A Population of Brown Dwarfs and Free-Floating Planets in Orion

Phil Lucas¹ & Pat Roche²

¹University of Hertfordshire, U.K.
²University of Oxford, U.K.

A large number of brown dwarfs have been discovered in recent years both in young Galactic Clusters and in the local field population of stars near the sun. These are objects which were too small to become stars, since their centres never became hot enough for the nuclear reactions of hydrogen which make stars shine. The image shown here reveals a large population of very young brown dwarfs in the Trapezium Cluster in Orion. This was one of the first projects undertaken with the infrared camera UFTI, on the UKIRT, in December 1998 and the results of this research have just been accepted by Monthly Notices of the Royal Astronomical Society. It is the most sensitive search yet for low-mass objects, observing three different near-infrared colours (I, J and H) to discover the mass, luminosity and temperature of the objects.

About two thirds of the 600 or so point sources seen in the data (the image accompanying this article has been cropped slightly) are stars, just under one third are brown dwarfs and about 13 are even smaller objects with masses similar to planets. Unlike ordinary planets, however, these do not orbit any star but float by themselves in space and shine by the residual heat left over from when they were born. The smallest object found so far has about 8 times the mass of Jupiter, quite massive by planetary standards, but still below the threshold for nuclear reaction of deuterium, which occurs at 13 Jupiter masses and is the minimum mass of a brown dwarf.

Orion is the nearest of the Giant Molecular Clouds - the places where most stars are thought to be born out of dense clouds of gas and dust like those seen here. Thus it is the best place to study in order to find out about the population of stars, brown dwarfs and free-floating planets that exist in the rest of the galaxy. Brown dwarfs and free floating planets are much easier to find when they are young and still retain some heat from the formation process. The objects in the Trapezium cluster are mostly between three hundred thousand and two million years old - very young compared to the 5 billion year age of the sun. The backdrop of the Orion Molecular Cloud obscures everything that lies behind it, which is very useful because it means that all the objects seen are members of the cluster, except for perhaps a handful which lie in the foreground.

An interesting feature of the study is that no smaller planets were found. This may indicate that there is a limit to how small these free-floating planets can be, although even more sensitive surveys will be needed to confim this. In the meantime, about 20 of the brown dwarfs and planets have been looked at again with UKIRT, examining their spectra to reveal their properties in more detail. The results are still being analysed but they show the signature of water that is expected in relatively cool stars and brown dwarfs, at a temperature of a mere 2800 degrees Centigrade.

Simultaneous Near-Infrared and X-Ray Variability in the Quasar 3C 273: The Origin of the X-Ray Seed Photons

Ian M^cHardy

University of Southampton, U.K.

INTRODUCTION

The low-energy (radio - optical) emission from blazars -- ie BL Lac objects and quasars which display some evidence of relativistic jets -- is universally agreed to be synchrotron emission from the relativistic electrons and magnetic fields in the jets. The high-energy (X-ray, Gamma-ray) emission from blazars is generally supposed to arise as a result of Compton scattering of low energy seed photons by relativistic electrons in the jet and the major question in blazar physics concerns the origin of those seed photons.

The most popular hypothesis is the Synchrotron Self-Compton (SSC) model in which the seed photons are the synchrotron photons from the jet, up-scattered by their parent electrons. Alternatively the seed photons may arise externally to the jet from the ambient optical/UV radiation field in the nucleus (the External Compton, EC, process) or, in a combination of the two models, photons from the jet may be mirrored back to the jet (the Mirror Compton, MC, model) from a gas cloud before scattering up to high energies. The various models make different predictions about the lags between the seed and Compton-scattered variations, and about the relative amplitudes of the two components and so, in principle, the models can be distiguished.

A great deal of observational effort has therefore been devoted to finding correlated variability between the high and low energy bands in blazars but the results previously have not been impressive. For example, in the case of 3C273, one of the brightest blazars, extensive searches have been carried out for a connection between the X-ray and millimetre bands on both daily (McHardy et al. 1993) and monthly (Courvoisier et al. 1990; McHardy et al. 1996) timescales but no correlation has been found. The problem has generally been the lack of well sampled lightcurves. Here we report on the results of two extensive observational campaigns, with infrared observations from UKIRT and X-ray observations from the Rossi X-ray Timing Explorer (RXTE), which have produced the clearest results so far obtained regarding the origin of the seed photons.

