Free Access
Issue
A&A
Volume 555, July 2013
Article Number A106
Number of page(s) 7
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201220861
Published online 09 July 2013

© ESO, 2013

1. Introduction

The current paradigm of cluster formation starts with a highly turbulent cloud in which dense clumps1 are close to virial equilibrium (e.g., McKee & Ostriker 2007; Bate 2009). This turbulence will decay in a free-fall time if there is no driving mechanism. Theoretical studies show that protostellar outflows might be one mechanism capable to drive turbulence in the cloud (e.g. Li & Nakamura 2006), which agrees with some observational results (Palau et al. 2007; Fontani et al. 2012). However, other studies have shown that outflows might not have enough energy and momentum to sustain turbulence (Arce et al. 2010), while shells driven by young stars might (Arce et al. 2011).

NGC 2264, located ~800 pc away (between 750−913 pc, Dahm & Simon 2005; Baxter et al. 2009; Naylor & Mayne 2010), is a well-studied cluster in which we can study the role of turbulence in star formation. Within this cluster there is a rich grouping of protostars, known as NGC 2264 D or as the Spokes cluster (we use both nomenclatures interchangeably in this paper). This region has been observed at (sub)millimeter wavelengths with JCMT/SCUBA and IRAM/30m, where several dense cores were identified (Ward-Thompson et al. 2000; Williams & Garland 2002; Wolf-Chase et al. 2003; Peretto et al. 2006). Spitzer observations of the region revealed an agglomeration of deeply embedded protostars (Class 0/I sources) identified primarily at 24 μm. Teixeira et al. (2006), using a nearest-neighbor analysis, found that these protostars have a characteristic projected spacing, 20   ±   5″, which is consistent with the thermal Jeans length of the region, 27″ or 0.104 pc at the distance of 800 pc (calculated using the mean filament density of 3   ×   104 cm-3 from Williams & Garland 2002; and temperature of 17 K from Ward-Thompson et al. 2000). Since the protostars are embedded in dense material, and the virial mass of the system is lower than the total gas and dust mass (Williams & Garland 2002; Wolf-Chase et al. 2003; Peretto et al. 2006), the protostars are bound to the filaments and are very likely still tracing the primordial substructure of the cluster. The measured regular spacing (consistent with the thermal Jeans length) of the protostars is thus interpreted as a fossil signature of thermal fragmentation of the parental filament (Teixeira et al. 2006).

Peretto et al. (2006) also observed the 15 cores in the Spokes cluster using dust continuum emission. These dense cores present a mean H2 density ranging between 105 and 106 cm-3, and masses of 1.8 to 17.3 M (Wolf-Chase et al. 2003; Peretto et al. 2006). Direct comparison with the Spitzer 24 μm image shows that the cores identified by Peretto et al. (2006) have 24 μm Spitzer counterparts (i.e., they harbor protostars) except for DMM-8 and DMM-11. Peretto et al. (2006) also observed these 15 dense cores in N2H+ (1–0), which traces high-density gas (2 × 105 cm-3), and found only lines with supersonic velocity dispersions (Mach ~3), which is at odds with the interpretation of thermal Jeans fragmentation. Follow-up observations by Teixeira et al. (2007) using the Submillimeter Array (Ho et al. 2004) of one of the cores, DMM-1, showed that it had fragmented into seven compact 1.3 mm sources, of masses ranging between 0.4 M and 1.2 M. The mean separation of these sources within DMM-1 is consistent with the thermal Jeans length of the core, which indicates that the core underwent thermal fragmentation.

