The fastest stars in the Galaxy

Abstract: We report a spectroscopic search for hypervelocity white dwarfs (WDs) that are runaways from Type Ia supernovae (SNe Ia) and related thermonuclear explosions. Candidates are selected from Gaia data with high tangential velocities and blue colors. We find six new runaways, including four stars with radial velocities (RVs) $>1000\,\rm km\,s^{-1}$ and total space velocities $\gtrsim 1300\,\rm km\,s^{-1}$. These are most likely the surviving donors from double-degenerate binaries in which the other WD exploded. The other two objects have lower minimum velocities, $\gtrsim 600\,\rm km\,s^{-1}$, and may have formed through a different mechanism, such as pure deflagration of a WD in a Type Iax supernova. The four fastest stars are hotter and smaller than the previously known "D$^6$ stars," with effective temperatures ranging from $\sim$20,000 to $\sim$130,000 K and radii of $\sim 0.02-0.10\,R_{\odot}$. Three of these have carbon-dominated atmospheres, and one has a helium-dominated atmosphere. Two stars have RVs of $-1694$ and $-2285\rm \,km\,s^{-1}$ -- the fastest systemic stellar RVs ever measured. Their inferred birth velocities, $\sim 2200-2500\,\rm km\,s^{-1}$, imply that both WDs in the progenitor binary had masses $>1.0\,M_{\odot}$. The high observed velocities suggest that a dominant fraction of the observed hypervelocity WD population comes from double-degenerate binaries whose total mass significantly exceeds the Chandrasekhar limit. However, the two nearest and faintest D$^6$ stars have the lowest velocities and masses, suggesting that observational selection effects favor rarer, higher-mass stars. A significant population of fainter low-mass runaways may still await discovery. We infer a birth rate of D$^6$ stars that is consistent with the SN Ia rate. The birth rate is poorly constrained, however, because the luminosities and lifetimes of $\rm D^6$ stars are uncertain.

• The paper identifies hypervelocity white dwarfs with radial velocities exceeding 1000 km/s as potential Type Ia supernova remnants.
• It employs Gaia astrometry and spectroscopic analysis to measure tangential and radial velocities for confirming runaway candidates.
• Findings support double-degenerate explosion models, with some stars reaching unprecedented speeds of up to 2500 km/s.

Insights from the Study on Hypervelocity White Dwarfs as Runaways from Type Ia Supernovae

The research article titled "The fastest stars in the Galaxy" by Kareem El-Badry and collaborators provides a focused investigation into the spectral and kinematic properties of hypervelocity white dwarfs (WDs), particularly those ejected as remnants from Type Ia supernovae (SNe Ia) in double-degenerate systems. Utilizing spectroscopic data from a selection of high-velocity candidates identified in the Gaia survey, the authors have identified and confirmed a subset of objects exceeding radial velocities of 1000 km/s, establishing them as among the fastest moving stars currently known within the Milky Way.

Methodology and Data Analysis

By selecting candidates exhibiting high tangential velocities and blue colors from Gaia's astrometric data, the study methodically narrows down potential hypervelocity white dwarfs. Conveniently, Gaia's precise measurements are leveraged to deduce the tangential velocities of these stars, which are further extrapolated to estimate total space velocities upon accounting for radial velocity components measured from spectroscopic observations using instruments like LRIS and MagE.

The spectroscopic analysis enables the authors to discern atmospheric compositions and temperatures implying diverse evolutionary pathways post-supernova detonation. Among the newly identified hypervelocity stars, a significant fraction exhibit carbon-dominated atmospheres, differentiating them from typical WDs and reinforcing the potential linkage to their progenitor binary's supernova activity.

Key Findings

The study's substantive outcome is the discovery of six hypervelocity WDs with radial velocities exceeding 1000 km/s. Notably:

• Two white dwarfs exhibit velocities of approximately 2200–2500 km/s, marking unprecedented systemic stellar velocities. The velocities infer a historical origin from double-degenerate binaries of masses exceeding the Chandrasekhar limit.
• Effective temperature analyses range from 20,000 K to 130,000 K, with radii constraints indicating a substantial degree of compactness for these remnants.
• A distinctive link is drawn between the obtained velocities and the likelihood of these being donor remnants where the less massive WD survives the explosion of a binary companion, reinforcing the double-detonation model of SNe Ia.

Implications and Future Direction

These findings carry critical implications for our understanding of the progenitor channels of SNe Ia. The work underscores the role of double-degenerate binaries, particularly emphasizing systems exceeding classical mass limits for CO cores. Further, the study posits that observable hypervelocity WDs primarily stem from mergers with donor masses greater than 1 M_⊙. This assertion challenges assumptions within existing white dwarf merger models often biased towards lower-mass scenarios due to observational constraints.

The research is poised to reshape our understanding of stellar dynamics post-supernova events, seen both in the implications of runaway velocities and the means by which thermonuclear supernovae impart relics into intergalactic space. As ongoing Gaia data releases provide progressively finer astrometric resolutions, subsequent endeavors can explore the properties and distribution of such runaway stars, refining birth rate estimates of D^6 stars relative to the overall SN Ia rates.

Overall, the paper serves as a strong testament to the synthesis of astrometric and spectroscopic data in unraveling the fates of stars subject to some of the universe's most energetic astrophysical phenomena. Exploration of the distinct classes such as LP 40-365 stars, alongside the D^6 candidates, invites broader inquiries into the post-supernova trajectories and enduring characteristics of such high-velocity objects. As theoretical models of binary evolution continue to advance, this work provides a cornerstone for testing the robustness of scenarios positing double-degenerate origins of thermonuclear SN Ia events.