This paper explores the epistemic status of models and simulations between theory, on the one hand, and observations, on the other. In particular, I will argue that the interpretation of an ...essentially invariant astrophysical model structure can change substantially over time. I will illustrate this claim using as an example the first 20 years (1985–2004) of development of the Paris-Durham shock code—a numerical model of slow interstellar shock waves (i.e. a disturbance of the medium that travel faster than the local speed of sound). I will show that the model’s interpretation and, in particular, its underlying representational ideal—the modeler’s (often implicit) goal governing the development and the use of the model—changed notably during this period. Whereas the code was originally used in a purely exploratory fashion, it was later taken to represent and encompass the target phenomenon as completely as possible. It is noteworthy that during this transition the model’s change of epistemic status was never explicitly acknowledged or in any way articulated. However, the impetus for the change can, I claim, be found in the role that observational data came to play in the later publications.
With the ever-growing quality of observational data in astronomy, the complexity of astrophysical models has been increasing in turn. This trend raises the question: Are there still reasons to prefer ...simpler models if the final goal is an actual model-target comparison? I argue for two aspects in which astrophysical research may favor models having reduced complexity: first, to address the problem of determining the values of adjustable parameters and, second, to pave the way for a validation of the model based on the modeler’s understanding of the scope of the model and the critical processes on the target’s side.
Context.
M 33 is a gas rich spiral galaxy of the Local Group. Its vicinity allows us to study its interstellar medium (ISM) on linear scales corresponding to the sizes of individual giant molecular ...clouds.
Aims.
We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum.
Methods.
We mapped the emission of gas and dust in M 33 using the far-infrared lines of C
II
and O
I
(63
μ
m) and the total infrared continuum. The line maps were observed with the PACS spectrometer on board the
Herschel
Space Observatory. These maps have 50 pc resolution and form a ∼370 pc wide stripe along its major axis covering the sites of bright H
II
regions, but also more quiescent arm and inter-arm regions from the southern arm at 2 kpc galacto-centric distance to the south out to 5.7 kpc distance to the north. Full-galaxy maps of the continuum emission at 24
μ
m from
Spitzer
/MIPS, and at 70
μ
m, 100
μ
m, and 160
μ
m from
Herschel
/PACS were combined to obtain a map of the TIR.
Results.
TIR and C
II
intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of
G
0, obs
∼ 2 to 200 in units of the average Galactic radiation field. The binned C
II
/TIR ratio drops with rising TIR, with large, but decreasing scatter. The contribution of the cold neutral medium to the C
II
emission, as estimated from VLA H
I
data, is on average only 10%. Fits of modified black bodies to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the O
I
data before comparing it with models of photon dominated regions. Most of the ratios of C
II
/O
I
and (C
II
+O
I
)/TIR are consistent with two model solutions. The median ratios are consistent with one solution at
n
∼ 2 × 10
2
cm
−3
,
G
0
∼ 60, and a second low-FUV solution at
n
∼ 10
4
cm
−3
,
G
0
∼ 1.5.
Conclusions.
The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ∼1. A fraction of the gas may, however, be represented by the second solution.
This article looks at philosophical aspects and questions that modern astrophysical research gives rise to. Other than cosmology, astrophysics particularly deals with understanding phenomena and ...processes operating at "intermediate" cosmic scales, which has rarely aroused philosophical interest so far. Being confronted with the attribution of antirealism by Ian Hacking because of its observational nature, astrophysics is equipped with a characteristic methodology that can cope with the missing possibility of direct interaction with most objects of research. In its attempt to understand the causal history of singular phenomena it resembles the historical sciences, while the search for general causal relations with respect to classes of processes or objects can rely on the "cosmic laboratory": the multitude of different phenomena and environments, naturally provided by the universe. Furthermore, the epistemology of astrophysics is strongly based on the use of models and simulations and a complex treatment of large amounts of data.
Supernova remnants (SNRs) are considered as being the sources of galactic cosmic rays. In order to understand the origin, acceleration, and composition of these cosmic rays, detailed knowledge of the ...physical conditions in the local interstellar medium is needed. The shock interaction of SNRs with molecular clouds that gives rise to strong molecular emission in the far-IR and sub-mm wavelength regimes can be used as a highly valuable tracer of these conditions. The application of MHD shock models in the interpretation of the resulting line emission can yield information on the energetic and chemical impact of supernova remnants. We have mapped two regions in the supernova remnant W44 with the APEX telescope in \({}^{12}\)CO (3-2), (4-3), (6-5), (7-6) and \({}^{13}\)CO (3-2). The extraction of integrated intensities on five different positions, corresponding to local maxima of CO emission, allows to compare these intensities to the outputs of a grid of models, which combine an MHD shock code with a radiative transfer module based on the 'large velocity gradient' approximation. We find that the observed CO line emission is compatible with non-stationary shocks and a pre-shock density of \(10^4\) cm\({}^{-3}\). Our models furthermore allow to constrain shock ages, velocities, the pre-shock magnetic field strength components perpendicular to the line-of-sight, and the full ladder of CO transitions. Finally, our analysis can be used to estimate the contribution of such SNRs to, e.g. the galactic energy balance and the momentum-injection into the surrounding interstellar medium.