The apparent disagreement between the observation of supersonic turbulence in the dense gas and the thermal fragmentation, suggested by the spatial separation of the young stellar objects (YSOs), can be tested. If thermal fragmentation indeed took place in the star formation process in this cluster, low levels of turbulence should still be seen using a higher-density tracer. The Spokes cluster contains high-density gas over an extended region, which could explain the N2H+ (1–0) observations by Peretto et al. (2006), which show supersonic turbulence in a dense gas tracer. These observations might not be tracing the densest gas in the star-forming cores, but instead it traces the dense environment containing the entire region. This interpretation can be readily tested with new observations using an even higher-density tracer at similar angular resolutions: N2H+ (3–2) or N2D+ (3–2). Moreover, the [N2D+/N2H+] abundance ratio is enhanced in the densest and coldest regions of cores (Caselli 2002), and it decreased after the protostar was formed (Emprechtinger et al. 2009). Therefore, the N2D+ (3–2) observations trace more pristine conditions than N2H+ (3–2).

A decrease in the velocity dispersion is expected when going to higher-density tracers, as shown by Myers (1983), Goodman et al. (1998), and Fuller & Myers (1992) in regions with lower average densities. Moreover, Pineda et al. (2010) recently presented observations of dense gas as traced by NH3 (1, 1) where a sharp transition from supersonic to subsonic turbulence is seen in the velocity dispersion. This result suggests that to form low-mass stars, the turbulence needs to be dissipated before the star can be formed. It is possible that in more massive and denser environments this transition would occur at higher densities, and therefore higher-density tracers need to be used. Foster et al. (2009) showed, from their observations of the dense cores in the Perseus molecular cloud, that the cores have subsonic NH3 (1, 1) linewidths regardless of their stellar content or cluster environment. Moreover, even though the effect of the molecular outflows is important in the dynamics of the low-density gas, the dense gas is only locally affected (Pineda et al. 2010, 2011, and in prep.).

Therefore, by observing the Spokes cluster in a denser-gas tracer than N2H+ (1–0), we can determine whether the emission arising from denser regions is less turbulent. This could provide a test of the thermal-fragmentation hypothesis of the quiescent gas.

2. Data

The observations of N2H+ and N2D+ (3–2) molecular lines were carried out on 5 September and 4, 8−9, and 11−13 November 2011 with APEX. Every dense core in the Spokes cluster identified by Peretto et al. (2006) that presented a good detection of the N2H+ (1–0) line (14 cores) was observed in N2H+ (3–2) using the APEX-2 receiver of the Swedish Heterodyne Facility Instrument (SHeFI; Vassilev et al. 2008). The cores with the narrowest N2H+ (1–0) lines (eight cores) were also observed in N2D+ (3–2) using the APEX-1 receiver of the SHeFI (Vassilev et al. 2008).

Figure 1 shows a JCMT/SCUBA 450 μm map of the Spokes cluster, including the locations of the previously identified protostars and our APEX observations. A summary of the sources is listed in Table 1. The observed transitions, their spectroscopic properties, and observational parameters are listed in Table 2. We use the following nomenclature for source identification: MM??, where “??” is the source number (e.g., source D-MM1 of Peretto et al. (2006) is referred to as MM01 hereafter).

The observations used position-switching, with the off-position at (αJ2000 = 06h40m52.57s, δJ2000 = 09°32′36.5″). They were calibrated by regularly measuring the sky brightness and cold/hot loads in the cabin. All observations used the eXtended bandwidth Fast Fourier Transform Spectrometer (XFFTS), which provides two 2.5 GHz units, with 32 768 channels each unit and 1 GHz overlap. This configuration provides an spectral resolution of 0.082 km   s-1 and 0.099 km   s-1 for N2H+ and N2D+ (3–2), respectively.

All spectra were reduced using CLASS902. Each spectrum had a first- or second-order polynomial fitted to remove the baseline before averaging all scans.

thumbnail Fig. 1

SCUBA 450 μm dust-continuum map for the Spokes cluster (Wolf-Chase et al. 2003). The observed dense cores identified in Peretto et al. (2006) are shown by the circles at their centers (color-coded according to which molecular line tracers were used in their observation). For comparison, the APEX beams corresponding to our observations are also shown in the lower right corner (following the same color-code convention).

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Table 1

List of observed sources.

Table 2

Observed transitions and observational parameters.

