# Absolute Nuv magnitudes of Gaia DR1 astrometric stars and a search for hot companions in nearby systems

## TL;DR

This study combines Gaia DR1 parallaxes with GALEX Nuv magnitudes to create an ultraviolet HR diagram, identifying stars with Nuv excess that may indicate hidden white dwarfs, young stars, or binaries.

## Contribution

It provides an empirical lower envelope fit for the main sequence in the Nuv band and identifies stars with significant Nuv excess, including potential hidden companions.

## Key findings

- Identified 50 nearby stars with Nuv excess, mainly late K and early M dwarfs.
- Presented a collection of 40 stars with bluer Nuv-G colors, including known binaries and unexplored objects.
- Revealed potential candidates for hidden white dwarfs or tight interacting binaries.

## Abstract

Accurate parallaxes from Gaia DR1 (TGAS) are combined with GALEX visual Nuv magnitudes to produce absolute Mnuv magnitudes and an ultraviolet HR diagram for a large sample of astrometric stars. A functional fit is derived of the lower envelope main sequence of the nearest 1403 stars (distance < 40 pc), which should be reddening-free. Using this empirical fit, 50 nearby stars are selected with significant Nuv excess. These are predominantly late K and early M dwarfs, often associated with X-ray sources, and showing other manifestations of magnetic activity. The sample may include systems with hidden white dwarfs, stars younger than the Pleiades, or, most likely, tight interacting binaries of the BY Dra-type. A separate collection of 40 stars with precise trigonometric parallaxes and Nuv - G colors bluer than 2 mag is presented. It includes several known novae, white dwarfs, and binaries with hot subdwarf (sdOB) components, but most remain unexplored.