M. Tafalla

Systematic Molecular Differentiation in Starless Cores

We present evidence that low-mass starless cores, the simplest units of star formation, are systematically differentiated in their chemical composition. Some molecules, including CO and CS, almost vanish near the core centers, where the abundance decreases by at least 1 or 2 orders of magnitude with respect to the value in the outer core. At the same time, the N2H+ molecule has a constant abundance, and the fraction of NH3 increases toward the core center. Our conclusions are based on a systematic study of five mostly round starless cores (L1498, L1495, L1400K, L1517B, and L1544), which we have mapped in C18O (1-0), CS (2-1), N2H+ (1-0), NH3 (1, 1) and (2, 2), and the 1.2 mm continuum [complemented with C17O (1-0) and C34S (2-1) data for some systems]. For each core we have built a spherically symmetric model in which the density is derived from the 1.2 mm continuum, the kinetic temperature is derived from NH3, and the abundance of each molecule is derived using a Monte Carlo radiative transfer code, which simultaneously fits the shape of the central spectrum and the radial profile of integrated intensity. Regarding the cores for which we have C17O (1-0) and C34S (2-1) data, the model fits these observations automatically when the standard isotopomer ratio is assumed. As a result of this modeling, we also find that the gas kinetic temperature in each core is constant at approximately 10 K. In agreement with previous work, we find that if the dust temperature is also constant, then the density profiles are centrally flattened, and we can model them with a single analytic expression. We also find that for each core the turbulent line width seems constant in the inner 0.1 pc. The very strong abundance drop of CO and CS toward the center of each core is naturally explained by the depletion of these molecules onto dust grains at densities of (2-6) × 104 cm-3. N2H+ seems unaffected by this process up to densities of several times 105 cm-3, or even 106 cm-3, while the NH3 abundance may be enhanced by its lack of depletion and by reactions triggered by the disappearance of CO from the gas phase. With the help of the Monte Carlo modeling, we show that chemical differentiation automatically explains the discrepancy between the sizes of CS and NH3 maps, a problem that has remained unexplained for more than a decade. Our models, in addition, show that a combination of radiative transfer effects can give rise to the previously observed discrepancy in the line width of these two tracers. Although this discrepancy has been traditionally interpreted as resulting from a systematic increase of the turbulent line width with radius, our models show that it can arise in conditions of constant gas turbulence.

CO DEPLETION IN THE STARLESS CLOUD CORE L1544

We present evidence for CO depletion toward the starless cloud core L1544. A comparison between C17O and the 1.3 mm continuum dust emission shows that CO is depleted by a factor of ~10 at the dust peak. Our observations are consistent with a model in which CO is condensed out onto dust grains at densities above nd ~ 105 cm-3. The corresponding radius of the depleted region is rd ~ 6500 AU, and we find that this depletion causes 2.3 M☉ of gas to be lost to view in molecular line emission. Optically thin high-density tracers, such as HC18O+ and D13CO+, show double-peaked profiles which suggest that we are observing superposed emission from the foreground and background undepleted layers with density below nd. We conclude from our data that the core is probably young (~104 yr old) and collapsing. For the component at VLSR = 7.1 km s-1 in this line of sight, we estimate [DCO+]/[HCO+] = 0.12 ± 0.02, which is larger by a factor of order 2 than values derived in other dense cloud cores.

