Mapping the Milky Way: UC Chile's New Catalog Identifies 5,000 Ancient Stars
Research conducted by Astrophysics PhD candidate and CATA astronomer Julio Olivares, published in Astronomy & Astrophysics, has led to the creation of a catalog containing nearly 5,000 RR Lyrae stars from the galactic bulge, or center. This could contribute to a better understanding of the history of the Universe.
photo_camera “We studied the central region of the galaxy, which is the bulge, a more challenging area to analyze because of the large number of stars. RR Lyrae stars allow us to calculate distances with much higher precision than with any other type of star,” explains astronomer Julio Olivares. (Image courtesy of CATA)
As astronomical studies continue to grow exponentially, a series of catalogs are being created to offer new possibilities for research.
A crucial aspect of this work involves integrating data from various surveys and studies, something that Julio Olivares, an astronomer at the Center for Astrophysics and Related Technologies (CATA) and a PhD candidate at the UC Chile Institute of Astrophysics, has successfully done.
His goal was to derive a rotation curve for the RR Lyrae stars of the galactic bulge in order to show that the ancient component traced by these stars is different from the two main components previously observed in the region.
“We studied the central region of the galaxy, which is the bulge, a more challenging area to analyze because of the large number of stars. RR Lyrae stars allow us to calculate distances with much higher precision than with any other type of star. These stars are ancient tracers, over 10 billion years old, compared to the Sun, which is only 4.5 billion years old. From many prior studies, we know that the bulge of the Milky Way also contains a bar where many stars rotate. So, it’s interesting that these stars are rotating, but not following the shape of the bar itself,” explains Julio Olivares.
In simple terms, his work consisted of combing three sets of data:
- the spectra from the APOGEE-2S survey;
- the light curves from the OGLE-IV survey;
- and the near-infrared photometry and proper motion data from the VISTA (Variables in the Milky Way) survey for 4,193 RR Lyrae stars.
He used the coordinates, velocities, proper motions, and distances to calculate orbits within an updated galactic potential and isolate the stars that belong to the bulge from those in other components of the galaxy.
This research led to the publication of his paper “Comparing Bulge RR Lyrae Stars with Bulge Giants” in the prestigious journal Astronomy & Astrophysics.
Stellar Catalog
An interesting result of this work is that, together with other astronomers, Julio Olivares created a new catalog of RR Lyrae stars from the bulge region, thus expanding existing surveys from about 2,000 stars to nearly 5,000.
“We’re increasing the number of stars available for study. Since each star has different parameters, I analyzed their velocities and also included their distance and metallicity. We started creating a dictionary that is growing over time, and we hope it continues to do so because these are the oldest stars in our galaxy and understanding them provides valuable insight into the history of the universe,” says Olivares.
Collaborative work was essential in this research process and will be for future studies as well. Julio Olivares expressed gratitude for the support he received by being part of CATA.
“The support I received ranged from access to facilities and resources to being connected with some of the top astronomers and astrophysicists in the country. Participating in conferences has also been a great learning opportunity. Most of the researchers I work with are also part of CATA, which made it much easier to work together on this project,” he notes.
Learning About Stars
The publication's conclusions reveal that stars within the bulge make up 57% of the sample. Additionally, RR Lyrae variables in the bulge rotate more slowly than the metal-rich red giant stars and display a lower velocity dispersion. Their kinematics are consistent with being part of the low-metallicity tail of the metal-poor component.
The study also confirms that a significant fraction of RR Lyrae stars from the halo and thick disk pass through the bulge within their orbits, contributing to the velocity dispersion. Therefore, proper orbital analysis is crucial to isolate the true bulge variables. Furthermore, the RR Lyrae stars of the bulge seem to trace a spheroidal component, although current data does not allow for a firm conclusion about their spatial distribution.
“One of the most interesting findings is that we’ve discovered many new stars of this type because we’re constantly observing the sky, watching how stars vary in brightness. Thanks to the VVV survey, we’ve discovered a new catalog of many more stars of this type in the infrared, but we still don’t have the radial velocities—we’re missing that kinematic data. It would be great to request observation time to obtain spectra of these stars and expand the catalog of stars in the bulge region,” concludes Olivares.
