HST/STIS analysis of the first main sequence pulsar CU Virginis

Abstract

Context. CU Vir has been the first main sequence star that showed regular radio pulses that persist for decades, resembling the radio lighthouse of pulsars and interpreted as auroral radio emission similar to that found in planets. The star belongs to a rare group of magnetic chemically peculiar stars with variable rotational period. Aims. We study the ultraviolet (UV) spectrum of CU Vir obtained using STIS spectrograph onboard the Hubble Space Telescope (HST) to search for the source of radio emission and to test the model of the rotational period evolution. Methods. We used our own far-UV and visual photometric observations supplemented with the archival data to improve the parameters of the quasisinusoidal long-term variations of the rotational period. We predict the flux variations of CU Vir from surface abundance maps and compare these variations with UV flux distribution. We searched for wind, auroral, and interstellar lines in the spectra. Results. The UV and visual light curves display the same long-term period variations supporting their common origin. New updated abundance maps provide better agreement with the observed flux distribution. The upper limit of the wind mass-loss rate is about 10 −12 M ⊙ yr −1 . We do not find any auroral lines. We find rotationally modulated variability of interstellar lines, which is most likely of instrumental origin. Conclusions. Our analysis supports the flux redistribution from far-UV to near-UV and visual domains originating in surface abundance spots as the main cause of the flux variability in chemically peculiar stars. Therefore, UV and optical variations are related and the structures leading to these variations are rigidly confined to the stellar surface. The radio emission of CU Vir is most likely powered by a very weak presumably purely metallic wind, which leaves no imprint in spectra.

Chandra X-ray study confirms that the magnetic standard Ap star KQ Vel hosts a neutron star companion

“…The wind mass-loss rate from an A0 star is very low (Babel 1996). An upper limit of 10 −12 M yr −1 has been determined for CU Vir (Krtička et al 2019), which has the same spectral type as KQ Vel. Based on spindown considerations and if Eq.…”

“…The MCWS model requires the presence of radiatively driven stellar winds. Very little is known about the winds of nonsupergiant A stars (Babel 1996;Krtička et al 2019). It is clear, however, that the winds of these stars are very weak.…”

Evidence for radio and X-ray auroral emissions from the magnetic B-type star ρ Oph A

“…Stellar ARE from early-type magnetic stars was detected as wide band strongly circularly polarized radio pulses arising from auroral rings above the magnetic poles (Trigilio et al 2011). While the auroral signature in the UV spectrum of the prototypical star CU Vir was not found (Krtička et al 2019), the search in X-rays gives promising results (Robrade et al 2018).…”

A scaling relationship for non-thermal radio emission from ordered magnetospheres: from the top of the Main Sequence to planets

“…The discrepancy is especially large at the lowest T eff corresponding to Ap stars. For instance, for a template A0p star, CU Vir (HD 124224), Krtička et al (2019) derive the upper limit Ṁ = 10 −12 M ⊙ yr −1 , which is orders of magnitude lower than that required by our radio spectra models.…”

“…Ultimately, the expected mass loss is still lower than the value required for the radio modeling. Independent evidence for a mass-loss inconsistency at these spectral types comes from HST observations of the wind sensitive UV lines of the early A-type star HD 124224 (Krtička et al 2019). Those data allow for only an upper limit to the wind mass loss, a limit that is discrepant with that required to explain the observed radio spectrum.…”