Molecular Ions in L1544. II. The Ionization Degree

The maps presented in Paper I are here used to infer the variation of the column densities of HCO+, DCO+, N2H+, and N2D+ as a function of distance from the dust peak. These results are interpreted with the aid of a crude chemical model that predicts the abundances of these species as a function of radius in a spherically symmetric model with radial density distribution inferred from the observations of dust emission at millimeter wavelengths and dust absorption in the infrared. Our main observational finding is that the N(N2D+)/N(N2H+) column density ratio is of order 0.2 toward the L1544 dust peak as compared to N(DCO+)/N(HCO+) = 0.04. We conclude that this result, as well as the general finding that N2H+ and N2D+ correlate well with the dust, is caused by CO being depleted to a much higher degree than molecular nitrogen in the high-density core of L1544. Depletion also favors deuterium enhancement, and thus N2D+, which traces the dense and highly CO-depleted core nucleus, is much more enhanced than DCO+. Our models do not uniquely define the chemistry in the high-density depleted nucleus of L1544, but they do suggest that the ionization degree is a few times 10-9 and that the ambipolar diffusion timescale is locally similar to the free-fall time. It seems likely that the lower limit, which one obtains to ionization degree by summing all observable molecular ions, is not a great underestimate of the true ionization degree. We predict that atomic oxygen is abundant in the dense core and, if so, H3O+ may be the main ion in the central highly depleted region of the core.

A Search for Infall Motions toward Nearby Young Stellar Objects

We report observations of 47 candidate protostars in two optically thick lines [H2CO (212-111) and CS (2-1)] and one optically thin line [N2H+ (1-0)] using the IRAM 30 m, SEST 15 m, and Haystack 37 m radio telescopes. The sources were selected for the redness of their spectra (Tbol < 200 K) and their near distance (d < 400 pc). Most of the sources have asymmetric, optically thick lines. The observed distribution of velocity differences, ?V = (Vthick - Vthin)/?Vthin, is skewed toward negative (blueshifted) velocities for both the H2CO and CS samples. This excess is much more significant for class 0 than for class I sources, suggesting that we detect infall motions toward class 0 and not toward class I sources. This indicates a difference in the physical conditions in the circumstellar envelopes around class I and class 0 sources, but it does not rule out the presence of infall onto class I sources by, for example, lower opacity gas. Bipolar outflows alone, or rotation alone, cannot reproduce these statistics if the sample of sources has randomly oriented symmetry axes. We identify 15 spectroscopic infall candidates, six of which are new. Most of these infall candidates have primarily turbulent rather than thermal motions and are associated with clusters rather than being isolated.

Probing the Evolutionary Status of Starless Cores through N2H+ and N2D+ Observations

We have undertaken a survey of N2H+ and N2D+ toward 31 low-mass starless cores using the IRAM 30 m telescope. Our main objective has been to determine the abundance ratio of N2D+ and N2H+ toward the nuclei of these cores and thus to obtain estimates of the degree of deuterium enrichment, a symptom of advanced chemical evolution according to current models. We find that the N(N2D+)/N(N2H+) ratio is larger in more centrally concentrated cores with larger peak H2 and N2H+ column density than the sample mean. The deuterium enrichment in starless cores is presently ascribed to depletion of CO in the high density (>3 × 104 cm-3) core nucleus. To substantiate this picture, we compare our results with observations in dust emission at 1.2 mm and in two transitions of C18O. We find a good correlation between deuterium fractionation and N(C18O)/N(H2)1.2 mm for the nuclei of 14 starless cores. We thus identified a set of properties that characterize the most evolved, or prestellar, starless cores. These are higher N2H+ and N2D+ column densities, higher N(N2D+)/N(N2H+), more pronounced CO depletion, broader N2H+ lines with infall asymmetry, higher central H2 column densities, and a more compact density profile than in the average core. We conclude that this combination of properties gives a reliable indication of the evolutionary state of the core. Seven cores in our sample (L1521F, Oph D, L429, L694, L183, L1544, and TMC 2) show the majority of these features and thus are believed to be closer to forming a protostar than are the other members of our sample. Finally, we note that the subsample of Taurus cores behaves more homogeneously than the total sample, an indication that the external environment could play an important role in the core evolution.