3. Results

We detected N2H+ (3–2) and N2D+ (3–2) emission toward all 14 and 8 observed cores respectively. The N2H+ (3–2) and N2D+ (3–2) spectra are shown in Figs. 2 and 3, respectively. In addition, we overplot on these figures the best-fit profiles to the lines.

thumbnail Fig. 2

Observed N2H+ (3–2) spectra toward the sources. The red curve is the best-fit model to the observations, whose fit parameters are shown in Table 3.

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thumbnail Fig. 3

Observed N2D+ (3–2) spectra toward the sources. The red curve is the best-fit model to the observations, whose fit parameters are shown in Table 3.

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The N2H+ and N2D+ (3–2) lines were fitted in IDL3 using a forward-fitting model that takes into account all hyperfine components, with the minimization performed using the mpfitfun program (Markwardt 2009). For a given centroid velocity (vLSR), velocity dispersion (σv), and total optical depth (τmain) we computed the optical depth at each velocity as (1)where wi and vi are the relative weight and offset velocity for each hyperfine component. The model spectrum is then generated as (2)where TMB is the main-beam temperature, ηf is the filling fraction of the emission in the beam, Tex is the excitation temperature, Tbg is the background brightness temperature (2.73 K), and

Since the extent of the emission is unknown and it is unclear if it is more or less extended than the continuum emission, we assumed a filling fraction of unity, ηf = 1. Therefore, Tex is obtained from the fit when τmain is well constrained. We also compared the results of fitting the spectra using the HFS mode within CLASS90, where we obtained the same results within the fitted parameter uncertainties. This procedure usually returns well-constrained parameters, but when the total optical depth is poorly constrained (τ < 3στ), we kept the excitation temperature as a fixed parameter. For the N2H+ (3–2) line, the excitation temperature was set to the average value obtained from all other spectra, i.e., 6.7 K, because this average excitation temperature provides the typical excitation conditions in the region. For N2D+ (3–2), the fit is only poorly constrained and therefore the excitation temperature was set to the value obtained from the N2H+ (3–2) fit, because both transitions are optically thin and have similar critical densities. This procedure is similar to that followed in Crapsi et al. (2005).

For most of the cores the fits are good, but there are two cores (MM03 and MM13) that present two distinct components along the line-of-sight. In this case two independent lines were fitted simultaneously, and each of the two components was labeled “a” or “b”. There are two other cores, MM08 and MM04, which show a clear asymmetry in the line profile that is suggestive of an additional component along their line-of-sight. Unfortunately, we were unable to separate the blended components. Table 3 shows the results of the hyperfine fitting procedure to the N2H+ (3–2) and N2D+ (3–2) lines of all sources.

Table 3

Best-fit line parameters for sources in NGC 2264-D.

The column densities were calculated using the constant excitation temperature approximation and that the beam filling factor is 1 for both lines, see Eqs. (A.1) and (A.2) from Appendix A, which are based on Caselli et al. (2002).

Table 4

Column densities.

Figure 4 compares the velocity dispersions obtained from the different molecular lines that we observed and the N2H+ (1–0) velocity dispersions reported in Peretto et al. (2006). The velocity dispersion, σv, has a thermal, σT, and a nonthermal, σNT, component and is given as . Each panel also indicates, for comparison purposes, expected velocity dispersions whose nonthermal components are σNT = cs(10   K) (dashed lines) and σNT = 2   cs(10   K) (dotted lines). Figure 4a shows the comparison of the velocity dispersion obtained from the two different transitions N2H+ (1–0) and (3–2); the plot clearly shows that there is a trend for the velocity dispersion to be narrower for the higher-J transition. Figure 4b shows the comparison of the velocity dispersions of the N2H+ and N2D+ (3–2) molecular lines. From this plot we see that the velocity dispersions of these two high-density tracers agree well.

thumbnail Fig. 4

Comparison of the velocity dispersion of different molecular lines. Cores listed as having two components (see Table 3) are shown as red symbols, cores that appear to have blended components are shown in black, and the rest are shown as blue symbols. Dashed and dotted lines show the expected velocity dispersion for the cases where the nonthermal velocity dispersion, σNT, equals cs(10   K) or 2   cs(10   K), where cs(10   K) is the sound speed of the average molecule at 10 K (for a mean molecular weight of μm = 2.33).

