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The Kinematic Properties of TŻO Candidate HV 11417 with Gaia DR3

Anna J. G. O'Grady

TL;DR

This study uses Gaia DR3 to reassess HV 11417, a Thorne-Żytkow Object (TŻO) candidate in the Small Magellanic Cloud (SMC). It confirms SMC membership with refined proper motions and computes HV 11417's local transverse velocity as $v_{\mathrm{loc}}=52\pm15$ km s$^{-1}$, qualifying it as a runaway star. The analysis shows HV 11417's kinematics are similar to HV 2112 and fall within the broad distribution of runaways among OB, RSG, and O-rich AGB populations, suggesting a kinematically disruptive history but not definitively diagnosing its nature as a TŻO. The results indicate that a runaway velocity alone cannot confirm a TŻO identity, especially given recent competing models, and highlight Gaia DR3's value for population-level kinematic studies in the SMC and the need for further observational constraints.

Abstract

HV 11417 is a candidate Thorne-Żytkow Object, a red supergiant with a neutron star core, located within the Small Magellanic Cloud (SMC). Previous studies have questioned, using Gaia DR2 data, whether HV 11417 was truly located at the distance of the SMC or was instead a foreground star. However, the proper motion measurement uncertainties for HV 11417 in DR2 were high. In this work, we use Gaia DR3 data to show that HV 11417 is very likely to be a true member of the SMC. We further analyze the kinematics of HV 11417 relative to its local environment, and compare it to populations of massive and evolved stars in the SMC. We find HV 11417 has a local transverse velocity of $52\pm15$ km/s, and thus qualifies as a runaway star (v$_\mathrm{loc}\geq$ 30 km/s). This runaway classification does not conclusively prove its nature as a TŻO, particularly given results from recent TŻO models, but does indicate that HV 11417 experienced a kinematic disruption in its evolution.

The Kinematic Properties of TŻO Candidate HV 11417 with Gaia DR3

TL;DR

This study uses Gaia DR3 to reassess HV 11417, a Thorne-Żytkow Object (TŻO) candidate in the Small Magellanic Cloud (SMC). It confirms SMC membership with refined proper motions and computes HV 11417's local transverse velocity as km s, qualifying it as a runaway star. The analysis shows HV 11417's kinematics are similar to HV 2112 and fall within the broad distribution of runaways among OB, RSG, and O-rich AGB populations, suggesting a kinematically disruptive history but not definitively diagnosing its nature as a TŻO. The results indicate that a runaway velocity alone cannot confirm a TŻO identity, especially given recent competing models, and highlight Gaia DR3's value for population-level kinematic studies in the SMC and the need for further observational constraints.

Abstract

HV 11417 is a candidate Thorne-Żytkow Object, a red supergiant with a neutron star core, located within the Small Magellanic Cloud (SMC). Previous studies have questioned, using Gaia DR2 data, whether HV 11417 was truly located at the distance of the SMC or was instead a foreground star. However, the proper motion measurement uncertainties for HV 11417 in DR2 were high. In this work, we use Gaia DR3 data to show that HV 11417 is very likely to be a true member of the SMC. We further analyze the kinematics of HV 11417 relative to its local environment, and compare it to populations of massive and evolved stars in the SMC. We find HV 11417 has a local transverse velocity of km/s, and thus qualifies as a runaway star (v 30 km/s). This runaway classification does not conclusively prove its nature as a TŻO, particularly given results from recent TŻO models, but does indicate that HV 11417 experienced a kinematic disruption in its evolution.

Paper Structure

This paper contains 9 sections, 1 equation, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Confirming SMC Membership
  3. Kinematic Properties Relative to Local Environment
  4. Local proper motion & transverse velocity of HV 11417
  5. Comparison to other stars
  6. Discussion
  7. Kinematic properties of HV 11417
  8. Implications for classification
  9. Conclusion

Figures (2)

  • Figure 1: The two dimensional proper motions of the local environments of HV 11417 (panel a) and comparison stars (panels b-f). The star of interest is a large gold circle, and all stars within 5' that are likely SMC sources are small blue circles, where darker blue indicates a higher density. A white cross indicates the mean proper motion for each group. Contours indicate the 1-, 2-, and 3-$\sigma$ densities of the local stars. The mean proper motion of the SMC has been subtracted, and the star of interest is not included in the local mean or density contour determination.
  • Figure 2: The tangential proper motion (left axis) and transverse velocity at the distance of the SMC (right axis) of HV 11417 and comparison stars, shown as gold circles with black error bars. The segmented blue shaded bars indicate the 50%, 75%, 90%, and 95% percentiles of the tangential proper motion cumulative density function of all likely SMC stars within 5' of each star of interest. The mean proper motion of the SMC and the weighted mean of each locale has been subtracted, allowing the proper motion of each star of interest to be compared to the average dispersion of their local environment.