N.B. Per survey policy, projects must be approved by the BDBS committee (R.M. Rich, C.I. Johnson, C. Pilachowski) bulge and posted on the Projects site in order to be proceeded with. Any team member may express interest to contribute. This policy is meant to prevent overlap and 'competition' within the collaboration.

Here is a list of currently defined projects, the abstracts and details are listed below:

1. R.M. Rich (UCLA): Mapping of the bulge extended horizontal branch; does the bulge contain massive He burning stars?

2. R. de Propris (Turku): N-rich stars and multiple populations

3. W.I. Clarkson (Dearborn): Accurate photometry from bleeding stars in wide-field imagers

4. A. Koch-Hansen (ARI): Verification and characterization of FSR cluster candidates

5. D. Lim (ARI): Globular cluster contribution to the Milky Way bulge formation: The fraction of second-generation populations on the HB

6. A. Savino (ARI): Horizontal branch morphological reconstruction through population ratios of helium-burning stars

7. (Proposed): W.I. Clarkson (Dearborn): Some lessons for LSST and comparisons to Gaia

8. (Proposed): T. Marchetti (ESO): Searching for clusters in BDBS + Gaia using clustering algorithms

9. (Proposed): A. Kunder (SMU): The spatial distribution of the BDBS red clump stars

10. J. Kader (Indiana): Multiple stellar populations in Milky Way bulge globular clusters

11. (Proposed): I. Simion (Shanghai): Tidal tails and faint substructures in the bulge

12. (Proposed): I. Simion (Shanghai): Study of the properties of the split RC

13. (Proposed): A. Kunder (SMU): Globular cluster extra-tidal stars

14. C. Johnson (STScI): Bulge Metallicity Maps

15. (Proposed): J. Kader (Indiana): Population tagging of tidally stripped globular cluster stars

16. C. Johnson (STScI): Searching for Young Stars in the Bulge

R. M. Rich: Mapping of the bulge extended horizontal branch; does the bulge contain massive He burning stars? (Adopted)

Collaboration: open to BDBS team members; current interest from Johnson, Clarkson, and de Propris

Abstract: We propose to map the morphology and extent of the blue horizontal branch in the Galactic bulge, using the globular clusters such as NGC 6522 (that has an EHB) in the BDBS footprint as templates. We seek to determine whether the morphology of the EHB depends on Galactic latitude or longitude, and we propose to match with Gaia and Galex archival data when possible. Saha et al. (2018) argue that a young, massive stellar population of He burning stars may be widespread int he bulge and we propose to use the BDBS u-band photometry to help set limits on such a population.

R. de Propris: N-rich stars and multiple populations (Adopted)

Abstract: My proposal, to elaborate on the one below, is to search for the presence of N-rich stars that are the hallmark of stars with MP2 abundances (in the parlance of Bastian & Lardo 2018), i.e., they are enriched in helium. We have shown that this is likely the cause of the UV excess in ETGs and that this is likely something that occurs in all stellar populations with ETG-like star-formation histories. Several authors have shown that the presence of these stars leads to a split or broadening of the red giant branch through the influence of N on the NH and CN lines (the effect is even stronger for main sequence stars).

My proposal is to compare the scatter on the principal sequences in u-g vs all other colours, accounting for differential reddening and for the photometric errors. Through mixture modelling we can then estimate whether these stars exist in the Galactic Bulge and their importance, and possibly explore the radial dependence (i.e., if they are centrally concentrated, as the UV upturn sources appear to be in ETGs).

Will Clarkson: Accurate photometry from bleeding stars in wide-field imagers (Adopted)

Collaboration: open to all who want to help; development so far has been performed by Clarkson, with testing by UM-Dearborn undergraduates and with some input from Kathy Vivas.

Abstract: All imaging observations in the Galactic plane are sensitive to charge bleeding from interlopers, in which a relatively small number of luminous and/or nearby objects are so saturated in typical (~1 minute) main-program exposures that they bleed across much of the detector. Summation along the bleed provides a promising method to recover the flux for bright objects from long exposures, although the application of this technique to crowded fields is not entirely trivial. With "long" (~1 minute), "short" (~5s) and "very short" (~0.5s) exposures in all filters, BDBS provides a beautiful dataset to develop the techniques of summing along the bleed in crowded regions, with typically thousands of strongly-bled objects across the focal plane in typical regions. The scientific payoff includes a much larger set of useful exposures for bright objects (such as RR Lyrae) in program observations.

