of the cluster census in the outer Milky Way, where old and young clusters exhibit different spatial distributions. We develop a method to generate realistic mock clusters as a function of their parameters, in addition to accounting for Gaia's selection function and astrometric errors. We then inject mock clusters into Gaia DR3 data, and attempt to recover them in a blind search using HDBSCAN. We find that the main parameters influencing cluster detectability are mass, extinction, and distance. Age also plays an important role, making older clusters harder to detect due to their fainter luminosity function. High proper motions also improve detectability. After correcting for these selection effects, we find that old clusters are $2.97\pm0.11$ times more common at a Galactocentric radius of 13~kpc than in the solar neighbourhood -- despite positive detection biases in their favour, such as hotter orbits or a higher scale height. The larger fraction of older clusters in the outer Galaxy cannot be explained by an observational bias, and must be a physical property of the Milky Way: young outer-disc clusters are not forming in the outer Galaxy, or at least not with sufficient masses to be identified as clusters in Gaia DR3. We predict that in this region, more old clusters than young ones remain to be discovered. The current presence of old, massive outer-disc clusters could be explained by radial heating and migration, or alternatively by a lower cluster destruction rate in the anticentre.

astro-ph.GA↗

arXiv2024

Non-radial oscillations mimicking a brown dwarf orbiting the cluster giant NGC 4349 No. 127

Several evolved stars have been found to exhibit long-period radial velocity variations that cannot be explained by planetary or brown dwarf companions. Non-radial oscillations caused by oscillatory convective modes have been put forth as an alternative explanation, but no modeling attempt has yet been undertaken. We provide a model of a non-radial oscillation, aiming to explain the observed variations of the cluster giant NGC 4349 No. 127. The star was previously reported to host a brown dwarf companion, but whose existence was later refuted in the literature. We reanalyzed 58 archival HARPS spectra, acquiring additional activity indicators using the SERVAL and RACCOON pipelines. We searched for periodicity in the indicators and correlations between the indicators and radial velocities. We further present a simulation code able to produce synthetic HARPS spectra, incorporating the effect of non-radial oscillations, and compare the simulated results to the observed variations. We find a positive correlation between chromatic index and radial velocity, along with closed-loop Lissajous-like correlations between radial velocity and each of the spectral line shape indicators (full width at half maximum, and contrast of the cross-correlation function and differential line width). Simulations of a low-amplitude, retrograde, dipole (l = 1, m = 1), non-radial oscillation can reproduce the observed behavior and explain the observables. Photometric variations below the detection threshold of the available ASAS-3 photometry are predicted. The oscillation and stellar parameters are largely in agreement with the prediction of oscillatory convective modes. The periodic variations of the radial velocities and activity indicators, along with the respective phase shifts, measured for the intermediate-mass cluster giant NGC 4349 No. 127, can be explained by a non-radial oscillation.

astro-ph.SR↗

arXiv2024

Most nearby young star clusters formed in three massive complexes

Efforts to unveil the structure of the local interstellar medium and its recent star formation history have spanned the past seventy years. Recent studies utilizing precise data from space astrometry missions have revealed nearby, newly formed star clusters with connected origins. Nonetheless, mapping young clusters across the entire sky back to their natal regions has been hindered by a lack of clusters with precise radial velocity data. Here we show that 155 out of 272 (57 percent) high-quality young clusters within one kiloparsec of the Sun arise from three distinct spatial volumes. This conclusion is based upon the analysis of data from the third Gaia release and other large-scale spectroscopic surveys. Currently dispersed throughout the Solar Neighborhood, their past positions over 30 Myr ago reveal that these families of clusters each formed in one of three compact, massive star-forming complexes. One of these families includes all of the young clusters near the Sun -- the Taurus and Sco-Cen star-forming complexes. We estimate that over 200 supernovae were produced from these families and argue that these clustered supernovae produced both the Local Bubble and the largest nearby supershell GSH 238+00+09, both of which are clearly visible in modern three-dimensional dust maps.

astro-ph.GA↗

arXiv2024

Improving the open cluster census. III. Using cluster masses, radii, and dynamics to create a cleaned open cluster catalogue

