| 1. |
- Wang, Xiaofeng, et al.
(författare)
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Evidence for type ia supernova diversity from ultraviolet observations with the hubble space telescope
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 749:2, s. 126-
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Tidskriftsartikel (refereegranskat)abstract
- We present ultraviolet (UV) spectroscopy and photometry of four Type Ia supernovae (SNe 2004dt, 2004ef, 2005M, and 2005cf) obtained with the UV prism of the Advanced Camera for Surveys on the Hubble Space Telescope. This data set provides unique spectral time series down to 2000 angstrom. Significant diversity is seen in the near-maximum-light spectra (similar to 2000-3500 angstrom) for this small sample. The corresponding photometric data, together with archival data from Swift Ultraviolet/Optical Telescope observations, provide further evidence of increased dispersion in the UV emission with respect to the optical. The peak luminositiesmeasured in the uvw1/F250W filter are found to correlate with the B-band light-curve shape parameter Delta m(15)(B), but with much larger scatter relative to the correlation in the broadband B band (e.g., similar to 0.4 mag versus similar to 0.2 mag for those with 0.8 mag < Delta m(15)(B) < 1.7 mag). SN 2004dt is found as an outlier of this correlation (at > 3 sigma), being brighter than normal SNe Ia such as SN 2005cf by similar to 0.9 mag and similar to 2.0 mag in the uvw1/F250W and uvm2/F220W filters, respectively. We show that different progenitor metallicity or line-expansion velocities alone cannot explain such a large discrepancy. Viewing-angle effects, such as due to an asymmetric explosion, may have a significant influence on the flux emitted in the UV region. Detailed modeling is needed to disentangle and quantify the above effects.
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| 2. |
- Sestito, Federico, et al.
(författare)
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The Pristine survey – X. A large population of low-metallicity stars permeates the Galactic disc
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966 .- 1745-3925 .- 1745-3933. ; 497:1, s. L7-L12
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Tidskriftsartikel (refereegranskat)abstract
- The orbits of the least chemically enriched stars open a window on the formation of our Galaxy when it was still in its infancy. The common picture is that these low-metallicity stars are distributed as an isotropic, pressure-supported component since these stars were either accreted from the early building blocks of the assembling Milky Way (MW), or were later brought by the accretion of faint dwarf galaxies. Combining the metallicities and radial velocities from the Pristine and LAMOST surveys and Gaia DR2 parallaxes and proper motions for an unprecedented large and unbiased sample of 1027 very metal poor stars at [Fe/H] ≤ −2.5 dex, we show that this picture is incomplete. We find that 31 per cent of the stars that currently reside spatially in the disc (|Z|≤3kpc) do not venture outside of the disc plane throughout their orbit. Moreover, this sample shows strong statistical evidence (at the 5.0σ level) of asymmetry in their kinematics, favouring prograde motion. The discovery of this population implies that a significant fraction of stars with iron abundances [Fe/H] ≤ −2.5 dex merged into, formed within, or formed concurrently with the MW disc and that the history of the disc was quiet enough to allow them to retain their disc-like orbital properties, challenging theoretical and cosmological models.
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| 3. |
- Fantin, Nicholas J., et al.
(författare)
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The Canada-France Imaging Survey : Reconstructing the Milky Way Star Formation History from Its White Dwarf Population
- 2019
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 887:2
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Tidskriftsartikel (refereegranskat)abstract
- As the remnants of stars with initial masses less than or similar to 8M(circle dot), white dwarfs contain valuable information on the formation histories of stellar populations. In this paper, we use deep, high-quality, u-band photometry from the Canada-France Imaging Survey, griz photometry from Pan-STARRS1, as well as proper motions from Gaia DR2, to select 25,156 white dwarf candidates over similar to 4500 deg(2) using a reduced proper motion diagram. We develop a new white dwarf population synthesis code that returns mock observations of the Galactic field white dwarf population for a given star formation history, while simultaneously taking into account the geometry of the Milky Way (MW), survey parameters, and selection effects. We use this model to derive the star formation histories of the thin disk, thick disk, and stellar halo. Our results show that the MW disk began forming stars (11.3 +/- 0.5) Gyr ago, with a peak rate of (8.8 +/- 1.4) M-circle dot yr(-1) at (9.8 +/- 0.4) Gyr, before a slow decline to a constant rate until the present day-consistent with recent results suggesting a merging event with a satellite galaxy. Studying the residuals between the data and best-fit model shows evidence for a slight increase in star formation over the past 3 Gyr. We fit the local fraction of helium-atmosphere white dwarfs to be (21 +/- 3)%. Incorporating this methodology with data from future wide-field surveys such as the Large Synoptic Survey Telescope, Euclid, The Cosmological Advanced Survey Telescope for Optical and ultraviolet Research, and the Wide Field Infrared Survey Telescope should provide an unprecedented view into the formation of the MW at its earliest epoch through its white dwarfs.
