Conclusions

As it can be seen on the left part of Figure 3.2, HCCO peaks towards the region where c-C₃H₂ peaks, while the line intensity decreases by almost a factor of 2 towards theCH₃OH peak.This is a clear evidence that HCCO follows a different chemistry than other oxygen-bearing species, showing instead a predominant hydrocarbon chemistry. This favours the fact that C₂H is chemically related to HCCO. However, higher-sensitivity observations are needed to prove this point and constrain the gas-grain chemical models.

• For both species we were able to determine all the quartic centrifugal distortion constants, one sextic parameter, H_KN, as well as all the quartic and one sextic spin-rotation constant, H_K^S.

• We expanded the centrifugal distortion analysis of HCCO by including a sextic con-stantH_K, an octic, L_KKN, and one decadic constant, P_KKKN, along with one sextic spin-rotation parameter, H_{KN K}^S . This extended set of parameters reduced the rms deviation by a factor of ∼ 2 with respect to the previous fit by Endo and Hirota [1987]. We also decreased the uncertainty on the rotational constants up to a factor of ∼40.

• We show that towards the pre-stellar core L1544, HCCO peaks within the region where c-C₃H₂ peaks, suggesting that the ketenyl formation is based predominantly on hydrocarbon chemistry.

• The spectral analysis of DCCO resulted to a rms deviation of 64 kHz. The rotational parameters are more strongly constrained in our fit with respect to Endo and Hirota [1987]. We provide for the first time a catalog of highly accurate frequencies for DCCO (uncertainties at 3 mm ∼15 kHz), which allows a future DCCO detection in cold sources, like starless and pre-stellar cores.

The Supersonic Jet Experiment

The Supersonic Jet Experiment is a new spectrometer that has been developed at the Center of Astrochemical Studies in Garching with the aim of studying unstable species in the millimeter and submillimeter range. The supersonic expansion of a selected gas mix-ture into a vacuum chamber results in strong rotational cooling of the molecular species, reaching very low temperatures (down to ∼5 K), which are typical for interstellar envin-ronments. Isolated in a collision-free molecular beam, ions and radicals are stabilized and become available for spectroscopic studies.

4.1 Introduction

In this experiment a gas sample is injected with a high pressure (P₀ >1 bar) through a pulsed valve into a vacuum chamber where it experiences very low pressures down to P_b ∼ 10⁻⁸ bar. At such large pressure gradients there is almost no heat transfer taking place, meaning that the thermal/internal energy of the species inside the mechanical valve almost entirely converts into the expansion energy of the molecular gas. Thus, the gas flow can be treated as an adiabatic expansion, which leads to a dramatic rotational cooling of the molecular species down to a few Kelvin. This process is illustrated in Fig. 4.1. The molecular thermal motion within the valve transforms into a strongly confined translational movement, leading to very narrow velocity distibutions and therefore substantially decreas-ing the Doppler broadendecreas-ing. The resultdecreas-ing cooldecreas-ing effect leads to a significant decrease of the local speed of sound vs of the expanding gas, since vs ∼√

T. The Mach number, M, on the other hand, which is given by the ratio of the flow velocity v_f to the local speed of sound v_s, is increasing, and thus the expanding gas reaches locally supersonic velocity.

In particular, there is a region within the jet expansion, where the density reduces to such a low level, that molecules are stabilised in a nearly collision-free environment called the zone of silence. Here, the molecular flow is travelling with supersonic velocity (M 1), without being influenced by the warm background gas.

The interaction of the molecular jet with the background gas is, however, disturbing the gas expansion in vertical and horizontal direction to the centerline of the beam, causing

P₀, T₀ M<<1

Zone of silence M>>1

Background pressure P_b

frontal shock front

M<1

lateral shock front

M>1

Figure 4.1: The supersonic jet expansion. In the so-called zone of silence the molecular flow is nearly collision-free and travels with supersonic velocity. The interaction of the jet with the background gas leads to the formation of lateral and frontal shock fronts with respect to the centerline of the beam [Scoles, 1988].

a rapid deceleration of the molecules. Expansion waves are reflected at the jet boundary, forming a lateral shock zone, also called barrel shock, as well as a frontal shock front, known as the Mach disk. The location of the Mach disk determines the expansion length x_m of the jet and is given by the following relation:

x_m

d = 0.67· rP₀

P_b, (4.1)

with d being the diameter of the valve’s orifice, P₀ the source pressure and P_b the back-ground pressure inside the vacuum chamber. The above formula indicates that a high pres-sure ratio results in an elongated jet expansion, which means a larger absorption length and thus a stronger molecular signal. The relation for an adiabatic thermodynamic process of the expanding gas is described as:

T T0

= (P_b P0

)

γ−1 γ

with γ = c_P cV

= f + 2

f , (4.2)

wherec_P,V is the specific heat capacity at constant pressure and volume, respectively, and f the degree of freedoms of the investigated gas. For all gases withγ = ⁵₃ and with ^x_d^m 1, the Mach number M is given by [Scoles, 1988]:

M = 3.232·(x_m

d )². (4.3)

This relation is only valid in the zone of silence with M 1. Using Eq. 4.2 we obtain for the jet temperature T according to Scoles [1988] the following expression:

T T0

= (1 +γ −1 2 M²)

−1

. (4.4)

The production and spectroscopic study of interstellar COMs in a cold molecular beam has proven to be beneficial for several reasons. The resulting low temperatures lead to the population of the lowest rotational levels, which simplifies dense and complex rotational spectra and subsequently makes spectral assignment easier. In addition, the collisions between molecules in a cold molecular beam are minimized, thus stabilizing sensitive and highly reactive molecules. In other words, once the molecules of interest are formed they are frozen-out within the jet, without being able to further react. Finally, the reduced Doppler broadening is significantly enhancing the achieved spectral resolution.