430 research outputs found

    Disrupted asteroid P/2016 G1. II. Follow-up observations from the Hubble Space Telescope

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    After the early observations of the disrupted asteroid P/2016 G1 with the 10.4m Gran Telescopio Canarias (GTC), and the modeling of the dust ejecta, we have performed a follow-up observational campaign of this object using the Hubble Space Telescope (HST) during two epochs (June 28 and July 11, 2016). The analysis of these HST images with the same model inputs obtained from the GTC images revealed a good consistency with the predicted evolution from the GTC images, so that the model is applicable to the whole observational period from late April to early July 2016. This result confirms that the resulting dust ejecta was caused by a relatively short-duration event with onset about 350 days before perihelion, and spanning about 30 days (HWHM). For a size distribution of particles with a geometric albedo of 0.15, having radii limits of 1 μ\mum and 1 cm, and following a power-law with index --3.0, the total dust mass ejected is \sim2×\times107^7 kg. As was the case with the GTC observations, no condensations in the images that could be attributed to a nucleus or fragments released after the disruption event were found. However, the higher limiting magnitude reachable with the HST images in comparison with those from GTC allowed us to impose a more stringent upper limit to the observed fragments of \sim30 m.Comment: 10 pages, 2 figures Accepted by Astronomical Journal, Nov. 2, 201

    Space Telescope Imaging Spectrograph slitless observations of Small Magellanic Cloud Planetary Nebulae: a study on morphology, emission line intensity, and evolution

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    A sample of 27 Planetary Nebulae (PNs) in the Small Magellanic Clouds (SMC) have been observed with the Hubble Space Telescope Imaging Spectrograph (HST/STIS) to determine their morphology, size, and the spatial variation of the ratios of bright emission lines. The morphologies of SMC PNs are similar to those of LMC and Galactic PNs. However, only a third of the resolved SMC PNs are asymmetric, compared to half in the LMC. The low metallicity environment of the SMC seems to discourage the onset of bipolarity in PNs. We measured the line intensity, average surface brightness (SB), and photometric radius of each nebula in halpha, hbeta, [O III] lambda4959 and 5007, [NII] 6548 and 6584, [S II] lambda6716 and 5731, He I 6678, and [OI] 6300 and 6363. We show that the surface brightness to radius relationship is the same as in LMC PNs, indicating its possible use as a distance scale indicator for Galactic PNs. We determine the electron densities and the ionized masses of the nebulae where the [S II] lines were measured accurately, and we find that the SMC PNs are denser than the LMC PNs by a factor of 1.5. The average ionized mass of the SMC PNs is 0.3 Msun. We also found that the median [O III]/hbeta intensity ratio in the SMC is about half than the corresponding LMC median. We use Cloudy to model the dependence of the [O III]/hbeta ratio on the oxygen abundance. Our models encompass very well the average observed physical quantities. We suggest that the SMC PNs are principally cooled by the carbon lines, making it hard to study their excitation based on the optical lines at our disposal.Comment: Accepted for publication in the Astrophysical Journal, 30 pages, 13 figures, 6 tables. For high resolution version of Figs 1 to 6, see http://archive.stsci.edu/hst/mcpn/home.htm

    Ultraviolet and visible photometry of asteroid (21) Lutetia using the Hubble Space Telescope

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    The asteroid (21) Lutetia is the target of a planned close encounter by the Rosetta spacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities. We used the Hubble Space Telescope (HST) to determine the albedo of Lutetia over a wide wavelength range, extending from ~150 nm to ~700 nm. Using data from a variety of HST filters and a ground-based visible light spectrum, we employed synthetic photometry techniques to derive absolute fluxes for Lutetia. New results from ground-based measurements of Lutetia's size and shape were used to convert the absolute fluxes into albedos. We present our best model for the spectral energy distribution of Lutetia over the wavelength range 120-800 nm. There appears to be a steep drop in the albedo (by a factor of ~2) for wavelengths shorter than ~300 nm. Nevertheless, the far ultraviolet albedo of Lutetia (~10%) is considerably larger than that of typical C-chondrite material (~4%). The geometric albedo at 550 nm is 16.5 +/- 1%. Lutetia's reflectivity is not consistent with a metal-dominated surface at infrared or radar wavelengths, and its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed for typical primitive, chondritic material. We derive a relatively high FUV albedo of ~10%, a result that will be tested by observations with the Alice spectrograph during the Rosetta flyby of Lutetia in July 2010.Comment: 14 pages, 2 tables, 8 figure

    Hubble Space Telescope observations of three very young star clusters in the Small Magellanic Cloud

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    We present Space Telescope Imaging Spectrograph (STIS) broad band imagery and optical slitless spectroscopy of three young star clusters in the Small Magellanic Cloud (SMC). MA 1796 and MG 2 were previously known as Planetary Nebulae, and observed as such within our Hubble Space Telescope (HST) survey. With the HST spatial resolution, we show that they are instead H II regions, surrounding very young star clusters. A third compact H II region, MA 1797, was serendipitously observed by us as it falls in the same frame of MA 1796. Limited nebular analysis is presented as derived from the slitless spectra. We find that MA 1796 and MG 2 are very heavily extincted, with c>1.4, defining them as the most extincted optically-discovered star forming regions in the SMC. MA 1796 and MG 2 are extremely compact (less than 1 pc across), while MA 1797, with diameter of about 3 pc, is similar to the ultra compact H II regions already known in the SMC. Stellar analysis is presented, and approximate reddening correction for the stars is derived from the Balmer decrement. Limited analysis of their stellar content and their ionized radiation shows that these compact H II regions are ionized by small stellar clusters whose hottest stars are at most of the B0 class. These very compact, extremely reddened, and probably very dense H II regions in the SMC offer insight in the most recent star formation episodes in a very low metallicity galaxy.Comment: The Astrophysical Journal, Part 1, in press 17 pages, and 3 figure

    How unique is the Asymptotic Normalisation Coefficient (ANC) method?

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    The asymptotic normalisation coefficients (ANC) for the vertex 10^{10}B \to 9^9Be + p is deduced from a set of different proton transfer reactions at different energies. This set should ensure the peripheral character of the reaction and availability of data for the elastic channels. The problems associated with the characteristics of the data and the analysis are discussed. For a subgroup of the set of available data, the uniqueness property of the extracted ANC is fulfilled. However, more measurements are needed before a definite conclusion can be drawn.Comment: 19 pages, 11 figures, to be published in Phys Rev
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