The Kinematics, Metallicities, and Orbits of Six Recently Discovered Galactic Star Clusters with Magellan/M2FS Spectroscopy

summary

The gist

The gist We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters and two newly discovered open clusters at low Galactic latitudes to determine their kinematics,

In short

Researchers used Magellan/M2FS spectroscopy and Gaia data to study six Milky Way star clusters. They found three globular clusters (Gran 3, Gran 4, Garro 01) and three open clusters (Gaia 9, Gaia 10, LP 866). The study determined their kinematics, chemical compositions ([Fe/H], [Mg/Fe]), and orbital properties. This work highlights the power of Gaia in mapping the star cluster population.

Key concepts

Magellan/M2FS Spectroscopy: This technique involves using a telescope (Magellan) equipped with a fiber system (M2FS) to take high-resolution spectra of stars. By analyzing these spectra, scientists measure crucial stellar properties like temperature, surface gravity, and chemical abundances ([Fe/H], [Mg/Fe]), which helps determine the cluster's age and origin.
Gaia Astrometry: Gaia is a space mission that precisely measures the positions and motions (proper motions) of stars. This data is used to calculate the 3D kinematics, systemic proper motion, and orbital paths of star clusters like Gran 3 and Gran 4, revealing how they move within the Milky Way's gravitational field.
Orbital Properties: This refers to the path a star cluster takes through space around the Galactic center. The study calculated these orbits using a galactic potential model. For instance, Gran 4 has an eccentric orbit suggesting it might have been involved in past merger events with other stellar streams, providing clues about its formation history.
Globular Cluster vs. Open Cluster: These are two types of star clusters distinguished by their size and age. Globular clusters are typically very old and dense, often found in the Galactic bulge or halo. Open clusters are younger and less dense, usually located in the disk. The paper uses metallicity ([Fe/H]) and age estimates to help classify Garro 01 as possibly an open cluster despite its similarity to a globular cluster.

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This episode discusses

The paper

The Kinematics, Metallicities, and Orbits of Six Recently Discovered Galactic Star Clusters with Magellan/M2FS Spectroscopy · Read on arXiv

Andrew B. Pace, Sergey E. Koposov, Matthew G. Walker, Nelson Caldwell, Mario Mateo, Edward W. Olszewski, Ian U. Roederer, John I Bailey III, Vasily Belokurov III, Kyler Kuehn III, Ting S Li, Daniel B Zucker

McWilliams Center for Cosmology at Carnegie Mellon University · Institute for Astronomy at the University of Edinburgh · Institute of Astronomy at the University of Cambridge · Kavli Institute for Cosmology at the University of Cambridge · Harvard-Smithsonian Center for Astrophysics · Department of Astronomy at the University of Michigan, Ann Arbor, Michigan State University, Department of Physics at the University of Santa Barbara, Santa Barbara California State University, Lowell Observatory

We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters (Gran 3/Patchick 125, Gran 4, Garro 01, LP 866) and two newly discovered open clusters (Gaia 9, Gaia 10) at low Galactic latitudes. We measure line-of-sight velocities and stellar parameters ([Fe/H],, T eff, [Mg/Fe]) from high resolution spectroscopy centered on the Mg triplet and identify 20-80 members per star cluster. We determine the kinematics and chemical properties of each cluster and measure the systemic proper motion and orbital properties by utilizing Gaia astrometry. We find Gran 3 to be an old, metal-poor (mean metallicity of [Fe/H]=-1.84) globular cluster located in the Galactic bulge on a retrograde orbit. Gran 4 is an old, metal-poor ([Fe/H]=-1.84) globular cluster with a halo-like orbit that happens to be passing through the Galactic plane. The orbital properties of Gran 4 are consistent with the proposed LMS-1/Wukong and/or Helmi streams merger events. Garro 01 is metal-rich ([Fe/H]=-0.30) and on a near circular orbit in the outer disk but its classification as an open cluster or globular cluster is ambiguous.. Gaia 9 and Gaia 10 are among the most distant known open clusters at R GC about 18, 21.2 kpc and most metal-poor with [Fe/H] -0.50,-0.46 for Gaia 9 and Gaia 10, respectively. LP 866 is a nearby, metal-rich open cluster ([Fe/H] =+0.1). The discovery and confirmation of multiple star clusters in the Galactic plane shows the power of Gaia astrometry and the star cluster census remains incomplete.