We have performed initial investigations into charge bleeding in BDBS data, producing a tool that isolates charge-bleed objects in long exposures, constructs shaped apertures (for both the object and sky), performs simple photometry, and compares the result to expectations from short exposures of the same field. This has been developed using a single, particularly crowded image in one field. Now we wish to (i) test at all filters and across a much wider range of crowding levels; (ii) complete the implementation of neighbor-subtraction photometry (in which scaled neighbor objects are subtracted from under the bleed), (iii) complete the testing of the methods, and (iv) write the results up. Stretch-goals include the construction of an "effective PSF" for strongly saturated objects (although it is not obvious at this stage if the fine structure of a charge-bled PSF is strictly reproducible - it would be good to test this).

Data required: Due to its focus on the pixel-data, this project mainly requires the imaging data, which we have been downloading from the NOAO archive as needed. However, the BDBS v2 catalog would be very useful as a truth-table to short-circuit certain parts of the testing.

Andreas Koch-Hansen: Verification and characterization of FSR cluster candidates (Adopted)

Collaboration: E.K. Grebel, D. Lim, A. Savino

Abstract: Our goal is to confirm or refute the nature of star clusters from the Froebrich, Scholz, & Raftery (2007; FSR) catalogs in the BDBS data as star clusters vs. asterisms or false positives using CMDs of these objects in all available filter combinations, aided by a crossmatch with Gaia. If confirmed as (young) clusters (possibly associated with dark clouds?), this will also provide interesting insight into the young populations in the bulge. For the cross-match of such suspected cluster candidates we will further employ thee public VPHAS+ (Drew et al. 2014). As another application of these clusters we will exploit their distances as independent estimates where RC-based distances are not available. If a substantial population of star clusters is identified, their ages, metallicities and distances inferred from the CMD modelling of these objects will constitute an independent archaeological record of the inner Milky Way formation history, complementing the information coming from other structural tracers, such as red clump stars and RR Lyrae.

Dongwook Lim: Globular cluster contribution to the Milky Way bulge formation: The fraction of second-generation populations on the HB (Adopted)

Collaboration: open to BDBS team member; Dongwook Lim, Andreas Koch-Hansen

Abstract: In the current hierarchical merging paradigm, the Milky way formed by numerous mergers and accretion of ancient subsystems. In particular, the recent studies of multiple stellar populations observed in globular clusters (GCs) have made GC important to understanding assembly history of the Galaxy, as building blocks. Koch et al. (2019), for instance, derived the fraction of halo stars originated from GCs based on CN-strong “second-generation (SG)” stars, and Chung et al. (2019) estimated the fraction of He-enhanced SG stars based on blue horizontal branch (HB) stars. We will extend this idea to the Galactic bulge and investigate the fraction of SG population originated from disrupted GCs using HB stars. In order to fulfill this, we will 1) generate synthetic HB model for metal-rich bulge stars, 2) define the selection criterions of the first-generation (FG) and SG stars on the various color-magnitude and color-color diagrams, 3) select the FG and SG stars from the best bulge sample of BDBS data combined with Gaia DR2, and 4) investigate the fraction of SG population depending on the Galactic position. This study would help to understand the formation of the Milky Way bulge, and the multi-band photometry of BDBS can be an effective tool to select HB stars in the bulge field.

Alessandro Savino: Horizontal branch morphological reconstruction through population ratios of helium-burning stars (Adopted)

Collaboration: A. Koch-Hansen, E.K. Grebel, D. Lim

Abstract: The horizontal branch (HB) is a stellar evolutionary phase whose morphology contains a wealth of information on the star formation and chemical enrichment history of the parent stellar population. Alas, when it comes to the bulge of the Milky Way, the large magnitude spread induced by the range of stellar distances renders a full morphological modelling of the HB problematic. However, certain types of helium-burning stars, such as blue HB stars (BHB), RR Lyrae (RRL - both fundamental and overtone pulsators) and red clump stars (RC), are robustly identifiable, using photometric data, in a distance-independent manner. We will use the BDBS dataset, complemented with variability-focused surveys like OGLE, to perform a census of these specific classes of helium burning stars. Carefully taking into account the selection functions of these surveys will allow us to recover the relative abundances of BHBs/RRLs/RCs as function of position in the bulge, providing a proxy for the local HB morphology. We will link these population ratios to physical quantities like age and chemical composition by means of a stellar population synthesis approach. This method alone will be sufficient to provide strong constraints on the shape of the metal-poor tail of the bulge metallicity distribution function and of its spatial gradients. When complemented with ancillary information from, e.g. current and future spectroscopic surveys, we will be able to put stringent constraints on quantities such as the age of the oldest bulge stars and the mass loss experienced by red giant branch stars.

