NICMOS Research Articles

Search for giant planets around seven white dwarfs in the Hyades cluster with the Hubble Space Telescope

ABSTRACT Only a small number of exoplanets have been identified in stellar cluster environments. We initiated a high angular resolution direct imaging search using the Hubble Space Telescope (HST) and its Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument for self-luminous giant planets in orbit around seven white dwarfs in the 625 Myr old nearby (≈45 pc) Hyades cluster. The observations were obtained with Near-Infrared Camera 1 (NIC1) in the F110W and F160W filters, and encompass two HST roll angles to facilitate angular differential imaging. The difference images were searched for companion candidates, and radially averaged contrast curves were computed. Though we achieve the lowest mass detection limits yet for angular separations ≥0.5 arcsec, no planetary mass companion to any of the seven white dwarfs, whose initial main-sequence masses were >2.8 M⊙, was found. Comparison with evolutionary models yields detection limits of ≈5–7 Jupiter masses (MJup) according to one model, and between 9 and ≈12 MJup according to another model, at physical separations corresponding to initial semimajor axis of ≥5–8 au (i.e. before the mass-loss events associated with the red and asymptotic giant branch phase of the host star). The study provides further evidence that initially dense cluster environments, which included O- and B-type stars, might not be highly conducive to the formation of massive circumstellar discs, and their transformation into giant planets (with m ≥ 6 MJup and a ≥6 au). This is in agreement with radial velocity surveys for exoplanets around G- and K-type giants, which did not find any planets around stars more massive than ≈3 M⊙.

A multiwavelength study of evolved massive stars in the Galactic Centre

The central region of the Milky Way Galaxy provides a unique laboratory for a systematic, spatially resolved population study of evolved massive stars of various types in a relatively high-metallicity environment. We have conducted a multiwavelength data analysis of 180 such stars or candidates, most of which were drawn from a recent large-scale Hubble Space Telescope (HST)/Near-Infrared Camera and MultiObject Spectrometer (NICMOS) narrow-band Paschen α survey, plus additional 14 Wolf–Rayet stars identified in earlier ground-based spectroscopic observations of the same field. The multiwavelength data include broad-band infrared (IR) photometry measurements from HST/NIC2, Simultaneous three-colour InfraRed Imager for Unbiased Surveys (SIRIUS), Two-Micron All-Sky Survey (2MASS), Spitzer/IRAC and X-ray observations from Chandra. We correct for extinctions towards individual stars, improve the Paschen α line equivalent width measurements, quantify the substantial mid-IR dust emission associated with carbon sequence Wolf–Rayet (WC) stars and find X-ray counterparts. In the process, we identify 10 foreground sources, some of which may be nearby cataclysmic variables. The nitrogen sequence Wolf–Rayet (WN) stars in the Arches and Central clusters show correlations between the Paschen α equivalent width and the adjacent continuum emission. However, the WN stars in the latter cluster are systematically dimmer than those in the Arches cluster, presumably due to the different ages of the two clusters. In the equivalent width–magnitude plot, late-type nitrogen sequence Wolf–Rayet (WNL) stars, WC stars and OB supergiants roughly fall into three distinct regions. We estimate that the dust mass associated with individual WC stars in the quintuplet cluster can reach 10−5 M⊙, or more than one order of magnitude larger than previous estimates. Thus, WC stars could be a significant source of dust in the galaxies of the early Universe. Nearly half of the evolved massive stars in the Galactic Centre are located outside the three known massive stellar clusters. The ionization of several compact H ii regions can be accounted for by their enclosed individual evolved massive stars, which thus likely formed in isolation or in small groups.

The extended ionized gas around the z = 2.44 radio galaxy MRC 0406–244: the nature of the superbubbles and the optical line brightness asymmetries

Abstract In this Letter, we investigate the nature of the dramatic ‘superbubble’ emission structures associated with the z= 2.44 radio galaxy MRC 0406−244, using rest-frame optical spectroscopy and an archival Hubble Space Telescope (HST) Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) image. Based on the optical line ratios and the HST morphology, we conclude that the gas in the superbubbles is photoionized by the obscured active nucleus. We suggest that this type of structure might be related to the spatially extended H i absorbers that are detected in front of many high-z radio galaxies. We also suggest that we may be witnessing the destruction of the extended emission-line region. In addition, we investigate the nature of the emission-line brightness asymmetry in MRC 0406−244: we conclude that this asymmetry is due to an asymmetry in the mass of ionized gas, confirming the scenario of McCarthy, van Breugel & Kapahi, and leading us to reject that of Gopal-Krishna & Wiita.

