On The Internal Structure Of Starless Cores. Physical and Chemical Properties of L1498 and L1517B

Tafalla, M.; Myers, Philip C.; Caselli, P.; Walmsley, C. M.

doi:10.1023/b:astr.0000045036.76044.bd

Cited by 25 publications

(42 citation statements)

References 82 publications

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“…However, in starless cores the values are not likely to be much higher (Hotzel et al 2001;Tafalla et al 2002;Crapsi et al 2007). Tafalla et al (2004) found that the excitation temperatures of the (1, 1) and (2, 2) lines varied by a factor of several. This variation depends on several factors, the cloud optical depth, the volume density, and the variation of the kinetic temperature.…”

Section: Discussionmentioning

confidence: 93%

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Reliability of NH₃as the temperature probe of cold cloud cores

Juvela¹,

Harju²,

Ysard³

et al. 2012

A&A

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Context. The temperature is a central parameter affecting the chemical and physical properties of dense cores of interstellar clouds and their potential evolution towards star formation. The chemistry and the dust properties are temperature dependent and, therefore, interpretation of any observation requires the knowledge of the temperature and its variations. Direct measurement of the gas kinetic temperature is possible with molecular line spectroscopy, the ammonia molecule, NH 3 , being the most commonly used tracer. Aims. We want to determine the accuracy of the temperature estimates derived from ammonia spectra. The normal interpretation of NH 3 observations assumes that all the hyperfine line components are tracing the same volume of gas. However, in the case of strong temperature gradients they may be sensitive to different layers and this could cause errors in the optical depth and gas temperature estimates. Methods. We examine a series of spherically symmetric cloud models, 1.0 and 0.5 M Bonnor-Ebert spheres, with different radial temperature profiles. We calculate synthetic NH 3 spectra and compare the derived column densities and temperatures to the actual values in the models. Results. For high signal-to-noise observations, the estimated gas kinetic temperatures are within ∼0.3 K of the real mass averaged temperature and the column densities are correct to within ∼10%. When the S /N ratio of the (2, 2) spectrum decreases below 10, the temperature errors are of the order of 1 K but without a significant bias. Only when the density of the models is increased by a factor of a few, the results begin to show significant bias because of the saturation of the (1, 1) main group. Conclusions. The ammonia spectra are found to be a reliable tracer of the real mass averaged gas temperature. Because the radial temperature profiles of the cores are not well constrained, the central temperature could still be different from this value. If the cores are optically very thick, there are no longer guarantees of the accuracy of the estimates.

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Section: Discussionmentioning

confidence: 93%

“…However, in the modelling of the NH 3 spectra, the fractional abundance of para-NH 3 is always kept at a constant value of 10 −8 (e.g. Hotzel et al 2001;Tafalla et al 2004). This applies also to the χ-model.…”

Section: Core Modelsmentioning

confidence: 99%

Reliability of NH₃as the temperature probe of cold cloud cores

Juvela¹,

Harju²,

Ysard³

et al. 2012

A&A

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“…Subsonic turbulence contributes less to the energy budget of the cloud than thermal pressure and so cannot provide sufficient support against gravitational collapse (Myers, 1983;Goodman et al, 1998;Tafalla et al, 2006). If cores are longer lasting entities there must be other mechanisms to provide stability.…”

