Category Archives: Uncategorized

IAU Symposium 353 Conference Proceedings are now published!

After months of editing and proof-reading, the proceedings of the IAU Symposium  that I co-organized in Shanghai in July 2019, have  finally been published. It was a great honor to edit them with Jerry Sellwood, who was a postdoc advisor when I was at Rutgers. The Symposium was organized by his former student Juntai Shen and I to celebrate Jerry’s 70th birthday and was an amazing conference.

The electronic version is here. I’m supposed to get a hard copy sometime. It’s probably sitting in a mailroom somewhere and will  continue to gather dust until we are out of COVID-19 purgatory! After which I will look at it for 5 minutes and it continue to will gather more dust on my book shelf. Sigh…

Hypervelocity star LAMOST-HVS1 was ejected from the Milky Way disk!

LAMOST HVS1 is a hypervelocity star which was originally thought to have been ejected from the Galactic Center by an interaction with the supermassive black hole. However, Gaia data for its proper motion indicates that it was ejected from the disk. The blue dots in the top panel show where this object was probably ejected from. The colored curves mark the locations of various spiral arms. It looks like this star was ejected with a velocity of > 560km/s from a massive star cluster in the Norma Spiral arm (purple curve) about 38Million years ago. This has important implications for other claimed Hypervelocity stars – which could be ejected from multiple-star interactions in clusters rather than by massive black holes! The paper is here:

Remnants of eight possible past accretion events in the halo found in Gaia data

Ian Roederer led this study of the kinematics (using Gaia proper motions) of some 30+  fields stars in the stellar halo which were selected because they were highly  r-process enhanced. Using Gaia kinematics we computed their orbits and their actions (which are quantities like angular momentum that are conserved). By looking for clustering in action space we identified 8 possible clusters with at least 3 stars. Interestingly the stars in each cluster have a much smaller spread in iron abundance relative to the spread of the whole sample implying that they could actually be signaling the mass of the satellite they came from. Using the standard mass-metallaicity relation this implies that these 8 possible satellites were of similar mass to known satellites. The paper is here:….156..179R

Old, Metal Poor, Extreme Velocity Stars in Gaia

Within three days of the Gaia DR2 data release Kohei Hattori discovered a population of extreme velocity stars – which at first sight appeared to be above previous estimates of the escape velocity (red dots). But these stars are unlike any other “hyper velocity stars” they are old and metal poor and much more like regular old halo stars. We looked into various ways to accelerate such a significant population of stars, and concluded that despite their high velocities, these stars must actually be bound to the Milky Way. This implies a total Milky Way mass about twice as large as previous estimates. Soon after two other papers used different Gaia tracers to come to the same conclusion. Our paper is here:…866..121H

Tangential anisotropy in the stellar halo as a signature of tidal interactions

Sarah Loebman found from her analysis of three different suites of simulations that the velocity anisotropy of the stellar halo is expected to be very radial – except when there has been some sort of tidal interaction. This might be a new and interesting way to find recent evidence of mergers in Gaia data. The ApJ paper is here…853..196L

Orbits that make the Box/Peanut X in the Milky Way bar

Our paper on the orbits that make up the X-shape in the Milky Way’s bar by my former undergraduate Caleb Abbott has just been accepted for publication in MNRAS. We show that box orbits and  their resonant cousins make the X-shape. Over 20% of the mass of the bar is associated with the X and most of the orbits (>70%) in the bar contribute to the Box/Peanut/X.

A preprint of this paper is available at:

Reliability of velocity anisotropy measurements in the Milky Way’s stellar halo

Kohei Hattori’s masterful paper on the reliability of velocity anisotropy measurements in the Milky Way’s stellar halo has just been accepted for publication in the Astrophysical Journal. The main conclusion is that using line-of-sight velocities to measure the velocity anisotropy profile beyond ~15 kpc from the Galactic center is extremely difficult. It’s a good thing that we will soon have Gaia!

The preprint is available at: