A three-dimensional map of the hot Local Bubble using diffuse interstellar bands

TLDR: In this article, the authors report the three-dimensional structure of the Local Bubble using two different Diffuse Interstellar Bands (DIB) tracers and reveal that DIB carriers are present within the Bubble.

Abstract: The Solar System is located within a low-density cavity, known as the Local Bubble, which appears to be filled with an X-ray emitting gas at a temperature of 10$^6$ K. Such conditions are too harsh for typical interstellar atoms and molecules to survive. There exists an enigmatic tracer of interstellar gas, known as Diffuse Interstellar Bands (DIB), which often appears as absorption features in stellar spectra. The carriers of these bands remain largely unidentified. Here we report the three-dimensional structure of the Local Bubble using two different DIB tracers ($\lambda$5780 and $\lambda$5797), which reveals that DIB carriers are present within the Bubble. The map shows low ratios of $\lambda$5797/$\lambda$5780 inside the Bubble compared to the outside. This finding proves that the carrier of the $\lambda$5780 DIB can withstand X-ray photo-dissociation and sputtering by fast ions, where the carrier of the $\lambda$5797 DIB succumbs. This would mean that DIB carriers can be more stable than hitherto thought and that the carrier of the $\lambda$5780 DIB must be larger than that of the $\lambda$5797 DIB. Alternatively, small-scale denser (and cooler) structures that shield some of the DIB carriers must be prevalent within the Bubble, implying that such structures may be an intrinsic feature of supernova-driven bubbles.

Figures

Figure 3: ζ and σ cloud distribution within the GP. The ratio of λ5797/λ5780 DIB equivalent width is thought to probe the UV radiation field. The σ sightlines, where have W (5797)/W (5780) < 0.3, sample regions with high UV intensity as the λ5797 DIB carrier is suppressed while the λ5780 DIB carrier is enhanced, and typically probe the envelopes of clouds. The ζ sightlines, where W (5797)/W (5780) > 0.3, sample regions where the λ5797 DIB carrier is protected from high energy photons and the λ5780 DIB carrier is suppressed, and typically probe the interiors of clouds (but not the highest densities). As is clear from the map, the LB and the passage towards Loop I are filled with σ clouds due to the harsh environment, but still some small ζ clouds can be found immersed within it.

Figure 2: λ5780 DIB distribution in three principal slices. The quantities are colored based on the logarithmic volume densities of the equivalent amount of neutral sodium, with redder regions tracing denser parts and bluer regions the rarefied mediums. Blue, gold, red and dark red contours correspond to log n (cm−3) =−11.6,−10.6,−10.1 and−9.7. The positions of nearby nebulae and star clusters are plotted with various symbols. Triangles represent the projection of observed stars with distances less than 30 pc to this particular slice: upward if located above the GP and downward if below it, and with the size proportional to the derived column density. Open circles are sightlines with zero DIB column densities assigned if the standard deviation within ±3 Å around the DIB position equaled the noise. The Sun is located in the center of the map, the name of each slice is printed in panel title. The distance scale is in units of parsec.17

Figure 1: Observed DIB specta within and around the LB. Normalized high S/N spectrum of strong (HD 43384 with EWλ5780 = 452±48 mÅ, EWλ5797 = 125±15 mÅ), mid (HD 175869 with EWλ5780 = 135±16 mÅ, EWλ5797 = 29 ± 5 mÅ), and weak (HD 23480 with EWλ5780 = 16 ± 4 mÅ, EWλ5797 = 8 ± 3 mÅ) DIBs as well as a sightline with absence of λ5797 (HD 12216 with EWλ5780 = 17± 3 mÅ) and an example of sightlines with no DIBs at all (HD193369). The red lines show Gaussian fits to the DIBs. All spectra are moved to interstellar rest frame. The HD 12216 (at d = 44 pc) is an example of DIBs located within the LB, and HD 23480 is located at the wall of the LB (at d = 105 pc), while HD 43384 is located well outside the LB at d = 641 pc.

Full text

A three-dimensional map of the hot Local Bubble using
diffuse interstellar bands
Amin Farhang
1,2
, Jacco Th. van Loon
3
, Habib G. Khosroshahi
1
, Atefeh Javadi
1
& Mandy Bailey
4
1
School of Astronomy, Institute for Research in Fundamental Sciences, 19395–5531 Tehran, Iran
2
Department of Physics and Astronomy, The University of Western Ontario, N6A 3K7, Canada
3
Lennard-Jones Laboratories, Keele University, ST5 5BG, UK
4
The Open University, Associate Lecturer Services (STEM), 351, Altrincham Road, Sharston,
Manchester, M22 4UN, UK
The Solar System is located within a low-density cavity, known as the Local Bubble
1, 2, 3
,
which appears to be filled with an X-ray emitting gas at a temperature of 10
6
K
4
. Such con-
ditions are too harsh for typical interstellar atoms and molecules to survive
2, 3
. There exists
an enigmatic tracer of interstellar gas, known as Diffuse Interstellar Bands (DIB), which of-
ten appears as absorption features in stellar spectra
5, 6, 7
. The carriers of these bands remain
largely unidentified
8
. Here we report the three-dimensional structure of the Local Bubble us-
ing two different DIB tracers (λ5780 and λ5797), which reveals that DIB carriers are present
within the Bubble
9, 10, 11
. The map shows low ratios of λ5797/λ5780 inside the Bubble com-
pared to the outside. This finding proves that the carrier of the λ5780 DIB can withstand
X-ray photo-dissociation and sputtering by fast ions, where the carrier of the λ5797 DIB
succumbs. This would mean that DIB carriers can be more stable than hitherto thought,
and that the carrier of the λ5780 DIB must be larger than that of the λ5797 DIB
12
. Alterna-
1
arXiv:1907.07429v1 [astro-ph.GA] 17 Jul 2019

tively, small-scale denser (and cooler) structures that shield some of the DIB carriers must
be prevalent within the Bubble, implying that such structures may be an intrinsic feature of
supernova-driven bubbles.
The origin of the Local Bubble (LB) is unknown, but measurements of
60
Fe column densi-
ties can be explained by successive explosions of massive stars (supernovae) within the Scorpius–
Centaurus stellar group
13
. The high temperature within the cavity is hostile to atoms, molecules
and dust grains
2, 3, 4
. Observations aimed at detecting highly ionized gas, which could be present at
million-degree temperatures, reveal no such plasma within the LB
14
. The DIB carriers offer an al-
ternative tracer of the wall and interior of the LB. Although their nature is unknown, recent studies
indicate that DIB carriers are likely carbon-based organic molecules
8
. They are universal and have
been detected in different environments of Milky Way (MW) and within different galaxies
5, 6, 7
and
in earlier partial results from our survey
9, 10, 11
. Our 3D map of the distribution of DIB carriers
within the vicinity of the Sun opens a new window on a substantial fraction of interstellar carbon
that may be locked up in the molecular carriers and that may play an important role in interstellar
chemistry; this may include the C
+
60
(“buckyballs”) anion which may be responsible for the DIBs
at 9577
˚
A and 9632
˚
A wavelengths
15
. Therefore, we employed two of the strongest DIB tracers at
5780
˚
A and 5797
˚
A wavelengths, in direction of 359 different sightlines, to mapped the LB out to
a distance of 200 pc. While the λ5780 DIB could possibly persist under harsh conditions of the
LB
16, 17
, the λ5797 DIB cannot survive in such an environment and must be shielded within the
inner regions of clouds
10
.
2

The outlines of the LB have been mapped with absorption lines of singly ionized calcium
(Ca II) and neutral sodium (Na I) in the spectra of background stars
2
, as well as by the attenuation
of stellar light by interstellar dust based on E(B − V ) measurements
3, 18, 19
, revealing that the LB
cavity extends out to 80 pc in the Galactic plane (GP) and up to several hundred pc perpendiculars
to the GP. Some dust maps have been produced by translating the strength of λ15273 DIB into
E(B − V ) and merging it with other E(B − V ) measurements as priors; thus not strictly speaking
a DIB map, but really a dust map
18
. Also, a pseudo-3D map of the λ8620 DIB within 3 kpc from
the Sun have been produced recently
20
. They compared the DIB map with the dust distribution
and found the λ8620 DIB and dust to have a similar distribution, but the scale height of the DIB
exceeds that of the dust. This is a clear indication that DIBs and dust do not necessarily trace the
same interstellar material and/or conditions
9
. Their pseudo-3D map traces large-scale structure,
with the whole LB being confined to one voxel.
Because of the high temperature of the LB, it is difficult to detect ordinary atoms within
the LB. However, our observations show that DIB carriers are present within the LB
9, 10, 11
. An
example of different DIBs within and around the LB is shown in Fig. 1. To ensure that the observed
DIBs are physical features not noise, only those absorptions with a confidence level exceeding
3σ are accepted in the mapping. In the following, we present three principal slices of the 3D
distribution of the DIB pseudo volume density within 200 pc from the Sun. The maps are in
Galactic coordinates, i.e. the Sun is located at the origin and the primary direction is towards the
Galactic Center (GC). The slices show the maps in imaginary planes of the Galactic, meridian and
rotational planes. The GP slices the MW disk, the meridian plane is perpendicular to the GP with
3