Section: Star Formationmentioning

confidence: 99%

Physical Processes in the Interstellar Medium

Klessen

Glover

2015

Star Formation in Galaxy Evolution: Connecting Numerical Models to Reality

160

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Interstellar space is filled with a dilute mixture of charged particles, atoms, molecules and dust grains, called the interstellar medium (ISM). Understanding its physical properties and dynamical behavior is of pivotal importance to many areas of astronomy and astrophysics. Galaxy formation and evolution, the formation of stars, cosmic nucleosynthesis, the origin of large complex, prebiotic molecules and the abundance, structure and growth of dust grains which constitute the fundamental building blocks of planets, all these processes are intimately coupled to the physics of the interstellar medium. However, despite its importance, its structure and evolution is still not fully understood. Observations reveal that the interstellar medium is highly turbulent, consists of different chemical phases, and is characterized by complex structure on all resolvable spatial and temporal scales. Our current numerical and theoretical models describe it as a strongly coupled system that is far from equilibrium and where the different components are intricately linked together by complex feedback loops. Describing the interstellar medium is truly a multi-scale and multi-physics problem. In these lecture notes we introduce the microphysics necessary to better understand the interstellar medium. We review the relations between large-scale and small-scale dynamics, we consider turbulence as one of the key drivers of galactic evolution, and we review the physical processes that lead to the formation of dense molecular clouds and that govern stellar birth in their interior.

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“…In the calculation of total ammonia column density we assumed the NH 3 ortho-to-para ratio to follow the LTE conditions. The kinetic temperature of the gas was calculated from the rotational temperature using the semiempirically derived analytic expression from Tafalla et al (2004) …”

Section: Ammonia Column Densities and The Kinetic Temperatures Using mentioning

confidence: 99%

Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

Ruoskanen

Harju

Juvela

et al. 2011

A&A

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Context. The gas kinetic temperature in the centres of starless, high-density cores is predicted to fall as low as 5−6 K. The temperature gradient, which affects the dynamics and chemistry of these objects, should be discernible with radio interferometers reaching a spatial resolution of 1000 AU or better. Aims. The aim of this study was to determine the kinetic temperature distribution in the low-mass prestellar core Oph D where previous observations suggest a very low central temperature.Methods. The densest part of the Oph D core was mapped in the NH 3 (1, 1) and (2, 2) inversion lines using the Very Large Array (VLA). The physical quantities were derived from the observed spectra by fitting the hyperfine structure of the lines, and subsequently the temperature distribution of Oph D was calculated using the standard rotational temperature techniques. A physical model of the cores was constructed, and the simulated spectra after radiative transfer calculations with a 3D Monte Carlo code were compared with the observed spectra. Temperature, density, and ammonia abundance of the core model were tuned until a satisfactory match with the observation was obtained.Results. The high resolution of the interferometric data reveals that the southern part of Oph D comprises of two small cores in consistence with the 1.3 mm dust continuum map. The gas kinetic temperatures, as derived from ammonia towards the centres of the southern and northern core are 7.4 and 8.9 K, respectively. These values represent line-of-sight averages using the LTE assumption. A model using modified Bonnor-Ebert spheres, in which the temperature decreases to 6.1 K and 8.9 K in the centres of southern and northern core, matched the observed values satisfactorily. The southern core, which has more steep temperature gradient, has central density of n c = 4 × 10 6 cm −3 , and the data suggests depletion of ammonia within 700 AU from the centre. The northern core, which is almost isothermal, seems to be less dense. The radial velocity gradients in these cores are almost opposite in direction, which may be an indication that turbulent fragmentation has a role in the formation of these cores. The observed masses of the cores are only ∼0.2 M . Their potential collapse could lead to formation of brown dwarfs or low-mass stars.

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On The Internal Structure Of Starless Cores. Physical and Chemical Properties of L1498 and L1517B

Cited by 25 publications

References 82 publications

Reliability of NH₃as the temperature probe of cold cloud cores

Reliability of NH₃as the temperature probe of cold cloud cores

Physical Processes in the Interstellar Medium

Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

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On The Internal Structure Of Starless Cores. Physical and Chemical Properties of L1498 and L1517B

Cited by 25 publications

References 82 publications

Reliability of NH3as the temperature probe of cold cloud cores

Reliability of NH3as the temperature probe of cold cloud cores

Physical Processes in the Interstellar Medium

Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

Contact Info

Product

Resources

About

Reliability of NH₃as the temperature probe of cold cloud cores

Reliability of NH₃as the temperature probe of cold cloud cores