HST/COS Far Ultraviolet Spectroscopic Analysis of U Geminorum Following a Wide Outburst

TL;DR

This study uses HST/COS FUV spectroscopy to monitor the cooling of the white dwarf in U Geminorum after an outburst, revealing temperature decline, abundance anomalies, and phase-dependent absorption features linked to disk overflow.

Contribution

First detailed FUV spectroscopic analysis of U Geminorum's post-outburst cooling, showing temperature evolution, abundance anomalies, and phase-dependent absorption features.

Findings

White dwarf cools from 41,500 K to 36,250 K over 56 days.

Supra-solar nitrogen abundances confirm anomalous N/C ratio.

Orbital phase modulation indicates material overflowing the disk rim.

Abstract

We have used HST/COS to obtain a series of 4 FUV (915-2148A) spectroscopic observations of the prototypical dwarf nova U Geminorum during its cooling following a two-week outburst. Our FUV spectral analysis of the data indicates that the white dwarf (WD) cools from a temperature of 41,500 K, 15 days after the peak of the outburst, to 36,250 K, 56 days after the peak of the outburst, assuming a massive WD (log(g)=8.8) and a distance of 100.4 pc. These results are self-consistent with a 1.1 solar mass WD with a 5,000 km radius. The spectra show many absorption lines of but no emission features. We find supra-solar abundances of nitrogen confirming the anomalous high N/C ratio. The FUV lightcurve reveals a 5% modulation with the orbital phase, showing dips near phase 0.25 and 0.75, where the spectra exhibit an increase in the depth of some absorption lines and in particular strong absorption lines from Si, Al, and Ar. The phase dependence we observe is consistent with material overflowing the disk rim at the hot spot, reaching a maximum elevation near phase 0.75, falling back at smaller radii near phase 0.5 where it bounces off the disk surface and again rising above the disk near phase 0.25. There is a large scatter in the absorption lines' velocities, especially for the silicon lines, while the carbon lines seem to match more closely the orbital velocity of the WD. This indicates that many absorption lines are affected by- or form in- the overflowing stream material veiling the WD, making the analysis of the WD spectra more difficult.