DOI: 10.1093/mnras/stad2760

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "The Kinematics, Metallicities, and Orbits of Six Recently Discovered Galactic Star Clusters with Magellan/M2FS Spectroscopy".

Jocelyn: The gist We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters and two newly discovered open clusters at low Galactic latitudes to determine their kinematics,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: We just covered how this paper confirms the identities of six star clusters using M2FS spectroscopy to get detailed data on their velocities and chemical compositions. The main point is that they are not just random stellar overdensities found by Gaia DR2; they are real objects >

Jocelyn: They use the M2FS system to select members, specifically for Gran three they selected stars with line-of-sight velocities between eight two km s minus one and nine seven km s minus one > <ref:2304.06904#pg1>

Subrahmanyan: That velocity selection is a way to confirm membership because the stellar parameters they get from Teff and log g also reinforce the idea that these stars belong together as a single population >

Vera: And for Gran three specifically, the members selected from that velocity range were consistent with single stellar populations, which aligns with its description as an old, metal-poor globular cluster > <ref:2304.06904#pg1>

Jocelyn: For Garro one they found that the members had metallicities that were consistent with a near circular orbit in the outer disk and its classification remains ambiguous because of the age estimate being around four gigayears > <ref:2304.06904#pg1>

Subrahmanyan: That ambiguity is what makes it interesting for structure formation studies; we need to know if it's an open cluster or a globular cluster because that affects how we model the stellar population >

Vera: And they use Gaia astrometry to compute the orbits, and for Gran four they find an eccentric orbit with a eccentricity of about zero point six three > <ref:2304.06904#pg1>

Jocelyn: That orbit has a small pericenter around eight kiloparsecs and a large apocenter around thirty-three point nine kiloparsecs >

Subrahmanyan: Those orbital parameters for Gran four are very suggestive of those proposed LMS-one/Wukong and or Helmi streams merger events, which is what links these local objects to the larger galactic environment > <ref:2304.06904#pg1>

Vera: So, the paper's thesis is that spectroscopic confirmation and orbital modeling provide the necessary data to determine the kinematics and chemical properties of these candidates, which reveals their true nature >

Jocelyn: And it matters because it shows that Gaia astrometry, when paired with high-resolution spectroscopy from M2FS, can help us identify star clusters we might otherwise miss >

Subrahmanyan: And for the world this means we can refine our models of how these local structures evolve within the Milky Way potential, using these six objects as test cases >

Conclusion: Vera: So to wrap up on "The Kinematics, Metallicities, and Orbits of Six Recently Discovered Galactic Star Clusters with Magellan/M2FS Spectroscopy," the authors have successfully confirmed three globular clusters and three open clusters using their combined data sets.

Jocelyn: The main implication is that this combination of spectroscopic measurements and astrometry gives us a much more complete picture of the kinematics and chemistry of these nearby star cluster candidates >

Subrahmanyan: It solidifies the understanding that these objects aren't just scattered stars but have specific, traceable orbits within our galaxy, which helps us refine our models for structure formation >

Vera: For an observer listening, it means we can start to see how local groups connect to the larger galactic architecture using these detailed kinematic and chemical constraints >

Jocelyn: It really shows that the census of star clusters is still quite incomplete and that tools like Gaia are powerful but require follow-up work from telescopes like M2FS to get the full picture >

Subrahmanyan: So, this work helps us test how local clusters behave in a way that informs how we model structure formation on larger scales >

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