CARMA LARGE AREA STAR FORMATION SURVEY: OBSERVATIONAL ANALYSIS OF FILAMENTS IN THE SERPENS SOUTH MOLECULAR CLOUD

The large-scale structure of the Serpens South molecular cloud is defined by parsec-scale filamentary structures around a central hub where a dense cluster of protostars lies. Figure 1 shows a color map of the CLASSy N₂H⁺ (J = 1 → 0) integrated emission, where the white contours represent the Herschel 350 μm continuum emission (25'' beam). The figure shows that the N₂H⁺ emission generally follows the dust emission detected with Herschel in the submillimeter, tracing the cold and dense gas in elongated structures commonly seen as filaments, and identified in this case by visually inspecting the N₂H⁺ integrated intensity map. The most prominent filament runs southeast of the hub (which we label FSE); there are also filaments due east (FE) and southwest (FSW; Figure 1). North of the hub, the high-resolution CLASSy N₂H⁺ observations clearly separate the main dusty filamentary structure into at least two separate filaments not resolved in the Herschel images: filaments FNE and FNW (Figure 1). Although most of the filaments are seen in both N₂H⁺ and dust emission, there are a few filaments only detected in dust (e.g., southwest or northeast structures on the map). We speculate that this can be due to the lower density of these filaments and/or a decrease in the N₂H⁺ abundance due to the reaction with increased CO abundances in regions of lower density and/or higher temperature (e.g., Bergin et al. 2001, 2002).

Analysis of the N₂H⁺ CLASSy data provides a detailed view of the morphology and kinematics of these filaments. In this Letter we focus on three filaments (FE, FSE, and FNW, Figure 1). The N₂H⁺ filaments are seen as elongated structures, with slightly curved morphologies (Figure 2(a)). The N₂H⁺ filament length is similar to that of its associated dusty filament, but its path is often separated by lower intensity emission. The widths of the N₂H⁺ filaments are narrower than those estimated from the Herschel data. In Figure 3(a) we compare the width of the dust and the N₂H⁺ (J = 1 → 0) integrated intensity emission of the FE filament by constructing an average intensity profile perpendicular to the filament main axis. A Gaussian fit to the N₂H⁺ intensity profile of this filament gives a FWHM = 0.035 pc, while a fit to the 350 μm dust continuum emission gives a FWHM = 0.110 pc. A similar analysis for the FN filament (not shown) also shows that the gas filament width is about one-half of the dusty filament width.

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Herschel observations of molecular cloud filaments find that the filament width distribution, for a sample of 278 filaments in different molecular clouds with varying distances, has a strong peak at 0.09 ± 0.04 pc (Arzoumanian et al. 2011; André et al. 2013). André et al. conclude that this characteristic width is a "quasi-universal" value, which implies that 0.1 pc must be an intrinsic scale of the filament formation process. Figure 3(a) shows how the N₂H⁺ filament profile convolved (violet dashed line) with the 350 μm Herschel beam matches quite well the dust profile (black line), while the 7'' CLASSy profile (green curve) shows a prominent centrally peaked filament. Why is the N₂H⁺ emission more peaked than the dust continuum?

There are key differences in how the dust continuum and N₂H⁺ emission trace the material in filaments. The dust continuum emission at 300–500 μm increases with increasing temperature and column density, with no dependence on density. The N₂H⁺ emission depends on density, N₂H⁺ column density, and gas temperature through the line excitation and opacity. Using the RADEX program (van der Tak et al. 2007) to calculate large(-scale) velocity gradient models with T_K = 12 K, we found that the N₂H⁺ J = 1 → 0 emission is a fairly linear function of N₂H⁺ column density for densities of 10⁵–10^6.3 cm⁻³. Below density 10⁵ cm⁻³, the N₂H⁺ column density needs to increase rapidly with decreasing density to emit a fixed line temperature. This means that the emission efficiency of the gas per unit path length decreases rapidly. In the context of a filament with fixed diameter, the observable emission will decrease unless the N₂H⁺ fractional abundance increases rapidly.

The chemistry of N₂H⁺ favors a drop in N₂H⁺ abundance with decreasing density (Bergin et al. 2001, 2002), which translates into a line emission drop. CO destroys N₂H⁺, so in regions less dense than the CO depletion threshold (2–6  ×  10⁴ cm⁻³, e.g., Tafalla et al. 2002), the abundance of N₂H⁺ decreases. For estimated N₂H⁺ abundances of 3–8 × 10⁻¹⁰ (e.g., Shirley et al. 2005), and a filament path length of 0.03 pc, gas at a density of 10⁵ cm⁻³ produces N₂H⁺ J = 1 → 0 lines of 1.5–4 K (at the observed FWHM of 0.35 km s⁻¹), comparable to the strongest observed lines. Hence the observations are consistent with the N₂H⁺ emission accentuated by the increase in density and abundance. The N₂H⁺ emission is therefore found as filamentary structures that are narrower than the dust filaments. Two alternatives are suggested. First, it could be that Herschel resolution is not enough to resolve the substructure inside filaments. Second, it could be that real filaments are broader than observed in N₂H⁺, but density, temperature, and abundance effects contribute to the different width measurements.