its x-axis pointing towards the GC, and the rotational plane is perpendicular to the GP and faced
towards the GC.
In upper left panel of Fig. 2 (λ5780 in the GP) the bulk of the material lies in front of the
Scutum and Aquila dark nebulae (in the 1st quadrant in the upper-right sector) at distances of
30–120 pc from the Sun. The outer layer of this dense DIB structure coincides with a dense
concentration in the dust map, however, the inner region is stretched within the LB and continued
to interrupt the whole LB where there is not any dust
3
(see Supplementary Fig. 1 for an RGB map
of dust, DIB and Na I). A lower-density DIB filament lies in front of the Cygnus rift molecular
clouds. In this direction, X-rays with energies of order keV probably affect the DIB carriers beyond
a distance of approximately 100 pc
21
. In the 2nd quadrant (upper-left sector) DIB carriers are
abundant in the direction of the Taurus dark clouds and molecular. The 3rd quadrant (lower-left
sector) is characterized by a general paucity of the DIB carrier, in the direction of the β Canis
Majoris (CMa) interstellar tunnel and the GSH 238+00+09 supershell (l = 260
◦
). However, a low-
density DIB trunk defies the odds; it may be associated with the photo-ionizing effect of βCMa
2
.
Finally, the 4th quadrant (lower-right sector) features the most famous cavity connected to the LB
(Loop I) in the direction of the GC (l = 345
◦
) at a distance of 200 pc. A narrow tunnel connects
the LB with Loop I, as revealed in previous LB maps
2, 3
; our map reveals that it is filled with a
DIB filament and that Loop I itself is filled with DIB material as well (see Supplementary Fig. 2
for more slices).
The vertically extended structures are seen in the meridian plane (middle left panel of Fig. 2),
4

one towards the GC in front of the Ophiuchus and Lupus complexes and R Coronae Australis (CrA)
and another in the opposite direction in front of the Taurus star formation complex in between the
Hyades and Pleiades star clusters. The atomic maps of the LB in this view show a striking open-
ended tunnel known as the Local Chimney, tilted at an angle of 35
◦
from the North Galactic Pole
(NGP)
22, 2
, but our map shows that the tunnel is not entirely devoid of DIB carriers. Interestingly,
there are some high latitude clouds (H I shells) in this direction
23
. The tilted tunnel can also be
discerned in the rotational plane, but most noticeable is the DIB complex in the 3rd quadrant and
material some 150 pc below the GP in the direction of the South Galactic Pole.
In the right column of Fig. 2, we present the λ5797 distributions. The carrier of the λ5797
DIB is highly susceptible to the destructive effect of energetic photons
9, 10
. The GP view shows
the largest concentrations lie between 30–120 pc distance in the direction of Scutum and a filament
extends over 200 pc to meet Loop I, both akin to what was seen in the λ5780. Likewise, the Local
Chimney is visible in the meridian plane and a thin filament coincides with the densest λ5780 DIB
concentration in the 3rd quadrant of the rotational plane. Apart from the similarity between the
two DIB carriers there are also notable differences, with the λ5797 DIB distribution generally more
fragmented and more tenuous. The λ5780 DIB is known to trace relatively energetic environments
as compared to the λ5797 that traces more neutral, shielded regions (see Supplementary Fig. 3
where the strength of the λ5780 saturates at high Na I densities). Indeed, the ratio of the two DIB
carrier distributions shows that the interior of the LB is depleted in the λ5797 DIB carrier, more so
than the λ5780 DIB carrier (Fig. 3) though more neutral, shielded cloudlets seem to persist within
the LB.
5