L1544: A Starless Dense Core with Extended Inward Motions

We present a multiline study of the dense core L1544 in the Taurus molecular complex. Although L1544 does not harbor an embedded star, it presents several characteristics of cores that have already undergone star formation, suggesting that it may be rather advanced in its evolution toward becoming a star-forming core. The spectral lines from L1544 present an interesting dichotomy, with the thick dense gas tracers su†ering very strong self absorption while CO and its isotopes are not being absorbed at all. The presence of the self absorptions allows us to study both the density structure and kinematics of the gas in detail. A simple analysis shows that the core is almost isothermal and that the self absorptions are due to very subthermal excitation of the dense gas tracers in the outer layers. The density has to decrease outward rapidly, and a detailed radiative transfer calculation that simultaneously -ts three iso- topes of CO and two of CS shows that the density approximately follows a r~1.5 power law. The self absorptions, in addition, allow us to measure the relative velocity between the inner and outer layers of the core, and we -nd that there is a global pattern of inward motions (background and foreground approaching each other). The relative speed between the foreground and background changes with posi- tion, and we use a simple two-layer model to deduce that while the foreground gas has a constant veloc- ity, the background material presents systematic velocity changes that we interpret as arising from two velocity components. We explore the origin of the inward motions by comparing our observations with models of gravitational collapse. A model in which the infall starts at the center and propagates outward (as in the inside-out collapse of Shu) is inconsistent with the large extension of the absorption (that sug- gests an advanced age) and the lack of a star at the core center (that suggests extreme youth). Ambipolar di†usion seems also ruled out because of the large amount of the inward speed (up to 0.1 km s~1) and the fact that ionized species move with speeds similar to those of the neutrals. Other infall models seem also to have problems -tting the data, so if L1544 is infalling, it seems to be doing so in a manner not contemplated by the standard theories of star formation. Our study of L1544 illustrates how little is still known about the physical conditions that precede star formation and how detailed studies of starless cores are urgently needed. Subject headings: ISM: individual (L1544) E ISM: kinematics and dynamics E stars: formation

A Simple Model of Spectral-Line Profiles from Contracting Clouds

A simple analytic model of radiative transfer in two parts of a contracting cloud matches a wide range of line profiles in candidate infall regions and provides a sensitive estimate of Vin, the characteristic inward speed of the gas forming the line. The model assumes two uniform regions of equal temperature and velocity dispersion σ, whose density and velocity are attenuation-weighted means over the front and rear halves of a centrally condensed, contracting cloud. The model predicts two-peak profiles for slow infall, Vin σ, and red-shoulder profiles for fast infall, Vin ~ σ. A simple formula expresses Vin solely in terms of σ and of observable parameters of a two-peak line. We apply the model to fit profiles of high and low optical depth lines observed in a dense core with no star (L1544, Vin = 0.006 km s-1), with an isolated protostar (L1527, 0.025 km s-1), and with a small group of stars (L1251B, 0.35 km s-1). The mass infall rate obtained from Vin and the map size varies from (2-40) × 10-6 M☉ yr-1 and agrees within a factor ~2 in each core with the independently determined rate ~σ3 G-1 for a gravitationally collapsing isothermal sphere. This agreement suggests that the inward motions derived from the line profiles are gravitational in origin.

A Survey of Infall Motions toward Starless Cores. I. CS (2-1) and N2H+ (1-0) Observations

We present the first results of a survey of 220 starless cores selected primarily by their optical obscuration and observed in CS (2-1), N2H+ (1-0), and C18O (1-0) using the Northeast Radio Observatory Corporation (NEROC) Haystack 37 m telescope. We have detected 163 out of 196 sources observed in CS, 72 out of 142 in N2H+, and 30 out of 30 in C18O. In total, 69 sources were detected in both CS and N2H+. The isolated component of the N2H+ (1-0) spectrum (F1F = 0,1-1,2) usually shows a weak symmetric profile that is optically thin. In contrast, a significant fraction of the CS spectra show non-Gaussian shapes, which we interpret as arising from a combination of self-absorption due to lower excitation gas in the core front and kinematics in the core. The distribution of the normalized velocity difference (δVCS) between the CS and N2H+ peaks appears significantly skewed to the blue (δVCS < 0), as was found in a similar study of dense cores with embedded young stellar objects (YSOs). The incidence of sources with blue asymmetry tends to increase as the total optical depth or the integrated intensity of the N2H+ line increases. This overabundance of blue over red sources suggests that inward motions are a significant feature of starless cores. We identify seven strong infall candidates and 10 probable infall candidates. Their typical inward speeds are subsonic, approximately 0.04-0.1 km s-1, so they contain thermal infall motions, unlike the faster inward speeds associated with most YSOs. We discuss the importance of the choice of a consistent set of line frequencies when using the velocity shift between an optically thick and a thin line as a tracer of infall, and show how the results of the survey depend on that frequency choice.

