The evolutionary status of the UX Orionis star RZ Piscium

Abstract

The star RZ Psc is one of the most enigmatic members of the UX Ori star family. It shows all properties that are typical for these stars (the light variability, high linear polarization in deep minima, the blueing effect) except for one: it lacks any signatures of youth. With the lithium line 6708 Å as a rough estimate for the stellar age, we show that the "lithium" age of RZ Psc lies between the age of stars in the Pleiades (∼70 Myr) and the Orion (∼10 Myr) clusters. We also roughly estimated the age of RZ Psc based on the proper motion of the star using the Tycho-2 catalog. We found that the star has escaped from its assumed birthplace near to the Galactic plane about 30−40 Myr ago. We conclude that RZ Psc is a post-UXOr star, and its sporadic eclipses are caused by material from the debris disk.

An in-depth analysis of the RZ Piscium atmosphere

“…Since our spectroscopic observations were carried out during the star's bright state (out of eclipse), the obtained value V r = −1.2 ± 0.5 km s −1 is the real radial velocity of the star. It should be noted that the precise value of V r is very important for calculations of the star's kinematic age (Grinin et al 2010;.…”

“…The refined age estimations were based on the analysis of the RZ Psc motion in the gravitational potential of Galaxy. Grinin et al (2010) and provided the value t k = 25 ± 5 Myr, which significantly exceeds protoplanetary-disk-dissipation timescale (Williams & Cieza 2011). On the other hand, the results of the RZ Psc spectroscopic monitoring clearly demonstrate the presence of the remnants of the CS gas in the nearest vicinity of the star manifested itself in the NaI resonance doublet lines .…”

“…This allowed us to use the matching between the observed and calculated gravity-sensitive MgIb profiles as the criteria for our log g choice. The strong lithium line 6708 Å is presented in the RZ Psc spectrum (Grinin et al 2010) and provides an abundance [Li/H] = 1.75 ± 0.1 relative to the Sun. Also, we determined the projected rotation velocity V sin i = 12.0 ± 0.5 km s −1 using SME package.…”

“…The strong Li 6708 Å (EW = 0.202 Å) absorption was found in the first RZ Psc spectrum obtained in November 2009. This finding allowed us to roughly estimate the star's age in the range of 10−70 Myr (Grinin et al 2010). The refined age estimations were based on the analysis of the RZ Psc motion in the gravitational potential of Galaxy.…”

Flares of accretion activity of the 20 Myr old UXOR RZ Psc

“…RZ Piscium (sp:K0 IV) is located at high Galactic latitude (b ≈ −35 • ) in isolation from the presently active star-forming regions. However, with stellar parameters T e f = 5350 ± 150K, lg g = 4.2 ± 0.2 dex, [Fe/H] = −0.3 ± 0.05 dex, and V sin i = 12 km s −1 (Potravnov et al 2014b), absorption at Li i 6708 Å (Grinin et al 2010), and X-Ray luminosity log L X /L bol ≈ −3.2 − 3.7 (Punzi et al 2018) (P18) imply that RZ Psc is a young solar-type star. Its UX Ori-type (UXOR) photometric variability (Zajtseva 1985;Kiselev et al 1991;Shakhovskoi et al 2003;Kennedy et al 2017) and bright (L IR /L bol ∼ 8%) mid-IR (λ 3µm) excess (de Wit et al 2013) indicate that the star hosts a circumstellar disc with a small (∼ 0.4 − 0.7 AU) central cavity.…”