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| 4. |
- Fernández-Alvar, Emma, et al.
(författare)
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The Pristine survey XIII : uncovering the very metal-poor tail of the thin disc
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 508:1, s. 1509-1525
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Tidskriftsartikel (refereegranskat)abstract
- We evaluate the rotational velocity of stars observed by the Pristine survey towards the Galactic anticentre spanning a wide range of metallicities from the extremely metal-poor regime ([Fe/H] < −3) to nearly solar metallicity. In the Galactic anticentre direction, the rotational velocity (Vφ) is similar to the tangential velocity in the galactic longitude direction (V). This allows us to estimate Vφ from Gaia early data release 3 (Gaia EDR3) proper motions for stars without radial velocity measurements. This substantially increases the sample of stars in the outer disc with estimated rotational velocities. Our stellar sample towards the anticentre is dominated by a kinematical thin disc with a mean rotation of ∼−220 km s−1. However, our analysis reveals the presence of more stellar substructures. The most intriguing is a well-populated extension of the kinematical thin disc down to [Fe/H] ∼ −2 . A scarcer fast-rotating population reaching the extremely metal-poor regime down to [Fe/H] ∼ −3.5 is also detected but without statistical significance to unambiguously state whether this is the extremely metal-poor extension of the thin disc or the high-rotating tail of hotter structures (like the thick disc or the halo). In addition, a more slowly rotating kinematical thick disc component is also required to explain the observed V distribution at [Fe/H] > −1.5 . Furthermore, we detect signatures of a ‘heated disc’, the so-called Splash, at metallicities higher than ∼−1.5. Finally, at [Fe/H] < −1.5 our anticentre sample is dominated by a kinematical halo with a net prograde motion.
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| 5. |
- Longeard, Nicolas, et al.
(författare)
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The pristine dwarf-galaxy survey - III. Revealing the nature of the Milky Way globular cluster Sagittarius II
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 503:2, s. 2754-2762
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Tidskriftsartikel (refereegranskat)abstract
- We present a new spectroscopic study of the faint Milky Way satellite Sagittarius II. Using multiobject spectroscopy from the Fibre Large Array Multi-Element Spectrograph, we supplement the data set of Longeard et al. with 47 newly observed stars, 19 of which are identified as members of the satellite. These additional member stars are used to put tighter constraints on the dynamics and the metallicity properties of the system. We find a low velocity dispersion of sigma(SgrII)(v) = 1.7 +/- 0.5 km s(-1), in agreement with the dispersion of Milky Way globular clusters of similar luminosity. We confirm the very metal-poor nature of the satellite ([Fe/H](spectro)(SgrII) = -2.23 +/- 0.07) and find that the metallicity dispersion of Sgr II is not resolved, reaching only 0.20 at the 95 per cent confidence limit. No star with a metallicity below -2.5 is confidently detected. Therefore, despite the unusually large size of the system (r(h) = 35.5(-1.2)(-1.4) pc), we conclude that Sgr II is an old and metal-poor globular cluster of the Milky Way.
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| 6. |
- Longeard, Nicolas, et al.
(författare)
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The Pristine dwarf galaxy survey - IV. Probing the outskirts of the dwarf galaxy Bootes I
- 2022
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 516:2, s. 2348-2362
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Tidskriftsartikel (refereegranskat)abstract
- We present a new spectroscopic study of the dwarf galaxy Boötes I (Boo I) with data from the Anglo-Australian Telescope and its AAOmega spectrograph together with the Two Degree Field multi-object system. We observed 36 high-probability Boo I stars selected using Gaia Early Data Release 3 proper motions and photometric metallicities from the Pristine survey. Out of those, 27 are found to be Boo I stars, resulting in an excellent success rate of 75 per cent at finding new members. Our analysis uses a new pipeline developed to estimate radial velocities and equivalent widths of the calcium triplet lines from Gaussian and Voigt line profile fits. The metallicities of 16 members are derived, including 3 extremely metal-poor stars ([Fe/H] < −3.0), which translates into a success rate of 25 per cent at finding them with the combination of Pristine and Gaia. Using the large spatial extent of our new members that spans up to 4.1 half-light radii and spectroscopy from the literature, we find a systemic velocity gradient of 0.40 ± 0.10 km s−1 arcmin−1 and a small but resolved metallicity gradient of −0.008 ± 0.003 dex arcmin−1. Finally, we show that Boo I is more elongated than previously thought with an ellipticity of ϵ = 0.68 ± 0.15. Its velocity and metallicity gradients as well as its elongation suggest that Boo I may have been affected by tides, a result supported by direct dynamical modelling.