HUBBLE SPACE TELESCOPENICMOS POLARIZATION OBSERVATIONS OF THREE EDGE-ON MASSIVE YOUNG STELLAR OBJECTS

Massive young stellar objects (YSOs), like low-mass YSOs, appear to be surrounded by optically thick envelopes and/or disks and have regions, often bipolar, that are seen in polarized scattered light at near-infrared wavelengths. We are using the 0.''2 spatial resolution of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on Hubble Space Telescope to examine the structure of the disks and outflow regions of massive YSOs in star-forming regions within a few kpc of the Sun. Here we report on 2 {mu}m polarimetry of NGC 6334 V and S255 IRS1. NGC 6334 V consists of a double-lobed bright reflection nebula seen against a dark region, probably an optically thick molecular cloud. Our polarization measurements show that the illuminating star lies {approx}2'' south of the line connecting the two lobes; we do not detect this star at 2 {mu}m, but there are a small radio source and a mid-infrared source at this location. S255 IRS1 consists of two YSOs (NIRS1 and NIRS3) with overlapping scattered light lobes and luminosities corresponding to early B stars. Included in IRS1 is a cluster of stars from whose polarization we determine the local magnetic field direction. Neither of the YSOs has its scattered light lobesmore » aligned with this magnetic field. The line connecting the scattered light lobes of NIRS1 is twisted symmetrically around the star; the best explanation is that the star is part of a close binary and the outflow axis of NIRS1 is precessing as a result of non-coplanar disk and orbit. The star NIRS3 is also offset from the line connecting its two scattered light lobes. We suggest that all three YSOs show evidence of episodic ejection of material as they accrete from dense, optically thick envelopes.« less

Constraining star formation and AGN inz∼ 2 massive galaxies using high-resolution MERLIN radio observations

We present high spatial resolution Multi-Element Radio-Linked Interferometer Network (MERLIN) 1.4-GHz radio observations of two high-redshift (z∼ 2) sources, RG J123623 (HDF 147) and RG J123617 (HDF 130), selected as the brightest radio sources from a sample of submillimetre-faint radio galaxies. They have starburst classifications from their rest-frame ultraviolet spectra. However, their radio morphologies are remarkably compact (<80 and <65 mas, respectively), demanding that the radio luminosity be dominated by active galactic nuclei (AGN) rather than starbursts. Near-infrared (IR) imaging [Hubble Space Telescope Near Infrared Camera and Multi-Object Spectrometer (NICMOS) F160W] shows large-scale sizes (R1/2∼ 0.75 arcsec, diameters ∼12 kpc) and spectral energy distribution (SED) fitting to photometric points (optical through the mid-IR) reveals massive (∼5 × 1011 M⊙), old (a few Gyr) stellar populations. Both sources have low flux densities at observed 24 μm and are undetected in observed 70 μm and 850 μm, suggesting a low mass of interstellar dust. They are also formally undetected in the ultradeep 2 Ms Chandra data, suggesting that any AGN activity is likely intrinsically weak. We suggest both galaxies have evolved stellar populations, low star formation rates and low accretion rates on to massive black holes (108.6 M⊙) whose radio luminosity is weakly beamed (by factors of a few). A cluster-like environment has been identified near HDF 130 by an overdensity of galaxies at z= 1.99, reinforcing the claim that clusters lead to more rapid evolution in galaxy populations. These observations suggest that high-resolution radio (MERLIN) can be a superb diagnostic tool of AGN in the diverse galaxy populations at z∼ 2.

HST /NICMOS DETECTION OF HR 8799 b IN 1998

Three planets have been directly imaged around the young star HR 8799. The planets are 5-13 M Jup and orbit the star at projected separations of 24-68 AU. While the initial detection occurred in 2007, two of the planets were recovered in a reanalysis of data obtained in 2004. Here we present a detection of the furthest planet of that system, HR 8799 b, in archival Hubble Space Telescope (HST)/Near Infrared Camera and Multi-Object Spectrometer (NICMOS) data from 1998. The detection was made using the locally optimized combination of images algorithm to construct, from a large set of HST/NICMOS images of different stars taken from the archive, an optimized reference point-spread function image used to subtract the light of the primary star from the images of HR 8799. This new approach improves the sensitivity to planets at small separations by a factor of ~10 compared to traditional roll deconvolution. The new detection provides an astrometry point 10 years before the most recent observations, and is consistent with a Keplerian circular orbit with a~ 70 AU and low orbital inclination. The new photometry point, in the F160W filter, is in good agreement with an atmosphere model with intermediate clouds and vertical stratification, and thus suggests the presence of significant water absorption in the planet's atmosphere. The success of the new approach used here highlights a path for the search and characterization of exoplanets with future space telescopes, such as the James Webb Space Telescope or a Terrestrial Planet Finder.