M33 is a gas rich spiral galaxy of the Local Group. We investigate the relationship between the two major gas cooling lines and the total infrared (TIR) dust continuum. We mapped the emission of gas ...and dust in M33 using the far-infrared lines of CII and OI(63um) and the TIR. The line maps were observed with Herschel/PACS. These maps have 50pc resolution and form a ~370pc wide stripe along its major axis covering the sites of bright HII regions, but also more quiescent arm and inter-arm regions from the southern arm at 2kpc galacto-centric distance to the south out to 5.7kpc distance to the north. Full-galaxy maps of the continuum emission at 24um from Spitzer/MIPS, and at 70um, 100um, and 160um from PACS were combined to obtain a map of the TIR. TIR and CII intensities are correlated over more than two orders of magnitude. The range of TIR translates to a range of far ultraviolet (FUV) emission of G0,obs~2 to 200 in units of the average Galactic radiation field. The binned CII/TIR ratio drops with rising TIR, with large, but decreasing scatter. Fits of modified black bodies (MBBs) to the continuum emission were used to estimate dust mass surface densities and total gas column densities. A correction for possible foreground absorption by cold gas was applied to the OI data before comparing it with models of photon dominated regions (PDRs). Most of the ratios of CII/OI and (CII+OI)/TIR are consistent with two model solutions. The median ratios are consistent with one solution at n~2x10^2 cm-3, G0~60, and and a second low-FUV solution at n~10^4 cm-3, G0~1.5. The bulk of the gas along the lines-of-sight is represented by a low-density, high-FUV phase with low beam filling factors ~1. A fraction of the gas may, however, be represented by the second solution.
Supernovae constitute a critical source of energy input to the interstellar medium (ISM). In this short review, we focus on their latest phase of evolution, the supernova remnants (SNRs). We present ...observations of three old SNRs that have reached the phase where they interact with the ambient ISM: W28, IC443, and 3C391. We show that such objects make up clean laboratories to constrain the physical and chemical processes at work in molecular shock environments. Our studies subsequently allow us to quantify the impact of SNRs on their environment in terms of mass, momentum, and energy dissipation. In turn, their contribution to the energy balance of galaxies can be assessed. Their potential to trigger a further generation of star formation can also be investigated. Finally, our studies provide strong support for the interpretation of gamma-ray emission in SNRs, a crucial step to answer questions related to cosmic rays population and acceleration.
During the formation of a star, material is ejected along powerful jets that impact the ambient material. This outflow regulates star formation by e.g. inducing turbulence and heating the surrounding ...gas. Understanding the associated shocks is therefore essential to the study of star formation. We present comparisons of shock models with CO, H2, and SiO observations in a 'pure' shock position in the BHR71 bipolar outflow. These comparisons provide an insight into the shock and pre-shock characteristics, and allow us to understand the energetic and chemical feedback of star formation on Galactic scales. New CO (Jup = 16, 11, 7, 6, 4, 3) observations from the shocked regions with the SOFIA and APEX telescopes are presented and combined with earlier H2 and SiO data (from the Spitzer and APEX telescopes). The integrated intensities are compared to a grid of models that were obtained from a magneto-hydrodynamical shock code which calculates the dynamical and chemical structure of these regions combined with a radiative transfer module based on the 'large velocity gradient' approximation. The CO emission leads us to update the conclusions of our previous shock analysis: pre-shock densities of 1e4 cm-3 and shock velocities around 20-25 km s-1 are still constrained, but older ages are inferred ( 4000 years). We evaluate the contribution of shocks to the excitation of CO around forming stars. The SiO observations are compatible with a scenario where less than 4% of the pre-shock SiO belongs to the grain mantles. We infer outflow parameters: a mass of 1.8x1e-2 Msun was measured in our beam, in which a momentum of 0.4 Msun km s-1 is dissipated, for an energy of 4.2x1e43erg. We analyse the energetics of the outflow species by species. Comparing our results with previous studies highlights their dependence on the method: H2 observations only are not sufficient to evaluate the mass of outflows.
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