Will Clarkson: Some lessons for LSST and comparisons to Gaia (Proposed)

Collaboration: Open to BDBS team members

BDBS allows us to answer several critical questions for future imaging surveys towards crowded regions. For example: to what photometric depth can we expect an LSST-like imager to reach in each filter in crowded regions? What astrometric systematics are typical in the delivered catalog from such a survey? To what photometric depth does Gaia actually reach in these regions? I propose a short paper addressing these questions, possibly to PASP. The paper will present a spatial comparison of BDBS’ photometric depth between the BDBS and Gaia DR2 catalogs. Residual astrometric systematics will be presented for the entire survey, derived using a comparison to Gaia DR2 positions. A comparison will also be presented between BDBS’ 5-sigma depth and the predictions for LSST’s current “baseline” strategy.

Tommaso Marchetti: Searching for clusters in BDBS + Gaia using clustering algorithms (Proposed)

Collaboration: Open to BDBS team members

Globular clusters are among the oldest stellar systems in our Milky Way, and they are key objects to study the formation and evolutionary history of the Galactic bulge. The combined photometric and astrometric information available for more than 75 million stars in common between the BDBS survey and Gaia is a unique dataset to reveal this hidden population. Following Gran et al. (2019), we propose to develop a clustering algorithm to detect overdensities in spatial coordinates, proper motions, and color, with the goal of identifying nearby stars that move together and that define a narrow sequence in the color-magnitude diagram. BDBS optical colors can then be used to infer the clusters' metallicity. This method can be used to confirm/reject previous claimed globular clusters, and to possibly discover new clusters in the southern Galactic bulge.

Andrea Kunder: The spatial distribution of the red clump BDBS stars (Proposed)

Collaboration: C.I. Johnson, Open to BDBS team members

It has been suggested that the Galactic bulge has populations of stars that exhibit different spatial distributions. Different studies have suggested that more metal-rich stars tend to have a spatial distribution that is boxy/barred while the more metal-poor stars tend to have a spatial distribution that is spheroidal. We recently found that at least within the bulge RR Lyrae population, metal-poor stars can show both a boxy/barred spatial distribution and a spheroidal spatial distribution. The difference in the spatial distributions are attributed to galactocentric distance. The red clump stars in BDBS are a factor of 10 more numerous than used in previous studies, enabling a study of greater resolution of the spatial distribution of the Galactic bulge as compared to previous studies. We will split the stars by both metallicity and by 3D distance from the center of the Galaxy to probe the spatial distribution of these stars. We will compare our results with the Milky Way simulation from Gardner et al. (2014) and Debattista et al. (2017) to see if the two components can arise naturally from disk buckling. We will also compare our results with Saha et al. (2016)5, to see if the two components are better explained from an early accretion event.

Justin Kader: Multiple stellar populations in Milky Way bulge globular clusters (Proposed)

Collaboration: C. Pilachowski

Photometric surveys searching for multiple populations in globular clusters (GCs) tend to avoid clusters toward the Galactic bulge because of the high degree of extinction, crowding, and superposition of stellar populations along the line of sight. The broad 6-band (ugrizY) reddening-corrected, astrometrically precise photometry of BDBS offers an unprecedented opportunity to carry out an extensive study aimed at identifying multiple stellar populations in the ~25 bulge GCs in the survey footprint and characterizing their main properties. We will use the Gaia DR2 matched catalog to determine cluster membership down to r ~ 19 mag, fainter than the main sequence turnoff of many of the clusters. We will take advantage of the near-UV, optical, and near-IR passbands to construct pseudo-CMDs, e.g. g vs. (u – g) – (g - i), which maximize separations of stellar populations in the main sequence and the RGB. We will compute populational number ratios and analyze the kinematics of individual populations in order to constrain the formation history of the poorly studied bulge globular clusters. Our analysis will allow us to explore the link among the properties of multiple populations, the clusters’ orbital parameters, and their internal kinematics.

