LP 543-25: A Rare Low-mass Runaway Disk Star

LP 543-25 or PSS 544-7 is a high proper-motion star, which was discovered by Humphreys et al. (1991) during a survey with the 2.1 m telescope at Kitt Peak National Observatory. LP 543-25 attracted some attention due to its high tangential velocity (de la Fuente Marcos & de la Fuente Marcos 2005; Lépine & Shara 2005); de la Fuente Marcos & de la Fuente Marcos (2005) argued that given its location and kinematic signature, LP 543-25 could be a candidate cannonball star ejected by a star cluster. However, the input data used in the original analysis by de la Fuente Marcos & de la Fuente Marcos (2005) were rather uncertain and so were the results: a tangential velocity of 399 ± 127 km s⁻¹ and a W-component >350 km s⁻¹. Here, we revisit the issue of the kinematics of this interesting star using Gaia DR2.

Gaia DR2 (Gaia Collaboration et al. 2016, 2018b) provides the necessary, high-quality data—right ascension and declination, stellar parallax, radial velocity, proper motions in right ascension and declination, and their respective standard errors—to investigate the kinematics of LP 543-25, which is source Gaia DR2 3137866242753594624. Such an input data set can be used to compute Galactic space velocities and their uncertainties following the procedure outlined by Johnson & Soderblom (1987). Here, we consider values of the Solar motion from Schönrich et al. (2010) and use a right-handed coordinate system for U, V, and W; axes are positive in the directions of the Galactic Center, Galactic rotation, and the North Galactic Pole.

Gaia DR2 does not list a value for the radial velocity of LP 543-25, therefore we will assume that its radial motion is negligible in comparison with its tangential one; its distance is ${458}_{-42}^{+51}$ pc. Computed as described above, the Heliocentric Galactic velocity components are U = 206 ± 21 km s⁻¹, V = −289 ± 29 km s⁻¹, and W = 30 ± 30 km s⁻¹ (Figure 1, left-hand side panels); the corresponding Galactocentric Galactic velocity components are U = −195 ± 21 km s⁻¹, V = −34 ± 29 km s⁻¹, and W = 37 ± 30 km s⁻¹. The star is therefore moving in the Galactic plane and away from the Galactic Center at a rate of nearly 200 km s⁻¹, i.e., it is bound to the Galaxy. With such kinematic properties it is not unreasonable to speculate that LP 543-25 may come from any of the multiple star clusters that inhabit the inner regions of the Milky Way. Leonard & Duncan (1990) showed numerically that star clusters can produce dynamically ejected runaway stars with a maximum velocity close to or above 200 km s⁻¹. A preliminary analysis of the photometric data of this star provided by Gaia DR2 (Figure 1, right-hand side panel) confirms the conclusions in de la Fuente Marcos & de la Fuente Marcos (2005): LP 543-25 could be a late (Population I) K dwarf/early M dwarf or a Population II dwarf/subdwarf.

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LP 543-25 appears to be a member of an elusive class of stars, the low-mass runaway stars (see e.g., Poveda et al. 2005; Hawkins et al. 2015). Poveda et al. (2005) discussed three low-mass runaway stars from the Orion Nebula, but Gaia DR2 does not provide data on any of these objects. The catalog of young runaway stars compiled by Tetzlaff et al. (2011) includes six stars—BD-11 569, BD-15 1776, BD+41 3931, LHS 3199, LHS 3340, and HD 149414—with probable masses under 1 M_⊙ and tangential velocity above 100 km s⁻¹. These six stars have data in Gaia DR2 and they are plotted in Figure 1. Using these well-studied runaway stars, it is possible to confirm that LP 543-25 is indeed a low-mass runaway star, perhaps one of the closest and less massive runaway stars identified so far. Although certainly promising, these results must be considered with some caution, spectroscopic information, including radial velocity determination, is required in order to constrain better the overall properties of this interesting star.