It has been known for over 40 years that our Sun sits in a region of unusually low gas density, which we call the Local Bubble (LB). The LB extends for ∼ 100 pc in the plane of the Galaxy and for hundreds of parsecs vertically, presumably as far as the Galactic Halo (Welsh et al. 2010). The presence of the LB was initially indicated by the very low values of interstellar extinction measurements of stars within 100 pc of the Sun (Welsh & Shelton 2009).
The origin is probably the result of one or more supernovae and the first evidence for the LB was provided by the observation of a diffuse soft-X-ray background by the ROSAT satellite (Lallement 2004). Initially this was interpreted as a scenario in which the Sun was surrounded by a region of hot (∼ 106
K) gas. However, such a volume of hot gas should emit copious energy in EUV emission lines, which were searched for with the CHIPS satellite but not detected (Sasseen, Hurwitz & Team 2004). Hot gas should also strongly absorb in the O vi 1032˚A line, which is far weaker towards nearby sources than expected (Barstow et al. 2010; Cox 2005). To date this conundrum has not been solved. It is clear there is a hot (∼ 106
K) gas in the region of the Sun, it seems likely there will be a hot gas at the edges of the LB where the walls of the LB have formed creating an interface between the hot gas and the warm/cool neutral gas of the LB walls. How the temperature of the ISM varies between the Sun and the walls of the LB is simply not known (private discussion with Rosine Lallement at the IAU Symposium IAUS 297: The diffuse interstellar bands, Leiden, May 2013).
ionized atoms or molecules. This poses a problem with understanding the nature of the gas within the LB. The temperature is high in the region of the Sun and it may be as hot throughout the whole of the cavity of the LB. If that is the case, then the neutral or singly-ionized atoms or molecules are simply not expected to survive. To overcome this problem it is necessary to probe the LB using species which may survive under these conditions. It was shown by van Loon et al. (2009) that Diffuse Interstellar Bands (DIBs) are seen in the relatively harsh environments such as the Disc-Halo interface, the high λ5780/λ5797 ratio indicating the existence of interfaces between cool/warm and hot gas.
Welsh et al. (2010) created 3-D gas density maps of Na i and Ca ii interstellar absorption with in 300 pc of the sun. With the Na i maps Welsh et al. (2010) revealed a central region of very low neutral gas absorption out to a distance of about 80 pc in most Galactic directions, thereby tracing the well known Local cavity region with the wall of neutral Na i gas beyond the cavity. However, their Ca ii maps did not show any sharp increase in absorption at a distance of 80 pc. Instead there was a slowly increasing value of Ca ii equivalent width with increasing sight-line distance. Figure 3.1 shows the Na i maps created by Welsh et al. (2010).
In Figure 3.1 the white to dark shading represents low to high values of the Na i volume density. The triangles represent the sight-line positions of stars used to produce the map with the size of the triangle being proportional to the derived Na i column density. Stars plotted with vertex upwards are located above the Galactic Plane and stars plotted with vertex downwards are located below the Galactic Plane. The regions of these maps with a matrix of dots represent areas of uncertain neutral gas density measurements.
I used targets from the Na i D Local Bubble survey of Lallement et al. (2003), as the catalogue of Welsh et al. (2010) was not publicly available at the time, to make an all-sky survey and use DIBs to probe the LB to try to reveal some structure that was not traced by the Na i and Ca ii interstellar absorption maps created by Welsh et al. (2010). As well as probing many different sight-lines, some targets, which where in a similar direction but varying distances from the Sun were observed to try and reveal
Figure 3.1: Plot of 3-D spatial distribution of interstellar Na i absorption in the (Top:) Galactic Plane projection and (Bottom:) the Meridian Plane projection. Distances are in parsecs and the symbols are described in the text. Credit Welsh et al. (2010).
a change in the environment along a particular sight-line and so reveal the small scale structure of the ISM.
Observations at the NTT
For this project I used the EFOSC2 instrument at the NTT as it is an efficient low- /medium-resolution spectrograph, which when used with Grism 20 and a 0.3′′
slit width was well suited to my required spectral resolution. The location of the NTT at La Silla allowed me to observe stars in both the Northern and Southern hemispheres which is essential for mapping the Local Bubble. I observed early type stars which are bright at optical wavelengths and so provide a relatively clean continuum against which interstellar features stand out. The aim was to take a large sample of high signal-to- noise measurements of the equivalent widths of strong, highly-diagnostic DIBs (5780, 5797, 6196 and 6614 ˚A) towards nearby stars in and surrounding the Local Bubble. The DIBs are used to probe the interaction region (the wall of the LB) between the hot gas in the bubble with its cooler surroundings as well as looking for neutral structures located within the bubble itself. Our target stars have well known high proper motions so this survey also offers a first point of reference suitable for a systematic investigation of time-varying interstellar absorption. This will probe the ISM on scales of a dozen au. The observed stars are listed in Table B.1 in Appendix B on the accompanying CD. The distribution of the targets is shown in Figure 3.2.
There are four sizes of symbol on the map. The largest symbol represents targets that are within 50 parsecs of the Sun; the next size down represents targets that are between 50 and 100 parsecs away; the third size represents targets that are between 100 and 200 parsecs away while the smallest symbol represents targets that are over 200 parsecs away from the Sun. The targets are plotted using their right ascension (RA) and declination (Dec) coordinates. Two large regions of the map and two smaller regions are sparsely populated with targets. The larger regions have an RA of 0–8 hours with Dec below −25◦
and an RA of 18–24 hours with Dec below −45◦
. The smaller regions have an RA of 7–13 hours with Dec above −25◦
and an RA of 8–11 hours with Dec between −26◦
and −40◦
Figure 3.2: Locations of the observed target stars in RA and Dec. Distance is repre- sented by the size of the symbol as given in the legend.
to fog in August 2011 and the restrictions due to cloud coverage and high wind in August 2011 and 2012. The restrictions meant that either part of the sky could not be observed or longer exposure times were needed so fewer targets could be observed during the night. The targets had been chosen to cover the whole sky for the survey and also to overlap with targets at different distances but in similar sight-lines to probe the change along the line-of-sight with distance. Some targets of similar distances were chosen to probe the small scale structure of the ISM in those regions. With the loss of observing time priority was given to choosing targets to populate the sky for the all sky survey over those that might show small scale structure, either with depth or in the same neighbourhood. This was to ensure some view of the whole sky could be obtained and areas of interest, as identified by strong DIB absorption lines, could be marked out for priority observations in the future. A Northern extension to the survey is being carried out at the Isaac Newton Telescope (INT) on La Palma, Canary Islands in a collaboration with the Institute for Research in Fundamental Sciences (IPM) in Tehran, Iran.