OBSERVATIONS

During the 6 week period from 22 December 1996 to 5 February 1997, X-ray observations were carried out twice a day by RXTE and nightly near infrared service observations were made at UKIRT. The major instrument on RXTE is the Proportional Counter Array (PCA), giving high sensitivity in the 3-20 keV band. Typical observation lengths were 1ksec. Infrared images were obtained slightly less frequently with IRCAM 3, with gaps being mainly a result of bad weather. Typical observation lengths were 3 minutes in K-band. Some millimetre observations were also obtained with the JCMT and at Owens Valley (OVRO). We do not discuss those observations here but refer interested readers to our recent paper, McHardy et al. 1999.

FIGURE 1: Near-infrared (top) and X-Ray (bottom) lightcurves of 3C 273 from 1996-7. The X-ray counts are the total from 3 PCUs of the PCA. The infrared observations are from UKIRT service observations.

In Figure 1 we present the resultant X-ray and infrared lightcurves. We see two large outbursts, which are particularly well sampled in the X-ray band. The first outburst has a quite smooth X-ray profile but the second outburst is more ragged and seems to be made up of a small number of superposed short flares. Visual inspection shows very similar variability in the K-band lightcurve.

X-RAY/INFRARED RELATIONSHIP

We have investigated the relationship between the X-ray and infrared bands in two ways. We first computed the simple discrete cross-correlation and then modelled the X-ray emission in terms of the infrared fluxes. If we consider only the better sampled, and smoother, first flare, the infrared clearly leads the X-rays by 0.75 +/-0.25 days. For the second, more ragged and less well sampled outburst, a lag nearer to zero days is preferred. Interestingly we note that the X-ray spectral behaviour is different in the two outbursts. In the first outburst the spectrum hardens at the start of the outburst, but becomes very soft by the peak of the outburst. However in the second, fragmented outburst, the hardest spectral points correspond to the peaks in flux indicating an overlapping set of injections of fresh particles as opposed to a single injection of particles, followed by particle `ageing' at the start of the first outburst.

THE X-RAY EMISSION MECHANISM

The very close correspondence of the infrared and X-ray variations shows that the K-band data samples the seed photon distribution. The rapid variation of the K-band flux therefore rules out an origin for the seed photons externally to the jet, eg in the surrounding accretion disc, as timescales for infrared variations would be too long. Thus the original version of the EC model (Dermer and Schlickeiser 1993) in which the high energy variations are caused by variations in the external seed photons is eliminated. The next version, in which the electrons in the jet that produce the infrared synchrotron emission also scatter an all-pervading ambient nuclear photon field is also ruled out, at least for the first flare, since we would then expect exactly simultaneous X-ray and infrared variations.

However in the SSC process we expect the seed photons to lead by approximately the time it takes those photons to travel from their place of origin to their place of scattering in the jet, thus giving an indication of the size of the X-ray emission region. In the MC model, we also expect the seed photons to lead, in this case by the light travel time from the jet to the external mirroring cloud.

These two possibilities might be distinguished if the experiment was repeated. If the lag, for X-ray flares of similar amplitude and timescales as those discussed above, was similar to that already found it would favour the SSC model, assuming similar sized emission regions. However in the MC model we might expect different lags as the seed photons are reflected from a cloud at a different, random distance. We therefore repeated the observations and the results are shown in Figure 2 where we obtained better sampling in both the X-ray and infrared wavebands.

FIGURE 2: X-ray and infrared lightcurves of 3C273 from March 1999. In these new data better sampling in both bands was obtained.

From Figure 2 we can again very clearly see that infrared and X-ray variations are strongly correlated, a moderate strength flare being visible in both bands. These observations also show that, far from being lucky with the first set of observations in early 1997, easily measurable X-ray and infrared flares occur regularly in 3C273, contrary to what used to be thought. From these very well sampled lightcurves it is easy to show that the X-rays again lag the infrared, with the lag this time being 1.2 +/-0.15 days.

CONCLUSIONS

The EC model was ruled out by the 1997 observations. The similarity between the lags as measured in early 1997 and in March 1999 gives strong support to the SSC model for the origin of the X-ray emission. However, it does not entirely rule out the MC model since it might just be possible that the reflecting clouds, in both cases, were at a similar distance from the emitting region in the jet. The high brightness of 3C273, coupled with its proven regular variability, means that similar correlated variability observations of 3C273 are likely to be performed fairly easily in the future. Should the same lag be found every time, the SSC model will be extremely strongly favoured.

