An official website of the United States government
Two main mechanisms have classically been proposed for the formation of runaway stars. In the binary supernova scenario (BSS), a massive star in a binary explodes as a supernova, ejecting its companion. In the dynamical ejection scenario, a star is ejected during a strong dynamical encounter between multiple stars. We propose a third mechanism for the formation of runaway stars: the subcluster ejection scenario (SCES), where a subset of stars from an infalling subcluster is ejected out of the cluster via a tidal interaction with the contracting gravitational potential of the assembling cluster. We demonstrate the SCES in a star-by-star simulation of the formation of a young massive cluster from a 106M⊙gas cloud using theTORCHframework. This star cluster forms hierarchically through a sequence of subcluster mergers determined by the initial turbulent, spherical conditions of the gas. We find that these mergers drive the formation of runaway stars in our model. Late-forming subclusters fall into the central potential, where they are tidally disrupted, forming tidal tails of runaway stars that are distributed highly anisotropically. Runaways formed in the same SCES have similar ages, velocities, and ejection directions. Surveying observations, we identify several SCES candidate groups with anisotropic ejection directions. The SCES is capable of producing runaway binaries: two wide dynamical binaries in infalling subclusters were tightened through ejection. This allows for another velocity kick via subsequent via a subsequent BSS ejection. An SCES-BSS ejection is a possible avenue for the creation of hypervelocity stars unbound to the Galaxy. The SCES occurs when subcluster formation is resolved. We expect nonspherical initial gas distributions to increase the number of calculated runaway stars, bringing it closer to observed values. The observation of groups of runaway stars formed via the SCES can thus reveal the assembly history of their natal clusters.
more »
« less
This content will become publicly available on May 1, 2026
A systematic method to identify runaways from star clusters produced from single-binary interactions: A case study of M67
Context. One hypothesis for runaway stars (RSs) is that they are ejected from star clusters with high velocities relative to the cluster center-of-mass motion. There are two competing mechanisms for their production: supernova-based ejections in binaries, where one companion explodes, leaves no remnant, and launches the other companion at the instantaneous orbital velocity, and the disintegration of triples (or higher-order multiples), which produces a recoiled runaway binary (RB) and an RS. Aims. We search for RS candidates using data from the Gaia DR3 survey with a focus on triple disintegration since in this case the product is always a binary and a single star that should be moving in opposite directions. Methods. We created a systematic methodology to look for candidate RS-RB runaway pairs produced from the disintegration of bound three-body systems formed from single-binary interactions based on momentum conservation and causality. The method we use is general and can be applied to any cluster with a 5D kinematic data set. We used our criteria to search for these pairs in a 150 pc circular field of view surrounding the open cluster M67, which we used as a benchmark cluster to test the robustness of our method. Results. Our results reveal only one RS-RB pair that is consistent with all of our selection criteria out of an initial sample of 108pairs.
more »
« less
- Award ID(s):
- 2107738
- PAR ID:
- 10637569
- Publisher / Repository:
- Astronomy & Astrophysics
- Date Published:
- Journal Name:
- Astronomy & Astrophysics
- Volume:
- 697
- ISSN:
- 0004-6361
- Page Range / eLocation ID:
- A183
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract The dense environments in the cores of globular clusters (GCs) facilitate many strong dynamical encounters among stellar objects. These encounters have been shown to be capable of ejecting stars from the host GC, whereupon they become runaway stars, or hypervelocity stars (HVSs) if unbound to the galactic potential. We study high-speed stellar ejecta originating from GCs by using Monte CarloN-body models, in particular focusing on binary–single encounters involving compact objects. We pair our model-discriminated populations with observational catalogs of Milky Way GCs (MWGCs) to compose a present-day Galactic population of stellar ejecta. We find that these kinds of encounters can accelerate stars to velocities in excess of 2000 km s−1, to speeds beyond the previously predicted limits for ejecta from star-only encounters and in the same regime of Galactic center ejections. However, the same ejections can only account for 1.5%–20% of the total population of stellar runaways, and only 0.0001%–1% of HVS, with similar relative rates found for runaway white dwarfs. We also provide credible regions for ejecta from 149 MWGCs, which we hope will be useful as supplementary evidence when pairing runaway stars with origin GCs.more » « less
-
Abstract AQ Col (EC 05217-3914) is one of the first detected pulsating subdwarf B (sdB) stars and has been considered to be a single star. Photometric monitoring of AQ Col reveals a pulsation timing variation with a period of 486 days, interpreted as time delay due to reflex motion in a wide binary formed with an unseen companion with expected mass larger than 1.05M⊙. The optical spectra and color–magnitude diagram of the system suggested that the companion is not a main-sequence star but a white dwarf or neutron star. The pulsation timing variation also shows that the system has an eccentricity of 0.424, which is much larger than any known sdB long period binary system. That might be due to the existence of another short period companion to the sdB star. Two optical spectra obtained on 1996 December 5 show a radial velocity change of 49.1 km s−1in 46.1 minutes, which suggests the hot subdwarf in the wide binary is itself a close binary formed with another unseen white dwarf or neutron star companion; if further observations show this interpretation to be correct, AQ Col is an interesting triple system worthy of further study.more » « less
-
Abstract We present 18 yr of OGLE photometry together with spectra obtained over 12 yr revealing that the early Oe star AzV 493 shows strong photometric (ΔI< 1.2 mag) and spectroscopic variability with a dominant, 14.6 yr pattern and ∼40 day oscillations. We estimate the stellar parametersTeff= 42,000 K, ,M/M⊙= 50 ± 9, andvsini= 370 ± 40 km s−1. Direct spectroscopic evidence shows episodes of both gas ejection and infall. There is no X-ray detection, and it is likely a runaway star. The star AzV 493 may have an unseen companion on a highly eccentric (e> 0.93) orbit. We propose that close interaction at periastron excites ejection of the decretion disk, whose variable emission-line spectrum suggests separate inner and outer components, with an optically thick outer component obscuring both the stellar photosphere and the emission-line spectrum of the inner disk at early phases in the photometric cycle. It is plausible that AzV 493’s mass and rotation have been enhanced by binary interaction followed by the core-collapse supernova explosion of the companion, which now could be either a black hole or a neutron star. This system in the Small Magellanic Cloud can potentially shed light on OBe decretion disk formation and evolution, massive binary evolution, and compact binary progenitors.more » « less
-
Abstract We search for mass segregation in the intermediate-aged open cluster NGC 6819 within a carefully identified sample of probable cluster members. Using photometry from Gaia, Pan-STARRS, and the Two Micron All Sky Survey as inputs for a Bayesian statistics software suite, BASE-9, we identify a rich population of (photometric) binaries and derive posterior distributions for the cluster age, distance, metallicity, and reddening, as well as star-by-star photometric membership probabilities, masses, and mass ratios (for binaries). Within our entire sample, we identify 2632 cluster members and 777 binaries. We then select a main-sequence “primary sample” with 14.85 <G< 19.5, containing 1342 cluster members and 250 binaries with mass ratiosq> 0.5, to investigate mass segregation. Within this primary sample, we find the binary radial distribution is significantly shifted toward the cluster center as compared to the single stars, resulting in a binary fraction that increases significantly toward the cluster core. Furthermore, we find that within the binary sample, more massive binaries have more centrally concentrated radial distributions than less massive binaries. The same is true for the single stars. We verify the expectation of mass segregation for this stellar sample in NGC 6819 through both relaxation time arguments and by investigating a sophisticatedN-body model of the cluster. Importantly, this is the first study to investigate mass segregation of the binaries in the open cluster NGC 6819.more » « less
