Large Millimeter/Submillimeter Array (ALMA), resulting in an increase of more than one order of
magnitude in both sensitivity and angular resolution. This interferometric project, called EMoCA
(Exploring Molecular Complexity with ALMA), aims
to decipher the molecular content of Sgr B2(N) in
order to test the predictions of astrochemical numerical simulations and to gain insight into the
chemical processes at work in the ISM.
Sgr B2 is the most massive star-forming region
in our Galaxy. It is located close to the Galactic
Center, which is 8.34 T 0.16 (SEM) kpc from the
Sun (7). Sgr B2 contains two main sites of star
formation, Sgr B2(N) and Sgr B2(M), that, since
the early 1970s, have turned out to be the best
hunting ground for complex organic molecules
in the ISM. Their immense hydrogen column densities that signify large quantities of gas enable the
detection of low-abundance species. Sgr B2(N)
itself contains two dense, compact, hot cores that
are separated by about 5″ [~40,000 astronomical
units (AU) in projection] (6).
Propyl cyanide (C3H7CN, hereafter PrCN) is the
smallest alkyl cyanide that exists in several distinct
isomers (Fig. 1): the chain isomer normal- or n-PrCN
(also known as butyronitrile or 1-cyanopropane)
and the branched isomer iso- or i-PrCN (also
known as iso-butyronitrile or 2-cyanopropane).
n-PrCN is the smallest alkyl cyanide that exists
in several distinct conformations. The CN group can
be attached to the terminal C of the propyl group in
the CCC plane, trans to the CCC chain, leading to the
anti conformer, also known as trans (Fig. 1C); it can
also be attached to the propyl group rotated by
T120° with respect to the CCC plane, leading to
the gauche conformer (Fig. 1B). The rotational spectrum of i-PrCN, previously only studied to a limited
extent in the microwave region, has recently been
recorded extensively in the laboratory from the microwave to the submillimeter wave region along
with a redetermination of the dipole moment (8).
We used ALMA in 2012 to perform a full spectral line survey toward Sgr B2(N) in the 3-mm
atmospheric window between 84 and 111 GHz (9).
We identified the detected lines by modeling the
molecular emission under the assumption of local
thermodynamic equilibrium. By using predictions
from the Cologne Database for Molecular Spectroscopy (10), we assigned emission features to
i-PrCN or n-PrCN (9). To interpret the astronomical detections, we performed numerical simulations of the chemistry occurring during the
evolution of a hot core (9).
Many spectral lines are detected in the ALMA
data toward both of the hot cores embedded
in Sgr B2(N). These spectra are very close to the
confusion limit; that is, signal from a spectral
line is detected in nearly every spectral channel.
The lines are narrower toward the northern, less-prominent hot core (full width at half maximum,
FWHM, ~5 km s−1) than toward the southern,
more-prominent one. The detection of faint lines
from rare species is therefore easier toward the
former, and we focused on this one in the present
work. We constructed a preliminary model of
the emission of all molecules previously detected,
sciencemag.org 26 SEPTEMBER 2014 • VOL 345 ISSUE 6204 1585
Fig. 2. Examples of transitions of i-PrCN and n-PrCN toward the north-
ern hot core of Sgr B2(N). (A and B) Continuum-subtracted spectrum
observed with ALMA in black, the preliminary model including all identified
molecules in green, and the synthetic spectra of i-PrCN and n-PrCN, respec-
tively, in red. (C and E) Integrated intensity maps of the transitions of i-PrCN
and n-PrCN marked with a red arrow in (A) and (B), respectively. The con-
tinuum emission at 90.5 GHz is shown in (D). The negative contour (dotted
blue) is –3s, and the positive contours (black) are 2i × 3s, with i an integer
starting at 0 and s the root-mean-square noise level [23 mJy beam–1 km s–1,
8.3 mJy beam−1 km s−1, and 20 mJy beam–1 km s–1 with half-power beam
widths of 1.8″ × 1.6″, 1.8″ × 1.6″, and 1.9″ × 1.6″ in (C), (D), and (E), respectively].
The large cross indicates the position of the northern hot core as traced by both
molecules. The smaller cross marks the position of the main hot core that has a
lower systemic velocity, which implies that the contours outside the red box do
not trace the emission of i-PrCN and n-PrCN in (C) and (E), respectively. The
black ellipses show the size of the respective synthetic beams.