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| 7. |
- Malhan, Khyati, et al.
(författare)
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Butterfly in a Cocoon, Understanding the Origin and Morphology of Globular Cluster Streams : The Case of GD-1
- 2019
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 881:2
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Tidskriftsartikel (refereegranskat)abstract
- Tidally disrupted globular cluster (GC) streams are usually observed, and therefore perceived, as narrow, linear, and one-dimensional structures in the 6D phase space. Here, we show that the GD-1 stellar stream, which is the tidal debris of a disrupted GC, possesses a secondary diffuse and extended stellar component (similar to 100 pc wide) around it, detected at the >5 sigma confidence level. Similar morphological properties are seen in synthetic streams that are produced from star clusters that are formed within dark matter sub-halos and then accrete onto a massive host galaxy. This lends credence to the idea that the progenitor of the highly retrograde GD-1 stream was originally formed outside of the Milky Way in a now defunct dark satellite galaxy. We deem that in future studies, this newly found cocoon component may serve as a structural hallmark to distinguish between the in situ and ex situ (accreted) formed GC streams.
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| 8. |
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| 9. |
- Ni, Yuan Qi, et al.
(författare)
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Infant-phase reddening by surface Fe-peak elements in a normal type Ia supernova
- 2022
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Ingår i: Nature Astronomy. - : Springer Science and Business Media LLC. - 2397-3366. ; 6:5, s. 568-576
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Tidskriftsartikel (refereegranskat)abstract
- Type Ia supernovae are thermonuclear explosions of white dwarf stars. They play a central role in the chemical evolution of the Universe and are an important measure of cosmological distances. However, outstanding questions remain about their origins. Despite extensive efforts to obtain natal information from their earliest signals, observations have thus far failed to identify how the majority of them explode. Here, we present infant-phase detections of SN 2018aoz from a very low brightness of −10.5 AB absolute magnitude, revealing a hitherto unseen plateau in the B band that results in a rapid redward colour evolution between 1.0 and 12.4 hours after the estimated epoch of first light. The missing B-band flux is best explained by line-blanket absorption from Fe-peak elements in the outer 1% of the ejected mass. The observed B − V colour evolution of the supernova also matches the prediction from an over-density of Fe-peak elements in the same outer 1% of the ejected mass, whereas bluer colours are expected from a purely monotonic distribution of Fe-peak elements. The presence of excess nucleosynthetic material in the extreme outer layers of the ejecta points to enhanced surface nuclear burning or extended subsonic mixing processes in some normal type Ia SN explosions.
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| 10. |
- Ni, Yuan Qi, et al.
(författare)
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The Origin and Evolution of the Normal Type Ia SN 2018aoz with Infant-phase Reddening and Excess Emission
- 2023
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 946:1
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Tidskriftsartikel (refereegranskat)abstract
- SN 2018aoz is a Type Ia SN with a B-band plateau and excess emission in infant-phase light curves ≲1 day after the first light, evidencing an over-density of surface iron-peak elements as shown in our previous study. Here, we advance the constraints on the nature and origin of SN 2018aoz based on its evolution until the nebular phase. Near-peak spectroscopic features show that the SN is intermediate between two subtypes of normal Type Ia: core normal and broad line. The excess emission may be attributable to the radioactive decay of surface iron-peak elements as well as the interaction of ejecta with either the binary companion or a small torus of circumstellar material. Nebular-phase limits on Hα and He i favor a white dwarf companion, consistent with the small companion size constrained by the low early SN luminosity, while the absence of [O I] and He i disfavors a violent merger of the progenitor. Of the two main explosion mechanisms proposed to explain the distribution of surface iron-peak elements in SN 2018aoz, the asymmetric Chandrasekhar-mass explosion is less consistent with the progenitor constraints and the observed blueshifts of nebular-phase [Fe II] and [Ni II]. The helium-shell double-detonation explosion is compatible with the observed lack of C spectral features, but current 1D models are incompatible with the infant-phase excess emission, Bmax–Vmax color, and weak strength of nebular-phase [Ca II]. Although the explosion processes of SN 2018aoz still need to be more precisely understood, the same processes could produce a significant fraction of Type Ia SNe that appear to be normal after ∼1 day.
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