Probing the Local Bubble with diffuse interstellar bands

I. Project overview and southern hemisphere survey^⋆

¹ Lennard-Jones Laboratories, Keele University, ST5 5BG, UK
e-mail: mandybailey22@gmail.com
² Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, Liverpool L3 5RF, UK
³ School of Astronomy, Institute for Research in Fundamental Sciences (IPM), PO Box 19395-5746, Tehran, Iran
⁴ School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD, UK
⁵ Royal Astronomical Society, Burlington House, Piccadilly, London W1J 0BQ, UK

Received: 2 June 2015
Accepted: 26 September 2015

Abstract

Context. The Sun traverses a low-density, hot entity called the Local Bubble. Despite its relevance to life on Earth, the conditions in the Local Bubble and its exact configuration are not very well known. Besides that, there is some unknown interstellar substance that causes a host of absorption bands across the optical spectrum, called diffuse interstellar bands (DIBs).

Aims. We have started a project to chart the Local Bubble in a novel way and learn more about the carriers of the DIBs, by using DIBs as tracers of diffuse gas and environmental conditions.

Methods. We conducted a high signal-to-noise spectroscopic survey of 670 nearby early-type stars to map DIB absorption in and around the Local Bubble. The project started with a southern hemisphere survey conducted at the European Southern Observatory’s New Technology Telescope and has since been extended to an all-sky survey using the Isaac Newton Telescope.

Results. In this first paper in the series, we introduce the overall project and present the results from the southern hemisphere survey. We make available a catalogue of equivalent-width measurements of the DIBs at 5780, 5797, 5850, 6196, 6203, 6270, 6283, and 6614 Å, of the interstellar Na i D lines at 5890 and 5896 Å, and of the stellar He i line at 5876 Å. We find that the 5780 Å DIB is relatively strong throughout, as compared to the 5797 Å DIB, but especially within the Local Bubble and at the interface with a more neutral medium. The 6203 Å DIB shows similar behaviour with respect to the 6196 Å DIB. Some nearby stars show surprisingly strong DIBs, whereas some distant stars show very weak DIBs, indicating small-scale structure within, as well as outside, the Local Bubble. The sight lines with non-detections trace the extent of the Local Bubble especially clearly and show it opening out into the halo.

Conclusions. The Local Bubble has a wall that is in contact with hot gas and/or a harsh interstellar radiation field. That wall is perforated, though, causing leakage of radiation and possibly hot gas. On the other hand, compact self-shielded cloudlets are present much closer to the Sun, probably within the Local Bubble itself. As for the carriers of the DIBs, our observations confirm the notion that these are large molecules whose differences in behaviour are mainly governed by their differing resilience and/or electrical charge, with more subtle differences possibly related to varying excitation.