Spatial statistics in star-forming regions

Observational studies of star formation reveal spatial distributions of Young Stellar Objects (YSOs) that are `snapshots' of an ongoing star formation process. Using methods from spatial statistics it is possible to test the likelihood that a given distribution process could produce the observed patterns of YSOs. I determine the sensitivity of the spatial statistical tests Diggle's G function (G), the `free-space' function (F), Ripley's K and O-ring for application to astrophysical data. To do this I applied each test to simulated data containing 2D Gaussian clusters projected on a random distribution of background stars. By varying the number of stars within the Gaussian cluster and the number of background stars I determined the ability of the tests to reject complete spatial randomness (CSR) with changing signal-to-noise. Ripley's K and O-ring were shown to be much more sensitive to Gaussian clusters than G and F. I then apply the O-ring test to determine if column density alone is sufficient to explain the locations of Class 0/I YSOs within Serpens South, Serpens Core, Ophiuchus, NGC1333 and IC348. Star formation is known to occur more readily where more raw materials are available, a relationship that is often expressed in the form of a 'Kennicutt--Schmidt' relation where the surface density of Young Stellar Objects (YSOs) is proportional to column density to some power, μ. Using the O-ring test as a summary statistic, confidence envelopes were produced for different values of μ from probability models made using the Herschel column density maps. The YSOs were tested against four distribution models: the best-estimate of μ for the region, μ = 0 (i.e. random) above a column density threshold and zero probability elsewhere, μ = 1, and the power-law that best represents the five regions as a collective, μ = 2.05 ± 0.20. Serpens South and NGC1333 rejected the μ = 2.05 model on small scales of ~ 0.15 pc which implies that small-scale interactions may be influencing their distribution. On scales above 0.15 pc, the positions of YSOs in all five regions can be well described using column density alone.Science and Technology Facilities CouncilScience and Technology Facilities Counci

Interferometric Observations of V838 Monocerotis

We have used long-baseline near-IR interferometry to resolve the peculiar eruptive variable V838 Mon and to provide the first direct measurement of its angular size. Assuming a uniform disk model for the emission we derive an apparent angular diameter at the time of observations (November-December 2004) of $1.83 \pm 0.06$ milli-arcseconds. For a nominal distance of $8\pm2$ kpc, this implies a linear radius of $1570 \pm 400 R_{\odot}$. However, the data are somewhat better fit by elliptical disk or binary component models, and we suggest that the emission may be strongly affected by ejecta from the outburst.Comment: 12 pages, 1 two-part encapsulated postscript figure. Accepted by ApJL. Added a table of observation

Spatial Statistics in Star Forming Regions: Testing the Limits of Randomness

This is the author accepted manuscript. the final version is available from OUP via the DOI in this recordObservational studies of star formation reveal spatial distributions of Young Stellar Objects (YSOs) that are ‘snapshots’ of an ongoing star formation process. Using methods from spatial statistics it is possible to test the likelihood that a given distribution process could produce the observed patterns of YSOs. The aim of this paper is to determine the usefulness of the spatial statistics tests Diggle’s G function (G), the ‘free-space’ function (F), Ripley’s K and O-ring for application to astrophysical data. The spatial statistics tests were applied to simulated data containing 2D Gaussian clusters projected on random distributions of stars. The number of stars within the Gaussian cluster and number of background stars were varied to determine the tests’ ability to reject complete spatial randomness (CSR) with changing signal-to-noise. The best performing test was O-ring optimised with overlapping logarithmic bins, closely followed by Ripleys K. The O-ring test is equivalent to the 2-point correlation function. Both F and G (and the minimum spanning tree, of which G is a subset) performed significantly less well, requiring a cluster with a factor of two higher signal-to-noise in order to reject CSR consistently. We demonstrate the tests on example astrophysical datasets drawn from the Spitzer catalogue.Science and Technology Facilities Council (STFC

Evidence for large superhumps in TX Col and V4742 Sgr

Since the discovery of the largest positive superhump period in TV Col, we have started a program to search for superhumps in CVs with large orbital periods. Here, we summarize preliminary results of TX Col and V4742 Sgr. TX Col is an intermediate polar with a 5.7-h orbital period. V4742 Sgr is a recent nova with no known periods. CCD unfiltered continuous photometry of these 2 objects was carried out during 56 nights in 2002-3. In TX Col, in addition to the orbital period of 5.7 h, we found peaks at 7.1 h and 5.0 h. These are interpreted as positive and negative superhumps correspondingly, although the effects of the quasi-periodic oscillations at about 2 h were not taken into consideration. In the light curve of V4742 Sgr 2 long periods are detected -- 6.1 and 5.4 h as well as a short-term period at 1.6 h. This result suggests that V4742 Sgr is an intermediate polar candidate and a permanent superhump system with a large orbital period (5.4 h) and a superhump period excess of 13 percent. If these results are confirmed, TX Col, V4742 Sgr and TV Col form a group of intermediate polars with extremely large superhump periods. There seems to be now growing evidence that superhumps can occur in intermediate polars with long orbital periods, which is very likely inconsistent with the theoretical prediction that superhumps can only occur in systems with mass ratios below 0.33. Alternatively, if the mass ratio in these systems is nevertheless below the theoretical limit, they should harbour undermassive secondaries and massive white dwarfs, near the Chandrasekhar limit, which would make them excellent candidates for progenitors of supernovae type Ia.Comment: 9 pages, 8 figures, 3 sty files, To appear in the proceedings of IAU JD5, `White Dwarfs: Galactic and Cosmological Probes', eds. Ed Sion, Stephane Vennes and Harry Shipman, Full abstract in pape

