Protostars interact violently with their natal cocoons within dense molecular clouds. Characterizing this feedback is key to understanding the efficiency of the star formation process and the... Show moreProtostars interact violently with their natal cocoons within dense molecular clouds. Characterizing this feedback is key to understanding the efficiency of the star formation process and the chemical processing of material that will be available for planet formation. In this thesis, the imprints of physical processes on molecular gas are analyzed using state-of-the-art far-infrared spectroscopy from Herschel / PACS. Interpretation of the origin of far-infrared line emission allows us to quantify the physical conditions and the role of shocks and ultraviolet radiation during the 'kindergarten years' of low- and high-mass protostars. Show less
We present 50-210 {$μ$}m spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel-PACS, and 0.5-1000 {$μ$}m spectral energy distributions, as part of the Dust, Ice, and Gas in... Show moreWe present 50-210 {$μ$}m spectral scans of 30 Class 0/I protostellar sources, obtained with Herschel-PACS, and 0.5-1000 {$μ$}m spectral energy distributions, as part of the Dust, Ice, and Gas in Time Key Program. Some sources exhibit up to 75 H$_{2}$O lines ranging in excitation energy from 100 to 2000 K, 12 transitions of OH, and CO rotational lines ranging from J = 14 { arr} 13 up to J = 40 { arr} 39. [O I] is detected in all but one source in the entire sample; among the sources with detectable [O I] are two very low luminosity objects. The mean 63/145 {$μ$}m [O I] flux ratio is 17.2 {plusmn} 9.2. The [O I] 63 {$μ$}m line correlates with L $_{bol}$, but not with the time-averaged outflow rate derived from low-J CO maps. [C II] emission is in general not local to the source. The sample L $_{bol}$ increased by 1.25 (1.06) and T $_{bol}$ decreased to 0.96 (0.96) of mean (median) values with the inclusion of the Herschel data. Most CO rotational diagrams are characterized by two optically thin components (${$$ackslash$langle ${$ ${$N$}$$}$$ackslash$rangle$}$ = (0.70 +/- 1.12)${$${$$}$ $ackslash$times 10^{}${$49$}$$}$ total particles). ${$ ${$N$}$$}$_CO correlates strongly with L $_{bol}$, but neither T $_{rot}$ nor ${$ ${$N$}$$}$_CO(warm)/${$ ${$N$}$$}$_CO(hot) correlates with L $_{bol}$, suggesting that the total excited gas is related to the current source luminosity, but that the excitation is primarily determined by the physics of the interaction (e.g., UV-heating/shocks). Rotational temperatures for H$_{2}$O (${$$ackslash$langle ${$T_rot$}$$ackslash$rangle $}$ = 194 +/- 85 K) and OH (${$$ackslash$langle ${$T_rot$}$$ackslash$rangle $}$ =183 +/- 117 K) are generally lower than for CO, and much of the scatter in the observations about the best fit is attributed to differences in excitation conditions and optical depths among the detected lines. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Show less
Yıldız, U.; Kristensen, L.; Dishoeck, E.F. van; San Jose Garcia, I.; Karska, A.; Harsono, D.S.; ... ; Hogerheijde, M.R. 2013
Context. In the deeply embedded stage of star formation, protostars start to heat and disperse their surrounding cloud cores. The evolution of these sources has traditionally been traced through... Show moreContext. In the deeply embedded stage of star formation, protostars start to heat and disperse their surrounding cloud cores. The evolution of these sources has traditionally been traced through dust continuum spectral energy distributions (SEDs), but the use of CO excitation as an evolutionary probe has not yet been explored due to the lack of high-J CO observations. Aims: The aim is to constrain the physical characteristics (excitation, kinematics, column density) of the warm gas in low-mass protostellar envelopes using spectrally resolved Herschel data of CO and compare those with the colder gas traced by lower excitation lines. Methods: Herschel-HIFI observations of high-J lines of $^{12}$CO, $^{13}$CO, and C$^{18}$O (up to J$_u$ = 10, E$_u$ up to 300 K) are presented toward 26 deeply embedded low-mass Class 0 and Class I young stellar objects, obtained as part of the Water In Star-forming regions with Herschel (WISH) key program. This is the first large spectrally resolved high-J CO survey conducted for these types of sources. Complementary