L-DWARF BINARIES IN THE 20-PARSEC SAMPLE

We have used the NICMOS NIC1 camera on the Hubble Space Telescope to obtain high-resolution near-infrared images of 27 nearby ultracool dwarfs with formal distance estimates less than 20 parsecs. One target, 2MASS J22551861-5713056, is resolved as a binary system, with a 4:1 flux ratio in the F110W passband. The primary has spectral type L6, and the revised spectroscopic parallax places it at a distance of ~12 parsecs. The secondary is likely to have a spectral type between L8 and T1. The remaining 26 dwarfs are unresolved. Combining the present results with our earlier NICMOS (Near-Infrared Camera and Multi Object Spectrometer) observations and literature data, high-resolution imaging exists for 72 of the 85 L dwarfs in the current 20-parsec census. We derive a formal frequency of 15.3+5.1 −3.3% for resolved binaries in the local L-dwarf population. The frequency drops to 12.5+5.3 −3.0% if we take Malmquist bias into account. The NICMOS observations are capable of detecting systems with mass ratios q>0.2, but all known binaries are near equal-mass, suggesting that low-q systems either fail to form, or have short lifetimes.

Updating reionization scenarios after recent data

Abstract The recent release of data on (i) high-redshift source counts from the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) Hubble Ultra Deep Field (HUDF) and (ii) electron scattering optical depth from the Wilkinson Microwave Anisotropy Probe (WMAP) requires a re-examination of reionization scenarios. Using an improved self-consistent reionization model, based on Choudhury &amp; Ferrara, we determine the range of reionization histories which can match a wide variety of data sets simultaneously. The updated constraints from our analysis imply that hydrogen reionization starts at around z≈ 15, driven by the metal-free Population III (PopIII) stars (with normal Salpeter-like initial mass functions), and it is 90 per cent complete by z≈ 10. The photoionizing power of the PopIII stars fades for z≲ 10 because of concomitant action radiative and chemical feedbacks, which causes the reionization process to stretch considerably and to end only by z≈ 6. The combination of different data sets still favours a non-zero contribution from metal-free stars, with star-forming efficiencies larger than 2 per cent.

Constraints on the initial mass function of the first stars

Motivated by theoretical predictions that the first stars were predominantly very massive, we investigate the physics of the transition from an early epoch dominated by massive Pop III stars to a later epoch dominated by familiar low-mass Pop II/I stars by means of a numerically generated catalogue of dark matter haloes coupled with a self-consistent treatment of chemical and radiative feedback. Depending on the strength of the chemical feedback, Pop III stars can contribute a substantial fraction (several per cent) of the cosmic star formation activity even at moderate redshifts, z ≈ 5. We find that the three z ≈ 10 sources tentatively detected in Near Infrared Camera and Multi-Object Spectrometer (NICMOS) Ultra Deep Fields (UDFs) should be powered by Pop III stars, if these are massive; however, this scenario fails to reproduce the derived Wilkinson Microwave Anisotropy Probe (WMAP) electron scattering optical depth. Instead, both the UDFs and WMAP constraints can be fulfilled if stars at any time form with a more standard, slightly top-heavy, Larson initial mass function.

The NICMOS Polarimetric Calibration11Based on observations made with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS 5‐26555.

ABSTRACTAn accurate knowledge of the absolute calibration of the polarizing elements in the Near Infrared Camera and Multi‐Object Spectrometer (NICMOS) becomes especially important when conducting studies that require measuring degrees of polarization of close to 1% in the near‐infrared. We present a comprehensive study of all previously observed polarimetric standards using the NIC2 camera on NICMOS. Considering both pre‐ and post‐NICMOS Cooling System observations, we find variations in the polarimetry consistent with the effects of subpixel misalignments and the point‐spread function. We also measure nonzero results from unpolarized standards, indicating an instrumental polarization of p ≈ 1.2%, θ ≈ 88°. The lack of polarized and unpolarized standard stars with which to perform a comprehensive calibration study means that we cannot be confident that the current calibration will be effective for a number of recent large NICMOS GO programs. Further observations of polarimetric standards are needed in order to fully characterize the behavior of NICMOS at around p = 1%.

