Marina Kounkel

Vanderbilt University, Postdoctoral Scholar





Recent major works:

Untangling the Galaxy I: Local Structure and Star Formation History of the Milky Way

Kounkel, M. & Covey, K.; 2019, AJ, accepted

Link to the full paper

Interactive 3d figures

Abstract: Gaia DR2 provides unprecedented precision in measurements of the distance and kinematics of stars in the solar neighborhood. Through applying unsupervised machine learning on DR2's 5-dimensional dataset (3d position + 2d velocity), we identify a number of clusters, associations, and co-moving groups within 1 kpc and |b|<30 deg (many of which have not been previously known). We estimate their ages with the precision of ∼0.15 dex. Many of these groups appear to be filamentary or string-like, oriented in parallel to the Galactic plane, and some span hundreds of pc in length. Most of these string lack a central cluster, indicating that their filamentary structure is primordial, rather than the result of tidal stripping or dynamical processing. The youngest strings (<100 Myr) are orthogonal to the Local Arm. The older ones appear to be remnants of several other arm-like structures that cannot be presently traced by dust and gas. The velocity dispersion measured from the ensemble of groups and strings increase with age, suggesting a timescale for dynamical heating of ∼300 Myr. This timescale is also consistent with the age at which the population of strings begins to decline, while the population in more compact groups continues to increase, suggesting that dynamical processes are disrupting the weakly bound string populations, leaving only individual clusters to be identified at the oldest ages. These data shed a new light on the local galactic structure and a large scale cloud collapse.

Caption: 3-dimensional map of the exened solar neighborhood. Structures are color-coded according to their age, from youngest (red) to oldest (purple). Thin lines show the apparent kinematics proected over 5 Myr, without accounting for the galactic dynamics.
Caption: A ratio of the multiplicity fraction derived from the young stars observed with APOGEE relative to the field. Values above 1 show an excess of companions in young stars, less than 1 show the deficit. Each bin shows the derived multiplicity fraction with all the periods up to the one that is specified. The red line is derived from the RV variables, the black line is derived from SB2s.

Close Companions around Young Stars

Kounkel, M.; Covey, K.; Moe, M.; Kratter, K. M.; Suárez, G.; Stassun, K. G.; Román-Zúñiga, C.; Hernandez, J.; Kim, J. S.; Peña Ramírez, K.; Roman-Lopes, A.; Stringfellow, G. S.; Jaehnig, K. O.; Borissova, J.; Tofflemire, B.; Krolikowski, D.; Rizzuto, A.; Kraus, A.; Badenes, C.; Longa-Peña, P.; Gómez Maqueo Chew, Y.; Barba, R.; Nidever, D. L.; Brown, C.; De Lee, N.; Pan, K.; Bizyaev, D.; Oravetz, D.; Oravetz, A.; 2019, AJ, 157, 196

Link to the full paper

Abstract: Multiplicity is a fundamental property that is set early during stellar lifetimes, and it is a stringent probe of the physics of star formation. The distribution of close companions around young stars is still poorly constrained by observations. We present an analysis of stellar multiplicity derived from Apache Point Observatory Galactic Evolution Experiment-2 spectra obtained in targeted observations of nearby star-forming regions. This is the largest homogeneously observed sample of high-resolution spectra of young stars. We developed an autonomous method to identify double-lined spectroscopic binaries (SB2s). Out of 5007 sources spanning the mass range of ∼0.05–1.5 M ⊙, we find 399 binaries, including both radial velocity (RV) variables and SB2s. The mass ratio distribution of SB2s is consistent with being uniform for q < 0.95 with an excess of twins for q > 0.95. The period distribution is consistent with what has been observed in close binaries (<10 au) in the evolved populations. Three systems are found to have q ∼ 0.1, with a companion located within the brown dwarf desert. There are no strong trends in the multiplicity fraction as a function of cluster age from 1 to 100 Myr. There is a weak dependence on stellar density, with companions being most numerous at Σ* ∼ 30 stars/pc‑2 and decreasing in more diffuse regions. Finally, disk-bearing sources are deficient in SB2s (but not RV variables) by a factor of ∼2 this deficit is recovered by the systems without disks. This may indicate a quick dispersal of disk material in short-period equal-mass systems that is less effective in binaries with lower q.


The APOGEE-2 Survey of the Orion Star-forming Complex. II. Six-dimensional Structure

Kounkel, M.; Covey, K.; Suárez, G.; Román-Zúñiga, C.; Hernandez, J.; Stassun, K.; Jaehnig, K.; Feigelson, E.; Peña Ramírez, K.; Roman-Lopes, A.; Stringfellow, G.; Da Rio, N.; Kim, S.; Borissova, J.; Fernández-Trincado, J.; Burgasser, A.; García-Hernández, D. A.; Zamora, O.; Pan, K.; Nitschelm, C.; 2018, AJ, 156, 84

Link to the full paper

Interactive 3d figures

Contributed talk at Cool Stars 20

Abstract: We present an analysis of spectroscopic and astrometric data from APOGEE-2 and Gaia DR2 to identify structures toward the Orion Complex. By applying a hierarchical clustering algorithm to the six-dimensional stellar data, we identify spatially and/or kinematically distinct groups of young stellar objects with ages ranging from 1 to 12 Myr. We also investigate the star-forming history within the Orion Complex and identify peculiar subclusters. With this method we reconstruct the older populations in the regions that are currently largely devoid of molecular gas, such as Orion C (which includes the σ Ori cluster) and Orion D (the population that traces Ori OB1a, OB1b, and Orion X). We report on the distances, kinematics, and ages of the groups within the Complex. The Orion D group is in the process of expanding. On the other hand, Orion B is still in the process of contraction. In λ Ori the proper motions are consistent with a radial expansion due to an explosion from a supernova; the traceback age from the expansion exceeds the age of the youngest stars formed near the outer edges of the region, and their formation would have been triggered when they were halfway from the cluster center to their current positions. We also present a comparison between the parallax and proper-motion solutions obtained by Gaia DR2 and those obtained toward star-forming regions by the Very Long Baseline Array.

Caption: 3-dimensional map of the Orion Complex. Green shows Orion A, orange is Orion B, cyan is Orion C, red is Orion D, and blue is λ Ori.

All publications



Marina.Kounkel (at)

Department of Physics and Astronomy
6301 Stevenson Center
Nashville, TN 37240