A Survey for Infall Motions toward Starless Cores. II. CS (2-1) and N2H+ (1-0) Mapping Observations

We present the results of an extensive mapping survey of starless cores in the optically thick line of CS (2-1) and the optically thin lines of N2H+ (1-0) and C18O (1-0). The purpose of this survey was to search for signatures of extended inward motions. A total of 53 targets were observed in the three lines with the FCRAO 14 m telescope. Thirty-three regions were mapped in both CS and N2H+, and thirty seven well-defined N2H+ cores have been identified. The N2H+ emission is generally compact enough to find a peak, while the CS and C18O emissions are more diffuse. For each core, we have derived the normalized velocity difference (?VCS) between the thick CS and thin N2H+ peak velocities. We define 10 strong and nine probable infall candidates, based on ?VCS analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the ?VCS parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple two-layer radiative transfer model to fit the profiles, we derive one-dimensional infall speeds, the values of half of which lie in the range of 0.05-0.09 km s-1. These values are similar to those found in L1544 by Tafalla et al., and this result confirms that infall speeds in starless cores are generally faster than expected from ambipolar diffusion in a strongly subcritical core. In addition, the observed infall regions are too extended to be consistent with the inside-out collapse model applied to a very low mass star. In the largest cores, the spatial extent of the CS spectra with infall asymmetry is larger than the extent of the N2H+ core by a factor of 2-3. All these results suggest that extended inward motions are a common feature in starless cores, and that they could represent a necessary stage in the condensation of a star-forming dense core.

Molecular Ions in L1544. I. Kinematics

We have mapped the dense dark core L1544 in H13CO+ (1-0), DCO+ (2-1), DCO+ (3-2), N2H+ (1-0), N2H+ (3-2), N2D+ (2-1), N2D+ (3-2), C18O (1-0), and C17O (1-0) using the IRAM 30 m telescope. We have obtained supplementary observations of HC18O+ (1-0), HC17O+ (1-0), and D13CO+ (2-1). Many of the observed maps show a general correlation with the distribution of dust continuum emission, in contrast to C18O (1-0) and C17O (1-0), which give clear evidence for depletion of CO at positions close to the continuum peak. In particular, N2D+ (2-1) and (3-2) and to a lesser extent N2H+ (1-0) appear to be excellent tracers of the dust continuum. Our DCO+ maps have the same general morphology as the continuum while H13CO+ (1-0) is more extended. We find also that many apparently optically thin spectral lines such as HC18O+ and D13CO+ have double or highly asymmetric profiles toward the dust continuum peak. We have studied the velocity field in the high-density nucleus of L1544, putting particular stress on tracers such as N2H+ and N2D+, which trace the dust emission and which we therefore believe trace the gas with density of order 105 cm-3. We find that the tracers of high-density gas (in particular, N2D+) show a velocity gradient along the minor axis of the L1544 core and that there is evidence for larger line widths close to the dust emission peak. We interpret this using the model of the L1544 proposed by Ciolek and Basu and by comparing the observed velocities with those expected on the basis of their model. The results show reasonable agreement between observations and model in that the velocity gradient along the minor axis and the line broadening toward the center of L1544 are predicted by the model. This is evidence in favor of the idea that ambipolar diffusion across field lines is one of the basic processes leading to gravitational collapse. However, the double-peaked nature of the profiles is reproduced only at the core center and if a hole in the molecular emission, due to depletion, is present. Moreover, line widths are significantly narrower than observed and are better reproduced by the Myers & Zweibel model, which considers the quasi-static vertical contraction of a layer due to dissipation of its Alfvenic turbulence, indicating the importance of this process for cores on the verge of forming a star.