An Infrared Coronagraphic Survey for Substellar Companions

We have used the F160W filter (1.4–1.8 μm) and the coronagraph on the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope to survey 45 single stars with a median age of 0.15 Gyr, an average distance of 30 pc, and an average H magnitude of 7 mag. For the median age we were capable of detecting a 30MJ companion at separations between 15 and 200 AU. A 5MJ object could have been detected at 30 AU around 36% of our primaries. For several of our targets that were less than 30 Myr old, the lower mass limit was as low as 1MJ, well into the high mass planet region. Results of the entire survey include the proper-motion verification of five low-mass stellar companions, two brown dwarfs (HR7329B and TWA5B), and one possible brown dwarf binary (Gl 577B/C).

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

Abstract We report on the discovery of a fifth lensed image in the large separation lensed quasar system SDSS J1004$+$4112. A faint point source located ${0\rlap {.}{}^{\mathrm {\prime \prime }}2}$ from the center of the brightest galaxy in the lensing cluster was detected in images taken with the Advanced Camera for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth lensed image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves, and thereby the mass distribution of the lensing cluster.

Distant Red Galaxies in the Hubble Ultra Deep Field

We take advantage of the Hubble Ultra Deep Field data to study the rest-frame optical and UV morphologies of the novel population of distant red galaxies (DRGs). Six galaxies with J-Ks > 2.3 are found to Ks = 21.5, five of which have photometric redshifts zphot ≳ 2, corresponding to a surface density of 0.9 arcmin-2. The surface brightness distributions of the zphot ≳ 2 galaxies are better represented by exponential disks than R1/4 laws. Two of the zphot ≳ 2 galaxies are extended, while three have compact morphologies. The rest-frame optical morphology of the zphot ≳ 2 galaxies is quite different from the rest-frame UV morphology: All the galaxies have red central components that dominate in the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) H160-band images, as well as distinct off-center blue features that show up in (and often dominate) the Advanced Camera for Surveys (ACS) images. The mean measured effective radius of the zphot ≳ 2 galaxies is ⟨re⟩ = 1.9 ± 1.4 kpc, similar (within the errors) to the mean size of Lyman break galaxies at similar redshifts. All the DRGs are resolved in the ACS images, while four are resolved in the NICMOS images. Two of the zphot ≳ 2 galaxies are bright X-ray sources and hence host active galactic nuclei (AGNs). One of these galaxies is resolved in the ACS and NICMOS images, which means that the AGN does not dominate its rest-frame UV/optical spectral energy distribution (SED), while the other is unresolved in the NICMOS images and hence could have an AGN-dominated SED. The diverse rest-frame optical and UV morphological properties of DRGs derived here suggest that they have complex stellar populations consisting of both evolved populations, which dominate the mass and the rest-frame optical light, and younger populations that show up as patches of star formation in the rest-frame UV light, in many ways resembling the properties of normal local galaxies. This interpretation is supported by fits to the broadband SEDs, which for all five zphot ≳ 2 galaxies are best represented by models with extended star formation histories and substantial amounts of dust.

Galaxies at [FORMULA][F]z~7-8[/F][/FORMULA]: [FORMULA][F]z[INF]850[/INF][/F][/FORMULA]-Dropouts in the Hubble Ultra Deep Field

