The benefit of a multi-band high resolution spectroscopic monitoring for studying stellar transients: the NGC 300 OT2008-1 UVES spectrum as a test case

Abstract

This work advocates the benefit of high resolution spectroscopic monitoring in the study of transients (local group transients given the available collecting power). As an exemplary analysis, we focus on the optical transient NGC300OT2008-1. Searching the ESO archives, we found a low resolution (LR, FORS) and a high resolution (HR, UVES) spectrum that were separated by only one day with no changes between them. The independent analysis of the FORS and UVES spectra show that in the LR spectrum we can securely identify only a small sample of lines and miss the correct characterization of the ejecta energetics which remain at the level of speculation. In the HR data, we identify a larger sample of emission lines and analyze their profiles suggestive of a complex geometry and ejecta dynamics whose inferences are simply impossible in LR spectra. Line profile studies are not possible with LR spectra, and may lead to potentially misleading measures. The limited information available from LR data compromises the interpretation and prevents formulating realistic physical scenarios, favoring parameter fitting to oversimplified, biased, standard models. In this occasion only one epoch was available, but monitoring is fundamental to characterize the transient evolution. The new scenario derived from the analysis of the HR spectrum cannot be confirmed or dismissed lacking a HR spectroscopic monitoring. LR and HR spectra serve different but complementary purposes. With LR one does bolometric-like studies (SED, strengthening or weakening of the major transitions), while with HR one does dynamics and precise physical characterization. HR spectroscopic monitoring on different types of transient has already shown how it serves to disentangle the various components, constrain their physical parameters, the involved energy source, and derive the ejecta dynamics.