Full details of the 1997 observations are given in McHardy et al. 1999.

REFERENCES

Courvoisier, T.J-L. et al. 1990. A&A 234 73.
Dermer, C., Schlickeiser, R., 1993. ApJ, 416, 458.
McHardy, I.M., 1993. Proc IAU Symposium 159, eds Courvoisier, T. and Blecha, A., Kluwer, p193. McHardy, I.M., 1996. ASP Conf. Series, 110, 293.
McHardy, I.M., Lawson, A.J., Newsam, A.M., Marscher, A.P., Robson, E.I. and Stevens, J., 1999. MNRAS 310, 571.
Robson, E.I. et al., 1993. MNRAS 262, 249.

The power of UFTI with ORAC: new images of an old friend, HH 1.

Chris Davis

UKIRT/Joint Astronomy Centre

During a recent ORAC training session for staff astronomers at UKIRT, spectacular new narrow-band images of the archetypal Herbig-Haro flow HH 1 were obtained, through H₂ 2.122 micron and [FeII] 1.644 micron filters. To distinguish line emission features from patches of scattered light, and to facilitate the construction of the very attractive true-colour image shown here (Figure 1) a narrow-band continuum image was also obtained. The mosaics that comprise Figure 1 were each produced in real time (i.e. at the telescope) using the JITTER9_SELF-FLAT orac reduction recipe. These impressive data illustrate not only the first-rate capabilities of UKIRT and UFTI, but also the quality of on-line data reduction available to UKIRT observers.

The data are a dramatic improvement on what has appeared in the literature to date, even rivalling recently-obtained NICMOS images in resolution. The three separate UFTI images that are combined in the Figure each consist of just nine 60 second jittered frames, so the UFTI data come in at a few dollars cheaper than the NICMOS data, as well as covering a wider field on the sky!

Of particular interest in these new images is the fine detail observed in both the jet (extending northwestward from the embedded source) and the HH 1 bow shock (top-right in the Figure: note that the bright star central to the image is in fact a foreground source and is unrelated to the HH flow). The jet is rather knotty in both the [FeII] and H₂ data. The excitation in the jet also clearly decreases, as the relative brightness of the [FeII] and H₂ emission (colour-coded green and red respectively) decreases with distance from the source. The jet drives a number of molecular bow shocks, seen here between the outflow source and bright foreground star in the centre of the frame. Further out along the flow axis, the bright HH 1 bow shock is observed; the northeastern wing of the bow is fillamentary in structure and observed only in low-excitation molecular hydrogen emission. The bow shock head is, on the other hand, very fragmented, comprising a number of sub clumps. The bow head is seen largely in [FeII] emission, since the gas excitation conditions are more extreme here. Of note, however, is the compact H₂ knot near the very tip of the bow shock (seen here in yellow - that is, red H₂ superimposed onto green [FeII]). This feature was once thought to be a mach disk, i.e. an "internal" shock that decelerates the jet gas as it enters the bow shock working surface. However, it is now revealed to be merely the low-excitation, oblique edge of the leading sub-clump in the HH 1 bow shock. The mach disk thus remains as yet undiscovered.

A Chip off the Old Block

Its true! UKIRT support scientist John Davies has been relocated to the asteroid belt.

Minor planet 1993 BH8 has been allocated the permanent number of (9064) Johndavies. The object, estimated to be about 6.5 km in diameter, was discovered 21 January 1993 by the Spacewatch telescope in Arizona and has been re-observed a 5 subsequent oppositions. Its orbit ranges from 2.12 to 2.74 AU from the Sun, it has a surface temperature of about 220K and a V mag in the range 17.5-19.5. So we could describe Johndavies as being cool, although not very bright, mildly eccentric and having a generally rugged appearance. It is probably a chip off a much older block.

The official citation reads:

Discovered 1993 Jan. 21 by Spacewatch at Kitt Peak. John K. Davies (b. 1955) of the Joint Astronomy Centre was instrumental in the successful discovery and follow-up of (3200) Phaethon and several comets with the IRAS satellite in 1983. He has also carried out studies of the infrared nature of distant minor planets and authored a number of popular books and articles.