We have detected likely z ~ 7-8 galaxies in the 144'' × 144'' Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) observations of the Hubble Ultra Deep Field. Objects are required to be ≥3 σ detections in both NICMOS bands, J110 and H160. The selection criteria for this sample are (z850 - J110)AB > 0.8, (z850 - J110)AB > 0.66(J110 - H160)AB + 0.8, (J110 - H160)AB < 1.2 and no detection at less than 8500 Å. The five selected sources have total magnitudes H160, AB ~ 27. Four of the five sources are quite blue compared to typical lower redshift dropout galaxies and are clustered within a 1 arcmin2 region. Because all five sources are near the limit of the NICMOS data, we have carefully evaluated their reality. Each of the candidates is visible in different splits of the data and a median stack. We analyzed several noise images and estimate the number of spurious sources to be 1 ± 1. A search using an independent reduction of this same data set clearly revealed three of the five candidates and weakly detected a fourth candidate, suggesting that the contamination could be higher. For comparison with predictions from lower redshift samples, we take a conservative approach and adopt four z ~ 7-8 galaxies as our sample. With the same detection criteria on simulated data sets, assuming no evolution from z ~ 3.8, we predict 10 sources at z ~ 7-8, or 14 if we use a more realistic (1 + z)-1 size scaling. We estimate that the rest-frame continuum UV (~1800 Å) luminosity density at z ~ 7.5 (integrated down to 0.3L) is just 0.20 times that found at z ~ 3.8 (or 0.20 times this quantity including cosmic variance). Effectively this sets an upper limit on the luminosity density down to 0.3L and is consistent with significant evolution at the bright end of the luminosity function from z ~ 7.5 to 3.8. Even with the lower UV luminosity density at z ~ 7.5, it appears that galaxies could still play an important role in reionization at these redshifts, although definitive measurements remain to be made.

The Solar Neighborhood. IX.Hubble Space TelescopeDetections of Companions to Five M and L Dwarfs within 10 Parsecs of the Sun

We report the detections of low-mass companions to five M and L dwarfs within 10 pc of the Sun using the Hubble Space Telescope (HST) Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Follow-up observations using the HSTAdvanced Camera for Surveys (ACS) and Fine Guidance Sensor 1r (FGS1r) confirm our NICMOS discoveries of companions to the L4.5 dwarf GJ 1001B (LHS 102B) and the M5 dwarf LHS 224, respectively. Images obtained with the Astrophysical Research Consortium 3.5 m telescope at Apache Point Observatory verify our discovery of a companion to the M3 dwarf G239-25. Our NICMOS images confirm the previously suspected duplicity of the M3 dwarfs GJ 54 and GJ 84. The components of GJ 1001BC and LHS 224AB have nearly equal luminosities in all the ACS and/or NICMOS bandpasses in which they were observed. The magnitudes of GJ 54A and B in one FGS1r bandpass and four NICMOS bandpasses differ by P1. GJ 84B and G239-25B are � 4 mag fainter than their M3 companions in the NICMOS bandpasses. We estimate from our NICMOS photometry that GJ 84B and G239-25B have spectral types M7 and M8, respectively, and masses near the lower limit for sustained hydrogen burning. The apparent separations of GJ 1001BC, GJ 54AB, and LHS 224AB suggest orbital periods less than 5 yr. These binary dwarfs are ideal candidates for continued astrometric monitoring that will quickly yield accurate dynamic masses needed to constrain the mass-luminosity relation for low-mass stars and brown dwarfs.

Nuclear clusters, bulges and massive black holes in spiral galaxies

We report on some of the recent work that has come from our Hubble Space Telescope programme to find massive black holes in nearby spiral galaxies. Determining black hole masses from gas kinematics in spiral galaxies is difficult, but results have been reported for NGC 4041 and new results are soon to be reported for NGC 1300 and NGC 2748. Complementary programmes are also looking for the presence of nuclear star clusters in our Space Telescope Imaging Spectrograph (STIS) and archival Near-infrared Camera and Multi-Object Spectrometer (NICMOS) images. We are also currently analysing United Kingdom Infrared Telescope (UKIRT) Fast Track Imager (UFTI) near-infrared images to determine the surface brightness profiles for 37 of the galaxies in the sample. 1. Massive Black Holes in Nearby Spirals Massive black holes seem to be a common component of galactic nuclei. Attempts by the community to establish a demography for massive black holes has lead to various correlations between the mass of the black hole and other host galaxy properties (e.g. Ferrarese & Merritt 2000, Gebhardt et al. 2000, Marconi & Hunt 2003). However, for all these correlations spiral galaxies are relatively under-represented with respect to elliptical galaxies. To address this problem, and in an attempt to quantify the demography of massive black holes in nearby spiral galaxies, we had an HST/STIS programme (GO:8228, PI:Axon) to determine black hole masses using gas dynamics of the nuclear discs in 54 spirals. Marconi et al. (2003) reported the first black hole mass determination from this programme for NGC 4041, while an atlas of the spectra with reasonable signal to noise are presented in Hughes et al. (2003). Examples of some of the spectra and images, plus colour maps made using archival NICMOS images are shown in figure 1. The next paper to report black hole masses from the programme is now near completion (Atkinson et al., in prep) and will present results for NGC 1300 and NGC 2748. 2. Complimentary Work While the main emphasis of the program was to find black hole masses, other complimentary work is underway to understand the environments of the centres of these galaxies. One of the principle host-galaxy properties that has been shown to be correlated with black hole mass is stellar velocity dispersion (Ferrarese & Merritt 2000, Gebhardt 181 182 M. Hughes et al.