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4. Discussion and conclusion

As stated in Sect. 1, we aimed to test whether the previous N2H+ (1–0) observations of the Spokes cluster were in fact probing the larger extended emission and not the densest gas in the star-forming cores. If this scenario is correct, we would expect the linewidths of higher-J transition emission, tracing higher-density gas, to be narrower. The results presented in the previous section indicate that the denser gas does indeed present narrower linewidths. For N2H+ (1–0) lines, as previously discussed in Peretto et al. (2006), the nonthermal velocity dispersions are supersonic and in most of them the σNT > 2   cs(10  K). However, in the higher-density gas, traced by N2H+ (3–2), the nonthermal velocity dispersion is lower than that of N2H+ (1–0). In fact, the N2H+ (3–2) velocity dispersions are ≈70% of the value derived from N2H+ (1–0), and more than half of the total sample shows σNT ≤ 2   cs(10  K). There are, however, three outlier data points where the velocity dispersion derived from N2H+ (3–2) is higher than those from N2H+ (1–0). Close inspection reveals that a) one of these outliers corresponds to a poorly constrained second component (MM03b); b) another outlier presents asymmetries in the line profile that clearly suggests the presence of a second unresolved component along the line-of-sight (MM08), and finally; c) the remaining outlier data point is obtained from the spectrum with the lowest signal-to-noise ratio with a large associated uncertainty (MM11). Finally, the velocity dispersions obtained with N2D+ (3–2) either agree with those obtained with N2H+ (3–2) or are lower. It is important to note there are two cores in our sample that show velocity dispersions consistent (within the 1-σ uncertainties) with a sonic nonthermal component: MM03a in N2H+ (3–2) and MM08 in N2D+ (3–2).

These results thus support the thermal-fragmentation scenario presented in Teixeira et al. (2006). Although the nonthermal velocity dispersions here reported are still broad, they are much narrower than those found previously using N2H+ (1–0) single-dish observations (Peretto et al. 2006). It also is important to note that these single-dish observations have a coarse angular resolution (≈17   840   AU = 0.086   pc at 800 pc), and the substructure or several components along the line-of-sight could increase the observed linewidth. It is clear that more high-angular resolution interferometric observations using high-density tracers are needed to truly assess the kinematics and substructure within NGC 2264-D.

Deuteration can be used as a proxy for the chemical evolution of a core (Crapsi et al. 2005; Emprechtinger et al. 2009; Fontani et al. 2011). For prestellar cores, the deuteration fraction increases as these cores evolve, reaching a maximum at the onset of star formation; the deuteration fraction of the core then begins to decrease as the protostar evolves. For this reason, a high-deuteration fraction in a protostellar core is a particular indicator of youth of the protostar, and a high-deuteration fraction of a prestellar core is an indicator that the core may soon collapse to form a protostar. Table 4 shows that there are four cores (MM03, MM05, MM11, and MM12) with a deuteration fraction at least a factor of 3 higher than the rest of the sample, where the largest difference is a factor of 125. Three of these cores are spatially separated from the main cluster (see Fig. 1) and are embedded in a less dense region. Furthermore, Table 1 shows that no protostar is identified in MM11, and both MM05 and MM12 have faint 24 μm sources that have no counterparts at other infrared wavelengths, meaning that these two cores could potentially be harboring either very young or very low mass protostars (additional investigation is necessary to confirm the protostellar nature of MM05 and MM12). Our results thus suggest that these three cores are the least evolved in the Spokes cluster, i.e., closest to the very earliest star formation evolutionary phase. Friesen et al. (2010) found similar results in their analysis of the cores in the Ophiuchus B2 region, where the deuteration fraction decreases with proximity to protostellar cores. The spectrum of core MM03, which is located within the dense region of the Spokes cluster, shows two components, as mentioned. Additional investigation, requiring higher angular and spectral resolution data, is needed to understand the deuteration fraction in this particular core. Moreover, we found no correlation between the level of deuteration and velocity dispersion. Although these few data points make this result uncertain, they agree with what was found in Friesen et al. (2013) in a larger study of nearby low-mass cores.