The census of open clusters has exploded in size thanks to data from the Gaia satellite. However, it is likely that many of these reported clusters are not gravitationally bound, making the open cluster census impractical for many scientific applications. We test different physically motivated methods for distinguishing between bound and unbound clusters, using them to create a cleaned cluster catalogue. We derived completeness-corrected photometric masses for 6956 clusters from our earlier work. Then, we used these masses to compute the size of the Roche surface of these clusters (their Jacobi radius) and distinguish between bound and unbound clusters. We find that only 5647 (79%) of the clusters from our previous catalogue are compatible with bound open clusters, dropping to just 11% of clusters within 250 pc. 3530 open clusters are in a strongly cut high quality sample. The moving groups in our sample show different trends in their size as a function of age and mass, suggesting that they are unbound and undergoing different dynamical processes. Our cluster mass measurements constitute the largest catalogue of Milky Way cluster masses to date, which we also use for further science. Firstly, we inferred the mass-dependent completeness limit of the open cluster census, showing that the census is complete within 1.8 kpc only for objects heavier than 230 M$_\odot$. Next, we derived a completeness-corrected age and mass function for our open cluster catalogue, including estimating that the Milky Way contains a total of $1.3 \times 10^5$ open clusters, only ~4% of which are currently known. Finally, we show that most open clusters have mass functions compatible with the Kroupa initial mass function. We demonstrate Jacobi radii for distinguishing between bound and unbound star clusters, and publish an updated star cluster catalogue with masses and improved cluster classifications. (abridged)

astro-ph.GA↗

arXiv2023

Improving the open cluster census. II. An all-sky cluster catalogue with Gaia DR3

Data from the Gaia satellite are revolutionising our understanding of the Milky Way. With every new data release, there is a need to update the census of open clusters. We aim to conduct a blind, all-sky search for open clusters using 729 million sources from Gaia DR3 down to magnitude $G\sim20$, creating a homogeneous catalogue of clusters including many new objects. We used the Hierarchical Density-Based Spatial Clustering of Applications with Noise (HDBSCAN) algorithm to recover clusters. We validated our clusters using a statistical density test and a Bayesian convolutional neural network for colour-magnitude diagram classification. We inferred basic astrometric parameters, ages, extinctions, and distances for the clusters in the catalogue. We recovered 7167 clusters, 2387 of which are candidate new objects and 4782 of which crossmatch to objects in the literature, including 134 globular clusters. A more stringent cut of our catalogue contains 4105 highly reliable clusters, 739 of which are new. Owing to the scope of our methodology, we are able to tentatively suggest that many of the clusters we are unable to detect may not be real, including 1152 clusters from the Milky Way Star Cluster (MWSC) catalogue that should have been detectable in Gaia data. Our cluster membership lists include many new members and often include tidal tails. Our catalogue's distribution traces the galactic warp, the spiral arm structure, and the dust distribution of the Milky Way. While much of the content of our catalogue contains bound open and globular clusters, as many as a few thousand of our clusters are more compatible with unbound moving groups, which we will classify in an upcoming work. We have conducted the largest search for open clusters to date, producing a single homogeneous star cluster catalogue which we make available with this paper.

astro-ph.GA↗

arXiv2020

Improving the open cluster census. I. Comparison of clustering algorithms applied to Gaia DR2 data

The census of open clusters in the Milky Way is in a never-before seen state of flux. Recent works have reported hundreds of new open clusters thanks to the incredible astrometric quality of the Gaia satellite, but other works have also reported that many open clusters discovered in the pre Gaia era may be associations. We aim to conduct a comparison of clustering algorithms used to detect open clusters, attempting to statistically quantify their strengths and weaknesses by deriving the sensitivity, specificity, and precision of each as well as their true positive rate against a larger sample. We selected DBSCAN, HDBSCAN, and Gaussian mixture models for further study, owing to their speed and appropriateness for use with Gaia data. We developed a preprocessing pipeline for Gaia data and developed the algorithms further for the specific application to open clusters. We derived detection rates for all 1385 open clusters in the fields in our study as well as more detailed performance statistics for 100 of these open clusters. DBSCAN was sensitive to 50% to 62% of the true positive open clusters in our sample, with generally very good specificity and precision. HDBSCAN traded precision for a higher sensitivity of up to 82%, especially across different distances and scales of open clusters. Gaussian mixture models were slow and only sensitive to 33% of open clusters in our sample, which tended to be larger objects. Additionally, we report on 41 new open cluster candidates detected by HDBSCAN, three of which are closer than 500 pc. When used with additional post-processing to mitigate its false positives, we have found that HDBSCAN is the most sensitive and effective algorithm for recovering open clusters in Gaia data. Our results suggest that many more new and already reported open clusters have yet to be detected in Gaia data.

astro-ph.GA↗