arXiv subjects

Helmer H. Koppelman

Publications and source records attributed to Helmer H. Koppelman.

21 records · Page 2Linked to original sources

arXiv2018

One large blob and many streams frosting the nearby stellar halo in Gaia DR2

We explore the phase-space structure of nearby halo stars identified kinematically from Gaia DR2 data. We focus on their distribution in velocity and in "integrals of motion" space as well as on their photometric properties. Our sample of stars selected to be moving at a relative velocity of at least 210 km/s with respect to the Local Standard of Rest, contains an important contribution from the low rotational velocity tail of the disk(s). The $V_R$-distribution of these stars depicts a small asymmetry similar to that seen for the faster rotating thin disk stars near the Sun. We also identify a prominent, slightly retrograde "blob", which traces the metal-poor halo main sequence reported by Gaia Collaboration et al. (2018d). We also find many small clumps especially noticeable in the tails of the velocity distribution of the stars in our sample. Their HR diagrams disclose narrow sequences characteristic of simple stellar populations. This stream-frosting confirms predictions from cosmological simulations, namely that substructure is most apparent amongst the fastest moving stars, typically reflecting more recent accretion events.

astro-ph.GA↗

arXiv2018

Scratching the surface with TGAS and RAVE: disk moving groups in the Solar neighborhood

There is long tradition extending more than a century on the identification of moving groups in the Solar neighborhood. However, with the advent of large kinematic surveys, and especially of the upcoming Gaia data releases there is a need for more sophisticated and automated substructure finders. We analyze the TGASxRAVE dataset to identify moving groups in the Galactic disk. These groups of stars may then be used to map dynamical and star formation processes in the vicinity of the Sun. We use the ROCKSTAR algorithm, a "friends-of-friends"-like substructure finder in 6D phase-space, and analyze the Hertzsprung-Russell diagrams of the groups identified. We find 125 moving groups within 300 pc of the Sun, containing on average 50 stars, and with 3D velocity dispersions smaller than 10 km/s. Most of these groups were previously unknown. Our photometric analysis allows us to isolate a subsample of 30 statistically significant groups likely composed of stars that were born together.

astro-ph.GA↗

arXiv2016

The time evolution of gaps in tidal streams

We model the time evolution of gaps in tidal streams caused by the impact of a dark matter subhalo, while both orbit a spherical gravitational potential. To this end, we make use of the simple behaviour of orbits in action-angle space. A gap effectively results from the divergence of two nearby orbits whose initial phase-space separation is, for very cold thin streams, largely given by the impulse induced by the subhalo. We find that in a spherical potential the size of a gap increases linearly with time for sufficiently long timescales. We have derived an analytic expression that shows how the growth rate depends on the mass of the perturbing subhalo, its scale and its relative velocity with respect to the stream. We have verified these scalings using N-body simulations and find excellent agreement. For example, a subhalo of mass 10^8 Msun directly impacting a very cold thin stream on an inclined orbit can induce a gap that may reach a size of several tens of kpc after a few Gyr. The gap size fluctuates importantly with phase on the orbit, and it is largest close to pericentre. This indicates that it may not be fully straightforward to invert the spectrum of gaps present in a stream to recover the mass spectrum of the subhalos.

astro-ph.GA↗