Dense Cores in Dark Clouds. XIV. N2H+ (1-0) Maps of Dense Cloud Cores

By P. Caselli, P. Benson, P. Myers, M. Tafalla. 2002

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We present results of an extensive mapping survey of N2H+ (1-0) in about 60 low-mass cloud cores already mapped in the NH3 (1, 1) inversion transition line. The survey has been carried out at the FCRAO antenna with an angular resolution of 54'', about 1.5 times finer than the previous ammonia observations made at the Haystack telescope. The comparison between N2H+ and NH3 maps shows strong similarities in the size and morphology of the two molecular species, indicating that they are tracing the same material, especially in starless cores. Cores with stars typically have map sizes about a factor of 2 smaller for N2H+ than for NH3, indicating the presence of denser and more centrally concentrated gas compared to starless cores. The mean aspect ratio is ~2. Significant correlations are found between NH3 and N2H+ column densities and excitation temperatures in starless cores, but not in cores with stars, suggesting a different chemical evolution of the two species. Starless cores are less massive ( ≃ 3 M☉) than cores with stars ( ≃ 9 M☉). Velocity gradients range between 0.5 and 6 km s-1 pc-1, similar to what has been found with NH3 data, and the ratio β of rotational kinetic energy to gravitational energy has magnitudes between ~10-4 and 0.07, indicating that rotation is not energetically dominant in the support of the cores. "Local" velocity gradients show significant variation in both magnitude and direction, suggesting the presence of complex motions not interpretable as simple solid-body rotation. Integrated intensity profiles of starless cores present a "central flattening" and are consistent with a spherically symmetric density law n ∝ r-α, where α = 1.2 for r < rbreak and α = 2 for r > rbreak, with rbreak ~ 0.03 pc. Cores with stars are better modeled with single density power laws with α ≥ 2, in agreement with observations of submillimeter continuum emission. Line widths change across the core, but we did not find a general trend: there are cores with significant positive as well as negative linear correlations between Δv and the impact parameter b. The deviation in line width correlates with the mean line width, suggesting that the line of sight contains ~10 coherence lengths. The corresponding value of the coherence length, ~0.01 pc, is similar to the expected cutoff wavelength for MHD waves. This similarity may account for the increased "coherence" of line widths on small scales. Despite finer angular resolution, the majority of N2H+ and NH3 maps show a similar "simple" structure, with single peaks and no elongation.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