Iulia T. Simion: Tidal tails and faint substructures in the bulge (Proposed)

Collaboration: Z. Yuan, C. Johnson, R. Ibata, M. Rich open to the BDBS collaboration

We propose to search for spatially and kinematically coherent structures in the bulge using the Stars' Galactic Origin (StarGO) algorithm. StarGO is based on a popular unsupervised learning algorithm and an adaptive and quantitative group-identification scheme. We aim to apply StarGO to the Southern Bulge stars with BDBS x Gaia EDR3 5D phase-space information (x,y,z,pmra,pmdec) and photometric metallicity. The code will map the 5D phase-space onto a 2D neural network (Zhen et al. 2018) where it can then identify and visualize stellar groups, providing the significance of each cluster. First, we will test this method on known bulge globular clusters. If successful, we will then search for tidal stars in an area around each bulge cluster (~25 of them in the BDBS footprint). StarGO has already been successfully applied to 5D data to discover tidal tails in the nearby disrupting open cluster Coma Berenices (Tang et al. 2020). Johnson et al. 2020 showed that both M22 and FSR 175 clusters have many extra-tidal candidates in the BDBS x Gaia DR2 catalog. With StarGO we should be able to find the diffuse structures around GCs in the bulge, where the tidal effects are significant. We will obtain a list of candidates with high probabilities to belong to tidal structures for spectroscopic follow-up studies.

Iulia T. Simion: Study of the properties of the split Red Clump (Proposed)

Collaboration: J. Shen, C. Johnson, M. Rich open to the BDBS collaboration

We propose to study the kinematic properties of the split RC as a function of metallicity using Gaia EDR3 proper motions. RC stars are good standard candles and can be used to compute distances with uncertainties of up to 1 kpc in the bulge region. Over ~90% of these RC stars also have Bailer-Jones+ 2020 distance estimates and their spatial distribution seems reasonable. A careful study of the distance errors is needed to compile a catalog of reliable RC distances. We aim to investigate the spatial distribution of the split RC and cross-check the kinematic findings with the Shen et al. 2010 model (see also Qin et al. 2015).

Andrea Kunder: Extra-tidal stars and spectroscopic follow-up of BDBS GCs (Proposed)

Collaboration: Joanne Hughes Clark (SU), Kevin Covey (WWU), Andrea Kunder (SMU), Kristen Larson (WWU), I. Simion, C. Johnson, M. Rich open to the BDBS collaboration

We will use ASteCA to search for extra-tidal stars around inner Galaxy globular clusters. We will use BDBS photometry as well as the OGLE RRL database and APOGEE spectroscopy to search for signatures of tidal-stripping in the inner globular clusters. We will follow-up any extra-tidal stars with spectroscopy from APO and AAT, which we have institutional time on.

Christian Johnson: Bulge Metallicity Maps (Adopted)

Collaboration: Mike Rich, Iulia Simion, open to BDBS collaboration

Using the u-i metallicity calibration from Johnson et al. (2020), we will build metallicity maps across the BDBS footprint that include several million stars.

Justin Kader: Population Tagging of Tidally Stripped Globular Cluster Stars (Proposed)

Collaboration: C. Pilachowski, C. Johnson, open to BDBS collaboration

We propose to study the light element abundances of RGB stars in the extratidal environments of several inner Galaxy globular clusters using BDBS photometry. The (u-g)-(g-i) color is sensitive to variations in C,N,O,Na,Mg,Al abundances among cluster red giant stars and can be used to identify multiple stellar populations in GCs. The results can be used to place constraints on Galactic bulge formation scenarios.

Christian Johnson: Searching for Young Stars in the Bulge (Adopted)

Collaboration: R. M. Rich, M. Joyce, T. Marchetti, open to BDBS collaboration

We propose a two part project seeking to verify or refute the existence of young stars in the bulge. The first part aims to investigate the presence of very young blue loop stars in the bulge using a combination of EDR3 data and isochrone fitting. The second part seeks to apply updated isochrones and BDBS photometry to an existing sample of microlensed dwarfs to obtain updated ages.