Fig. 1 shows a reconstructed K image of the Serpens molecular cloud core. Black stars represent sources belonging to the cloud with their corresponding identification numbers, as quoted in Table 2 (see 3.1). Solid lines represent logarithmic contour plot of the central nebula. Filled circles correspond to the unidentified sources. Figs. 2 and 3 show the image of the Serpens Cloud Core in K and I wavebands; the scale adopted for these images is 0.25" per pixel.
3.1. Log of the sources of the survey
Table 2 presents the broad-band photometric data. The number in brackets is the uncertainty on the computed magnitude as described in 2.3. Crosses correspond to the regions of the sky that were not observed in the concerned photometric band. 'nd' corresponds to sources that were not detected. Identification numbers followed by a 'S' as superscript correspond to Serpens cloud members. Conversions from fluxes to magnitudes were made using relations from Landolt-Bornstein (1982).
Before drawing any conclusion on the distribution of near infrared sources detected by the survey, we had to distinguish the embedded sources from the background sources. K band photometry by itself will not discriminate between sources that are embedded in the cloud and sources that are background stars.
Methods that have previously been employed to help in the separation of reddened background sources from the true embedded PMS stars are: i) the observation of a control field located at approximately the same galactic coordinates as the survey field but off the associated molecular cloud that contains the embedded PMS population and, ii) the use of a model that produces a table of K source counts according to galactic coordinates. However, use of these techniques does not allow to determine which particular stars in the survey field are PMS stars i.e. it is statistical in nature. Considering our survey, the number of detected sources is not large enough and such statistical methods can obviously lead to inaccurate results.
The Serpens objects are identified by using the same basic criteria as those used by Eiroa and Casali (1992). (1) Stars associated with cometary or bipolar nebulae are considered as certain members of the Serpens population. (2) H emission-line stars can also be identified as cloud members since H emission must imply the presence of (ionized) circumstellar material. (3) PMS stars having IR excess in the near infrared colour-colour diagram (Rydgren & Cohen 1985). In such a diagram, stars located to the right of the reddening vector followed by an A0 star can unambiguously be identified as Serpens cloud members.
3.2. The colour-colour (J-H,H-K) diagram
The optical/NIR photometry derived from our survey allows us to study, via colour-colour (c-c) and colour-magnitude (c-m) diagrams, the combined effects of both the intrinsic properties of the sources and the overlying extinction. Thanks to NIR photometry we can theoretically penetrate deeper into the molecular clouds, observe a much larger fraction of the embedded population and learn more about the global properties of the star formation region and its individual sources.
The J-H, H-K c-c diagram presented in Fig. 4 provides a useful mean of distinguishing between the effects of interstellar reddening and IR excess. In this work, we make the assumption that the Rieke & Lebofsky (1985, hereafter RL85) reddening law can be applied to the Serpens cloud and represents a reasonable approximation of the NIR extinction caused by the associated molecular cloud since a) few sources lie above the upper of the two vectors, and b) the vectors generally follow the same slope as that implied by stars of different colours. Also plotted as solid line in Fig. 4, are the locations of both unreddened main-sequence and giant stars. From the extreme points of these curves we have plotted two dashed lines representing RL85 reddening vectors. The area between these lines corresponds to the reddening zone for normal stars. The crosses located on the reddening lines are separated by distances corresponding to 10 mag of visual extinction. Open circles correspond to the unidentified sources. Filled circles represent Serpens sources identified using criteria described in Sect. 3.1. It is clear from Fig. 4 that a significant fraction of the objects observed in the Serpens cloud is located between the two reddening vectors and is consistent with reddened background stars seen through the cloud. More than one third of the sources, however, lies at positions outside the reddening vectors. This region of the JHK colour-colour diagram is known as the infrared excess region (Lada & Adams 1992) and corresponds to the location of PMS stars. However, naked-T Tauri stars, post-T Tauri stars and some class I sources found in Ophiuchus by Wilking & Lada (1983) do not show any NIR excess, and will be found between the two reddening vectors in such a diagram.
Another interesting point that can be inferred from the diagram is that sources located in the reddening zone for normal stars are found spread along the reddening band. This indicates that the extinction caused by the cloud or by the circumstellar material is variable and can reach values up to 20 magnitudes of visual extinction.
This colour-colour diagram is somehow different to that presented by Eiroa & Casali (1992) and Sogawa et al. (1997). This is not surprising since their surveys cover an area larger than our, with lower sensitivities. Thus, sources plotted on these diagrams, do not correspond exactly to the same population.
3.3. The colour-magnitude (K,J-K) diagram
The K versus J-K colour-magnitude (c-m) diagram for all objects found in the Serpens cloud core is plotted in Fig. 5. In this diagram, ZAMS stars are plotted at the assumed distance of the Serpens cloud i.e. 310 pc (the solid line joining the open diamonds). A representative RL85 reddening vector ( magnitudes) is plotted as an arrow and the dashed line indicates the effective detection limit of the survey. Filled circles represent the Serpens objects while open circles are the unidentified sources.
With the K=16.3 detection limit, we would therefore observe unreddened and unextincted ZAMS stars down to spectral type M4 at 310 pc. Using the mass- relation for ZAMS shown in Zinnecker et al. (1993), this corresponds to a stellar mass of 0.3 . However, PMS stars are over-luminous for their mass which would lower the effective PMS mass detection limit significantly. Zinnecker & McCaughrean (1991) present age dependent mass-luminosity functions over the range years to years derived from homogeneous tracks calculated by I. Mazzitelli. These suggest that over this age range, 0.08 PMS objects would show a relatively small change in from 4.9 to 5.3. This corresponds to a range of 12.4-12.8 for stars in Serpens. This is over 3.5 magnitudes brighter than our detection limit and hence our survey would be sensitive enough to detect stars with masses less than 0.08 . But these values do not take into account the extinction due to the molecular cloud. At K, the effect of extinction is however minimized and the range of values between 0-20 magnitudes (as implied by our NIR c-c diagram) could lead the embedded population to be 2.3 magnitudes fainter than the values quoted above. This would suggest that 0.08 PMS stars would have values in the range 14.7-15.1. This is still below our detection limit, suggesting that we should be able to detect objects with masses significantly less than the 0.08 mass limit.
© European Southern Observatory (ESO) 1998
Online publication: January 27, 1998