Detection of orbital and superhump periods in Nova V2574 Ophiuchi (2004)

We present the results of 37 nights of CCD unfiltered photometry of nova V2574 Oph (2004) from 2004 and 2005. We find two periods of 0.14164 d (~3.40 h) and 0.14773 d (~3.55 h) in the 2005 data. The 2004 data show variability on a similar timescale, but no coherent periodicity was found. We suggest that the longer periodicity is the orbital period of the underlying binary system and that the shorter period represents a negative superhump. The 3.40 h period is about 4% shorter than the orbital period and obeys the relation between superhump period deficit and binary period. The detection of superhumps in the light curve is evidence of the presence of a precessing accretion disk in this binary system shortly after the nova outburst. From the maximum magnitude - rate of decline relation, we estimate the decay rate t_2 = 17+/-4 d and a maximum absolute visual magnitude of M_Vmax = -7.7+/-1.7 mag.Comment: 6 pages, 6 figures, 2 .sty files, AJ accepted, minor change to one of reference

Spectroscopic monitoring of the transition phase in nova V4745 Sgr

We present a spectroscopic monitoring of the transient nova V4745 Sagittarii (Nova Sgr 2003 #1) covering ten months after the discovery. During this period the light curve showed well expressed transient phase in the form of six rebrightenings, and the presented dataset is one of the best spectroscopic coverages of a classical nova during the transition phase. Low- and medium-resolution spectra revealed that the majority of spectral lines switched back to strong P-Cyg profiles during the mini-outbursts, similar to the spectra just after the major eruption. We interpret the observed phenomena as evidence for ``echo-outbursts'' resulting in episodes of secondary mass-ejections and propose that the transition phase in classical novae is driven by repetitive instabilities of the hydrogen shell burning on the surface of the white dwarf. From the available light curve data we estimate the absolute magnitude of the system of about -8.3 +/- 0.5 mag. All spectrophotometric pieces of evidence point toward a very low (E(B-V) < 0.1) interstellar reddening, leading to a rough distance estimate of V4745 Sgr (9 kpc < d < 19 kpc).Comment: 8 pages, 10 figures, A&A accepte

Spatial statistics in star forming regions: is star formation driven by column density alone?

This is the author accepted manuscript. The final version is available from the Royal Astronomical Society via the DOI in this recordThe Herschel Gould Belt survey (HGBS) data available in HGBS Archive, at http://www.herschel.fr/cea/gouldbelt/en/ . The Dunham et al. (2015) Young Stellar Object source data are available at https://doi.org/10.1088/0067-0049/220/1/11 . The Spitzer data underlying this article are available in NASA/IPAC Infrared Science Archive at https://irsa.ipac.caltech.edu/ data/SPITZER/C2D/images/Star formation is known to occur more readily where more raw materials are available. This is often expressed by a `Kennicutt--Schmidt' relation where the surface density of Young Stellar Objects (YSOs) is proportional to column density to some power, μ. The aim of this work was to determine if column density alone is sufficient to explain the locations of Class~0/I YSOs within Serpens South, Serpens Core, Ophiuchus, NGC1333 and IC348, or if there is clumping or avoidance that would point to additional influences on the star formation. Using the O-ring test as a summary statistic, 95 per cent confidence envelopes were produced for different values of μ from probability models made using the Herschel column density maps. The YSOs were tested against four distribution models: the best-estimate of μ for the region, μ=0 above a minimum column density threshold and zero probability elsewhere, μ=1, and the power-law that best represents the five regions as a collective, μ=2.05 ± 0.20. Results showed that μ=2.05 model was consistent with the majority of regions and, for those regions, the spatial distribution of YSOs at a given column density is consistent with being random.Serpens South and NGC1333 rejected the μ = 2.05 model on small scales of ˜0.15 pc which implies that small-scale interactions may be necessary to improve the model. On scales above 0.15 pc, the positions of YSOs in all five regions can be well described using column density alone.Science and Technology Facilities Council (STFC

The Peculiar 2004 Superoutburst in the Helium Dwarf Nova, 2003aw

We conducted a time-resolved photometric campaign of the helium dwarf nova, 2003aw in 2004 May--June. 2003aw stayed at 14.7--15.7 mag for the first several days during this campaign, which is the plateau phase of this superoutburst. This variable then rapidly decayed to $\sim$18.0 mag, still brighter by about 2 mag than its quiescence magnitude, and maintained this brightness for about 20 days, having short flares of $\sim$2 mag. A long fading tail followed it. We detected superhumps with a period of 0.02357(4) d [= 2036(3) s] during the plateau phase. The whole light curve of the superoutburst in 2003aw, taking into account the present data and those in the literature, perfectly resembles that of the 1996-1997 superoutburst of the peculiar WZ Sge-type hydrogen-rich dwarf nova, EG Cnc.Comment: 5 pages, 4 figures, PASJ Letter in pres

Smoothed Particle Hydrodynamics Simulations of Apsidal and Nodal Superhumps

In recent years a handful of systems have been observed to show "negative" (nodal) superhumps, with periods slightly shorter than the orbital period. It has been suggested that these modes are a consequence of the slow retrograde precession of the line of nodes in a disk tilted with respect to the orbital plane. Our simulations confirm and refine this model: they suggest a roughly axisymmetric, retrogradely-precessing, tilted disk that is driven at a period slightly less than half the orbital period as the tidal field of the orbiting secondary encounters in turn the two halves of the disk above and below the midplane. Each of these passings leads to viscous dissipation on one face of an optically-thick disk -- observers on opposite sides of the disk would each observe one brightening per orbit, but 180 degrees out of phase with each other.Comment: 11 pages. Accepted for publication in The ApJ Letter