The above discussion relates to dust filaments composed of single N₂H⁺ filaments; there are other broad dust filaments composed of several narrower N₂H⁺ filaments. For example, the FSE filament is composed of three N₂H⁺ filaments in the southern part that are positioned quasi-parallel to each other and extend over the entire width of the dusty filament (see Figures 2 and 3(b)). A similar morphology is seen in the FE filament, which runs parallel to two shorter N₂H⁺ filaments south of it. This is analogous to the bundles found by Hacar et al. (2013) within filaments in a region of the Taurus molecular cloud, which were only resolved in radial velocity (they lacked the resolution to spatially resolve them). The structures presented here are spatially resolved and their paths clearly distinguished and, unlike the bundles found by Hacar et al., most of the quasi-parallel N₂H⁺ filaments within a dust filament have similar radial velocities.

These results indicate that the dust filaments observed by Herschel do not always represent one uniform structure within physical filaments, but perhaps are related filaments along the line of sight that project into the appearance of a single filament. A more robust intensity profile analysis along with further high angular resolution line and continuum observations of more dust filaments in other regions are needed in order to determine if filaments have a common width and the frequency of subfilaments within filaments.

The CLASSy N₂H⁺ data also probe the gas kinematics, therefore adding a dynamical perspective to the analysis of these structures. An analysis of the line-of-sight velocity centroid map reveals that the filaments present smooth line-of-sight velocity variations. Some filaments display slight velocity gradients along their length (e.g., FSE and FE; Figure 1) while others do not (e.g., FNW). The strength of the observed gradients (∼1 km s⁻¹ pc⁻¹), corresponds to a crossing time of about a million of years. Line-of-sight velocity gradients along molecular cloud filaments have been identified in Serpens South and in other regions as well (e.g., Peretto et al. 2014). For example, Kirk et al. (2013) also detected a velocity gradient in FSE (0.9 km s⁻¹ pc⁻¹ assuming d = 415 pc), which is consistent with our measured FSE gradient of 0.8 km s⁻¹ pc⁻¹. In many cases, these gradients have been interpreted as evidence of material flowing inside the filaments, infalling toward the hubs or filamentary junctions—regions of active star formation. The typical velocity gradient ranges between 0.2–2 km s⁻¹ pc⁻¹, coinciding with our estimate of the gradients along the FSE and FE filaments in Serpens South. Although the observed velocities are consistent with those expected from gravitational infall, given the current data this is not the only possible (nor necessarily the preferred) explanation for the observed gradients. Most importantly, not all filaments in Serpens South show gradients along their length (see, for instance, the FNW filament in Figure 1), contrary to what is expected if the gravitational pull from the relatively massive hub is the main cause for the gradient. Moreover, even filaments with gradients do not show the symmetrical high velocity infall signature (e.g., Filament A of Figure 2 in Moeckel & Burcket 2014). A simple alternative for a gradient in radial velocity along a filament's length is that it is the mere projection effect in an elongated structure, perhaps originating from cloud-scale gas turbulence (N. Moeckel 2014, private communication). In this scenario, parsec-scale turbulent eddies of gas can produce smooth radial velocity gradients in structures with sizes comparable to the eddies. Another possibility is that the velocity variations in the filaments are due to the convergence of two or more flows (Csengeri et al. 2011), and even more sophisticated alternative can include oscillations inside the filament of the material produced by over-densities or starless cores within them (Hacar et al. 2013).

While some velocity gradients are parallel to the main length of the filament, the CLASSy N₂H⁺ data also show the existence of velocity gradients perpendicular to several filaments. These gradients are, in several cases, detected along the entirety of the filament. The velocity centroid map of the FNW and FE filaments display these gradients (Figures 1 and 2(b)). Figure 4 shows both the gradient perpendicular to the FE filament (11.9 km s⁻¹ pc⁻¹) and the gradient along the filament (0.9 km s⁻¹ pc⁻¹). Remarkably, one gradient is an order of magnitude larger than the other, indicating that the gas crossing time across the filament is 10 times shorter than along the filament (note that gas crossing time is estimated as the time that takes a given gas parcel to travel along or across the filament); this suggests the more dynamical nature of the velocity gradient across (perpendicular to) the filament than the one along (parallel to) the filament.

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To better understand perpendicular line-of-sight velocity gradients in a handful of filaments in Serpens South and other clouds as well, the data are being analyzed and compared against numerical simulations of filament formation via self-gravity in gas that has already been compressed into a locally planar structure by supersonic converging flows (e.g., Gong & Ostriker 2011, Chen & Ostriker 2014); the results will be published elsewhere (L. G. Mundy et al., in preparation). In these simulations, the velocity gradients are the result of the in-plane motions associated with forming and growing the filament. Gradients across filaments have already been observed in the DR21 filament (Schneider et al. 2010), but in that case, the filament width is about 1 pc, which is more than a factor of 10 wider than the filaments we are analyzing in Serpens South. Therefore, the nature of the gradients across the Serpens South FE and FNW filaments is likely different from those found in DR21.

More generally, gradients across filaments can be thought of as projection effects of kinematics (due to turbulence, self-gravity, or other forces; Schneider et al. 2010; Chen & Ostriker 2014) within a flat structure inclined with respect to the plane of the sky (but see Li & Goldsmith 2012). An alternative explanation is that two or more narrower filaments with slightly different radial velocities partially overlap on the plane of the sky forming the perpendicular velocity gradient. In any case, these perpendicular gradients are not consistent with the picture in which gradients in filaments trace the flow of material along filaments toward the hub, as the local dynamical evolution will occur on a much faster time than the inward flow timescale.