lower J CO maps were observed using ground-based telescopes, such as the JCMT and APEX and convolved to matching beam sizes. Results: The $^{12}$CO 10-9 line is detected for all objects and can generally be decomposed into a narrow and a broad component owing to the quiescent envelope and entrained outflow material, respectively. The $^{12}$CO excitation temperature increases with velocity from ~{}60 K up to ~{}130 K. The median excitation temperatures for $^{12}$CO, $^{13}$CO, and C$^{18}$O derived from single-temperature fits to the J$_u$ = 2-10 integrated intensities are ~{}70 K, 48 K and 37 K, respectively, with no significant difference between Class 0 and Class I sources and no trend with M$_{env}$ or L$_{bol}$. Thus, in contrast to the continuum SEDs, the spectral line energy distributions (SLEDs) do not show any evolution during the embedded stage. In contrast, the integrated line intensities of all CO isotopologs show a clear decrease with evolutionary stage as the envelope is dispersed. Models of the collapse and evolution of protostellar envelopes reproduce the C$^{18}$O results well, but underproduce the $^{13}$CO and $^{12}$CO excitation temperatures, due to lack of UV heating and outflow components in those models. The H$_{2}$O 1$_{10}$ - 1$_{01}$/CO 10-9 intensity ratio does not change significantly with velocity, in contrast to the H$_{2}$O/CO 3-2 ratio, indicating that CO 10-9 is the lowest transition for which the line wings probe the same warm shocked gas as H$_{2}$O. Modeling of the full suite of C$^{18}$O lines indicates an abundance profile for Class 0 sources that is consistent with a freeze-out zone below 25 K and evaporation at higher temperatures, but with some fraction of the CO transformed into other species in the cold phase. In contrast, the observations for two Class I sources in Ophiuchus are consistent with a constant high CO abundance profile. Conclusions: The velocity resolved line profiles trace the evolution from the Class 0 to the Class I phase through decreasing line intensities, less prominent outflow wings, and increasing average CO abundances. However, the CO excitation temperature stays nearly constant. The multiple components found here indicate that the analysis of spectrally unresolved data, such as provided by SPIRE and PACS, must be done with caution. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices C and D are available in electronic form at http://www.aanda.orgShow less
Context. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a... Show moreContext. Understanding the physical phenomena involved in the earlierst stages of protostellar evolution requires knowledge of the heating and cooling processes that occur in the surroundings of a young stellar object. Spatially resolved information from its constituent gas and dust provides the necessary constraints to distinguish between different theories of accretion energy dissipation into the envelope. Aims. Our aims are to quantify the far-infrared line emission from low-mass protostars and the contribution of different atomic and molecular species to the gas cooling budget, to determine the spatial extent of the emission, and to investigate the underlying excitation conditions. Analysis of the line cooling will help us characterize the evolution of the relevant physical processes as the protostar ages. Methods. Far-infrared Herschel-PACS spectra of 18 low-mass protostars of various luminosities and evolutionary stages are studied in the context of the WISH key program. For most targets, the spectra include many wavelength intervals selected to cover specific CO, H$_{2}$O, OH, and atomic lines. For four targets the spectra span the entire 55-200 {$μ$}m region. The PACS field-of-view covers ~{}47'' with the resolution of 9.4''. Results. Most of the protostars in our sample show strong atomic and molecular far-infrared emission. Water is detected in 17 out of 18 objects (except TMC1A), including 5 Class I sources. The high-excitation H$_{2}$O 8$_{18}$-7$_{07}$ 63.3 {$μ$}m line (E$_u$/k$_B$ = 1071 K) is detected in 7 sources. CO transitions from J = 14-13 up to J = 49 - 48 are found and show two distinct temperature components on Boltzmann diagrams with rotational temperatures of ~{}350 K and ~{}700 K. H$_{2}$O has typical excitation temperatures of ~{}150 K. Emission from both