A high-resolution study of the interstellar medium in the luminousIRASgalaxy Arp 193

We present high-resolution multi-element radio-linked interferometer network (MERLIN) and Very Large Array (VLA) observations of the H I absorption towards the starburst nucleus of Arp 193 (IC 883, UGC 08387), a luminous IRAS merger. We compare the distribution of atomic gas with that of radio continuum emission, high-resolution CO data and near-infrared Hubble Space Telescope (HST) Near-Infrared Camera and Multi-object Spectrometer (NICMOS) data. We find, that for a constant spin temperature and CO flux to H 2 mass conversion factor, X CO , the interstellar medium (ISM) becomes progressively more H 2 rich towards the centre of the source. We also find that the projected velocity distribution of the atomic and molecular gas components are not the same. We discuss possible reasons for the observed differences and find that the principle difference is probably in the spatial distribution of the molecular and atomic ISM, although the addition of real dynamical differences provides a better explanation of the data. Evidence for free-free absorption towards the centre of the starburst nucleus is found, implying a large mass of ionized gas, and therefore young stars. Arp 193 is one of the very few IRAS luminous objects in which such a detailed study is possible.

An Extended Star Formation History for the Galactic Center fromHubble Space TelescopeNICMOS Observations

We present Hubble Space Telescope (HST) Near-Infrared Camera and Multiobject Spectrometer (NICMOS) observations as evidence that continuous star formation has created much of the central stellar cusp of the Galaxy. The data are the deepest ever obtained for a Galactic center population, being more than 50% complete for mF205W < 19.3, or initial stellar masses ≳2 M☉. We use Geneva and Padova stellar evolution models to produce synthetic luminosity functions for burst and continuous star formation scenarios, finding that the observations are fitted best by continuous star formation at a rate that is consistent with the recent star formation activity that produced the three massive young clusters in the central 50 pc. Further, it is not possible to fit the observations with ancient burst models, such as would be appropriate for an old population like that in Baade's window or NGC 6528.

Highlights from HST/NICMOS

The near-infrared camera and multi-object spectrometer (NICMOS) was installed in the Hubble Space Telescope in February 1997. After 22 months of operation the 110 kg supply of solid N 2, which cooled the NICMOS-3 HgCdTe detectors, was exhausted. The cryostat warmed up leaving the instrument in an electromechanically functional, but scientifically non-viable, passive state. Following a three year suspension of observations, an active cooling system featuring a reverse Brayton cycle micro-turbine was installed and integrated with NICMOS during HST servicing mission 3B. As a result the NICMOS detectors and cold optics have successfully been re-cooled to operationally effective temperatures and the science program has begun. We review the operating characteristics of the NICMOS instrument, its new cooling system, and its performance both in absolute terms and in comparison to its previous on-orbit incarnation. We discuss the scope and nature of the astronomical investigations that have been and are again enabled with NICMOS. Additionally, we present some of the first scientific results from programs executed with the re-cooled NICMOS.

A Comparative Anatomy of Dusty Disks Imaged by NICMOS

Using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope we have conducted a coronagraphic imaging survey of 18 main sequence stars with large infrared excesses, searching for circumstellar dust (debris) in scattered light. Dusty disks with radial and hemispheric brightness anisotropies and complex morphologies, both possibly indicative of dynamical interactions with unseen planetary mass companions, were spatially resolved and imaged around three young (≲ 10Myr old) stars. From these observations we describe the debris systems around: a) HR 4796A (A0V), a 70 AU radius ring less than 14 AU wide with unequal ansal flux densities; b) HD 141569A (Herbig Ae/Be), a 400 AU radius disk with a 40 AU wide gap; and c) TW Hya (K7 T-Tauri), a pole-on circularly symmetric disk with a radial break in its surface density of scattering particles. Additionally, our non-detection of scattered light and high precision photometry of a fourth system of similar age, HD 98800 A/B, coupled with mid and thermal IR measurements, greatly constrain a likely model for the debris about the B component.