HP2 survey III. The California Molecular Cloud: A sleeping giant revisited

We present new high resolution and dynamic range dust column density and temperature maps of the California Molecular Cloud derived from a combination of Planck and Herschel dust-emission maps, and 2MASS NIR dust-extinction maps. We used these data to determine the ratio of the 2 : 2 mu m extinction coefficient to the 850 mu m opacity and found the value to be close to that found in similar studies of the Orion B and Perseus clouds but higher than that characterizing the Orion A cloud, indicating that variations in the fundamental optical properties of dust may exist between local clouds. We show that over a wide range of extinction, the column density probability distribution function (pdf) of the cloud can be well described by a simple power law (i.e., PDFN proportional to A(K)(-n)) with an index (n = 4 : 0 +/- 0 : 1) that represents a steeper decline with A(K) than found (n approximate to 3) in similar studies of the Orion and Perseus clouds. Using only the protostellar population of the cloud and our extinction maps we investigate the Schmidt relation, that is, the relation between the protostellar surface density, Sigma(*), and extinction, AK, within the cloud. We show that Sigma(*) is directly proportional to the ratio of the protostellar and cloud pdfs, i.e., PDF* (A(K))/PDFN(A(K)). We use the cumulative distribution of protostars to infer the functional forms for both Sigma(*) and PDF*. We find that Sigma(*) is best described by two power-law functions. At extinctions A(K) less than or similar to 2 : 5 mag, Sigma(*) proportional to A(K)(beta) with beta = 3 : 3 while at higher extinctions beta = 2 : 5, both values steeper than those (approximate to 2) found in other local giant molecular clouds (GMCs). We find that PDF* is a declining function of extinction also best described by two power-laws whose behavior mirrors that of Sigma(*). Our observations suggest that variations both in the slope of the Schmidt relation and in the sizes of the protostellar populations between GMCs are largely driven by variations in the slope, n, of PDFN(A(K)). This confirms earlier studies suggesting that cloud structure plays a major role in setting the global star formation rates in GMCs