Class 0 and I sources is usually spatially extended along the outflow direction but with a pattern that depends on the species and the transition. In the extended sources, emission is stronger off source and extended on {amp}{ge}10,000 AU scales; in the compact sample, more than half of the flux originates within 1000 AU of the protostar. The H$_{2}$O line fluxes correlate strongly with those of the high-J CO lines, both for the full array and for the central position, as well as with the bolometric luminosity and envelope mass. They correlate less strongly with OH fluxes and not with [O I] fluxes. In contrast, [O I] and OH often peak together at the central position. Conclusions. The PACS data probe at least two physical components. The H$_{2}$O and CO emission very likely arises in non-dissociative (irradiated) shocks along the outflow walls with a range of pre-shock densities. Some OH is also associated with this component, most likely resulting from H$_{2}$O photodissociation. UV-heated gas contributes only a minor fraction to the CO emission observed by PACS, based on the strong correlation between the shock-dominated CO 24-23 line and the CO 14-13 line. [O I] and some of the OH emission probe dissociative shocks in the inner envelope. The total far-infrared cooling is dominated by H$_{2}$O and CO, with the fraction contributed by [O I] increasing for Class I sources. Consistent with previous studies, the ratio of total far-infrared line emission over bolometric luminosity decreases with the evolutionary state. Appendices A-J are available in electronic form at http://www.aanda.orgShow less
Context. The OH radical is a key species in the water chemistry network of star-forming regions, because its presence is tightly related to the formation and destruction of water. Previous studies... Show moreContext. The OH radical is a key species in the water chemistry network of star-forming regions, because its presence is tightly related to the formation and destruction of water. Previous studies of the OH far-infrared emission from low- and intermediate-mass protostars suggest that the OH emission mainly originates from shocked gas and not from the quiescent protostellar envelopes. Aims: We aim to study the excitation of OH in embedded low- and intermediate-mass protostars, determine the influence of source parameters on the strength of the emission, investigate the spatial extent of the OH emission, and further constrain its origin. Methods: This paper presents OH observations from 23 low- and intermediate-mass young stellar objects obtained with the PACS integral field spectrometer on-board Herschel in the context of the ''Water In Star-forming regions with Herschel'' (WISH) key program. Radiative transfer codes are used to model the OH excitation. Results: Most low-mass sources have compact OH emission ({lsim}5000 AU scale), whereas the OH lines in most intermediate-mass sources are extended over the whole 47.{Prime}0 { imes} 47.{Prime}0 PACS detector field-of-view ({gsim}20 000 AU). The strength of the OH emission is correlated with various source properties such as the bolometric luminosity and the envelope mass, but also with the [OI] and H$_{2}$O emission. Rotational diagrams for sources with many OH lines show that the level populations of OH can be approximated by a Boltzmann distribution with an excitation temperature at around 70 K. Radiative transfer models of spherically symmetric envelopes cannot reproduce the OH emission fluxes nor their broad line widths, strongly suggesting an outflow origin. Slab excitation models indicate that the observed excitation temperature can either be reached if the OH molecules are exposed to a strong far-infrared continuum radiation field or if the gas temperature and density are sufficiently high. Using realistic source parameters and radiation fields, it is shown for the case of Ser SMM1 that radiative pumping plays an important role in transitions arising from upper level energies higher than 300 K. The compact emission in the low-mass sources and the required presence of a strong radiation field and/or a high density to excite the OH molecules points toward an origin in shocks in the inner envelope close to the protostar. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are only available in electronic form at http://www.aanda.orgShow less