Effect of Magnetic Braking on Circumstellar Disk Formation in a Strongly Magnetized Cloud (2010, background): “The cloud cores also have angular momentum (Arquilla & Goldsmith 1986; Goodman et al. 1993; Caselli et al. 2002).”
Identifying the Low-Luminosity Population of Embedded Protostars in the c2d Observations of Clouds and Cores (2007, background): “…of n & 104 cm−3 (e.g., Enoch et al. 2007), inversion transition lines of NH3, which also trace regions with volume densities of n & 104 cm−3 (e.g., Ho & Townes 1983; Benson & Myers 1989), and rotational transition lines of N2H +, which trace similar material as the NH3 lines (Caselli et al. 2002).”
From molecular cores to planet-forming disks: An SIRTF legacy program (2003, background): “…core sample were that the distance is less than 400 pc, that the size is not too large (typically less than 5 ), and that the core has been mapped in a dense gas tracer, typically NH3 (Jijina, Myers, & Adams 1999), CS (Lee, Myers, & Tafalla 2001), and/or N2H (Lee et al. 2001; Caselli et al. 2002).”
On The Internal Structure Of Starless Cores. Physical and Chemical Properties of L1498 and L1517B (2004, unknown): “We have presented detailed models of the internal structure of two close-to-round starless cores in the nearby Taurus-Auriga star formation complex, L1498 and L1517B.”
Magnetically Regulated Star Formation in Three Dimensions: The Case of the Taurus Molecular Cloud Complex (2008, methodology): “For comparison, we superpose in the figure the specific angular momenta of 7 starless N2H + cores deduced by Caselli et al. (2002).”
CHEMISTRY IN INFRARED DARK CLOUD CLUMPS: A MOLECULAR LINE SURVEY AT 3 mm (2011, background): “However, since N2H + is thought to be a higher density tracer (e.g., Caselli et al. 2002; Pirogov et al. 2003), its emission should emanate mostly from the center of the clumps and thus we still can see a weak trend.”
SCUBA Polarization Measurements of the Magnetic Field Strengths in the L183, L1544, and L43 Prestellar Cores (2003, methodology): “…flux ratio in units of the critical ratio, kcor is kobs corrected by 1/3 from equation (4), MBcrit is the magnetic critical mass that can be supported by our measured B, and Mvir is the virial mass (e.g., Caselli et al. 2002), computed for TK ¼ 10 K and the r̄ and VNT listed in Table 1.”
VARIABLE PROTOSTELLAR ACCRETION WITH EPISODIC BURSTS (2015, methodology): “Despite limitations of exploration of parameter space in these models, we can combine the information there with the observational finding of Caselli et al. (2002) that β has a median value of 0.02 and most values in the range ∼ 10−4 − 10−1 to conclude that most cores with Mcore ≥ 1.0M⊙ will…”
DIRECT OBSERVATION OF A SHARP TRANSITION TO COHERENCE IN DENSE CORES (2010, background): “Goodman et al. (1998) and Caselli et al. (2002) coined the term “coherent core,” to describe the region where the non-thermal motions are subsonic and constant as “islands of calm in a more turbulent sea.””
Probing the Evolutionary Status of Starless Cores through N2H+ and N2D+ Observations (2004, methodology): “Another possibility is that depletion of N2 (and hence of N2H + andN2D +) may occur in regions with density higher than 5 ;105 cm 3 (see also Caselli et al. 1999, 2002c; Bergin et al. 2001; Crapsi et al. 2004; Belloche & André 2004).”
THE BURST MODE OF ACCRETION AND DISK FRAGMENTATION IN THE EARLY EMBEDDED STAGES OF STAR FORMATION (2010, methodology): “Typical rotation rates of ∼ 1 km s−1 pc−1 ≈ 10−14 rad s−1 measured in molecular cloud cores (Goodman et al. 1993; Caselli et al. 2002) are sufficient to provide a significant angular momentum barrier to star formation.”
On the diversity and statistical properties of protostellar discs (2018, background): “…be important since the velocity dispersion within dense molecular clouds cores is typically subsonic and independent of scale (Goodman et al. 1998; Caselli et al. 2002) and there is observational evidence that dense cores may be kinematically distinct from the large clouds in which they are…”
Our astrochemical heritage (2012, background): “Starless and prestellar cores present evidence of overall (slow) rotation (Arquilla and Goldsmith 1986; Goodman et al. 1993; Caselli et al. 2002), thus they possess an initial angular momentum.”
Molecular Evolution in Collapsing Prestellar Cores. III. Contraction of a Bonnor-Ebert Sphere (2004, methodology): “Caselli et al. (2002a) obtained column density ratios for N(DCO+)/N(HCO+) and N(N2D +)/N(N2H +) towards the dust peak of L1544 of 0.04 and 0.2, respectively.”
LIMITING ACCRETION ONTO MASSIVE STARS BY FRAGMENTATION-INDUCED STARVATION (2010, background): “For example, the mean values in the sample discussed by Goodman et al. (1993) are j ≈ 6 × 1021 cm2s−1 with masses up to several hundred solar masses, while Caselli et al. (2002) studied lowmass cores with typically only a few solar masses and find j ≈ 7 × 1020 cm2s−1.”
On the internal structure of starless cores II. A molecular survey of L1498 and L1517B (2006, background): “In the larger and better resolved L1498 core, the maps of C3H2, H2CO, CH3OH, SO, HCO+ (plus isotopes), HCN (plus isotope), HC3N, and C2S all present distributions that differ from the centrally concentrated dust or N2H+.”
Cold Dark Clouds: The Initial Conditions for Star Formation (2007, background): “The effects of freeze-out can alter both the pre-factor and the exponent of the density dependence (Bergin & Langer 1997; Caselli et al. 2002c; Walmsley et al. 2004), but see also Ciolek & Mouschovias (1998).”
Theory of Star Formation (2007, background): “Observations have established that the angular momentum, or equivalently, the rotational energy, of cores is indeed small (e.g., Goodman et al., 1993; Jijina, Myers, & Adams, 1999; Caselli et al., 2002; Pirogov et al., 2003).”