5. Summary

We summarize our main results and conclusions as follows:

  • 1.

    We observed in the Spokes cluster 14 cores in N2H+ (3−2) and 8 cores in N2D+ (3−2) with APEX; the measured linewidths are overall narrower than those of previous N2H+ (1−0) observations, σv[N2H+ (3−2)] ~ 0.7   σv[N2H+ (1−0)].

  • 2.

    The three cores (MM03b, MM08, and MM11) that do not show narrower N2H+ (3–2) linewidths than the N2H+ (1–0) linewidths are cores with more than one component along the line-of-sight or low signal-to-noise spectra.

  • 3.

    The denser gas, probed by higher J-transitions of N2H+ and N2D+, presents lower levels of turbulence (nonthermal velocity dispersion) and in two cases approaches the sound speed. These results support the scenario of thermal fragmentation in the expectation that higher-angular resolution observations and/or higher-density tracers would find more quiescent gas.

  • 4.

    Finally, we find that the three cores of our sample, spatially separated from the main Spokes cluster, are likely the youngest, based on their higher deuteration values.


1

Regions that may contain substructure and where several stars might form.

3

Interactive data language.

Acknowledgments

We thank Gary Fuller, Charlie Lada, Paola Caselli, and the anonymous referee for comments that improved the paper. We also thank the staff at the APEX telescope for performing the observations presented in this paper in service-mode. J.E.P. has received funding from the European Communitys Seventh Framework Programme (/FP7/2007-2013/) under grant agreement No. 229517. This publication is supported by the Austrian Science Fund (FWF).

References

Appendix A: Column density determination

The column density is calculated for the optically thick and thin cases using different expressions, but in both cases assuming a beam filling factor of 1. In the optically thick case, (A.1)and in the optically thin case, (A.2)which are the same as Eqs. (A.1) and (A.4) in Caselli et al. (2002) after rearranging terms and using that where T0 ≡ /k and Jν(T) = T0/ [exp(T0/T) − 1]. The partition function is calculated using the first 100 levels.

All Tables

Table 1

List of observed sources.

Table 2

Observed transitions and observational parameters.

Table 3

Best-fit line parameters for sources in NGC 2264-D.

Table 4

Column densities.

All Figures

thumbnail Fig. 1

SCUBA 450 μm dust-continuum map for the Spokes cluster (Wolf-Chase et al. 2003). The observed dense cores identified in Peretto et al. (2006) are shown by the circles at their centers (color-coded according to which molecular line tracers were used in their observation). For comparison, the APEX beams corresponding to our observations are also shown in the lower right corner (following the same color-code convention).

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In the text
thumbnail Fig. 2

Observed N2H+ (3–2) spectra toward the sources. The red curve is the best-fit model to the observations, whose fit parameters are shown in Table 3.

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In the text
thumbnail Fig. 3

Observed N2D+ (3–2) spectra toward the sources. The red curve is the best-fit model to the observations, whose fit parameters are shown in Table 3.

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In the text
thumbnail Fig. 4

Comparison of the velocity dispersion of different molecular lines. Cores listed as having two components (see Table 3) are shown as red symbols, cores that appear to have blended components are shown in black, and the rest are shown as blue symbols. Dashed and dotted lines show the expected velocity dispersion for the cases where the nonthermal velocity dispersion, σNT, equals cs(10   K) or 2   cs(10   K), where cs(10   K) is the sound speed of the average molecule at 10 K (for a mean molecular weight of μm = 2.33).

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In the text

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