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Ramstedt, S., Mohamed, S., Olander, T., Vlemmings, W. H., Khouri, T. & Liljegren, S. (2018). CO envelope of the symbiotic star R Aquarii seen by ALMA. Astronomy and Astrophysics, 616, Article ID A61.
Open this publication in new window or tab >>CO envelope of the symbiotic star R Aquarii seen by ALMA
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, article id A61Article in journal (Refereed) Published
Abstract [en]

The symbiotic star R Aqr is part of a small sample of binary AGB stars observed with the Atacama Large Millimeter/submillimeter Array (ALMA). The sample stars are: R Aqr, Mira, W Aql, and pi(1) Gru. The sample covers a range in binary separation and wind properties, where R Aqr is the source with the smallest separation. The R Aqr binary pair consists of an M-type AGB star and a white dwarf at a separation of 45 mas, equivalent to about 10 AU at 218 pc. The aim of the ALMA study is to investigate the dependence of the wind shaping on the binary separation and to provide constraints for hydrodynamical binary interaction models. R Aqr is particularly interesting as the source with the smallest separation and a complex circumstellar environment that is strongly affected by the interaction between the two stars and by the high-energy radiation resulting from this interaction and from the hot white dwarf companion. The CO(J = 3 -> 2) line emission has been observed with ALMA at similar to 0.5 '' spatial resolution. The CO envelope around the binary pair is marginally resolved, showing what appears to be a rather complex distribution. The outer radius of the CO emitting region is estimated from the data and found to be about a factor of 10 larger than previously thought. This implies an average mass -loss rate during the past similar to 100 yr of M approximate to 2x10(-7) M-circle dot yr(-1), a factor of 45 less than previous estimates. The channel maps are presented and the molecular gas distribution is discussed and set into the context of what was previously known about the system from multiwavelength observations. Additional molecular line emission detected within the bandwidth covered by the ALMA observations is also presented. Because of the limited extent of the emission, firm conclusions about the dynamical evolution of the system will have to wait for higher spatial resolution observations. However, the data presented here support the assumption that the mass -loss rate from the Mira star strongly varies and is focused on the orbital plane.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2018
Keywords
stars: AGB and post-AGB, binaries: symbiotic, circumstellar matter, stars: winds, outflows
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-363942 (URN)10.1051/0004-6361/201833394 (DOI)000441818900002 ()
Funder
EU, European Research Council, 614264Swedish Research Council
Available from: 2018-10-22 Created: 2018-10-22 Last updated: 2018-10-22Bibliographically approved
Saberi, M., Vlemmings, W. H., De Beck, E., Montez, R. & Ramstedt, S. (2018). Detection of CI line emission towards the oxygen-rich AGB star omi Ceti. Astronomy and Astrophysics, 612, Article ID L11.
Open this publication in new window or tab >>Detection of CI line emission towards the oxygen-rich AGB star omi Ceti
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id L11Article in journal (Refereed) Published
Abstract [en]

We present the detection of neutral atomic carbon CI(3P13P0) line emission towards omi Cet. This is the first time that CI is detected in the envelope around an oxygen-rich M-type asymptotic giant branch (AGB) star. We also confirm the previously tentative CI detection around V Hya, a carbon-rich AGB star. As one of the main photodissociation products of parent species in the circumstellar envelope (CSE) around evolved stars, CI can be used to trace sources of ultraviolet (UV) radiation in CSEs. The observed flux density towards omi Cet can be reproduced by a shell with a peak atomic fractional abundance of 2.4 × 10−5 predicted based on a simple chemical model where CO is dissociated by the interstellar radiation field. However, the CI emission is shifted by ~4 km s−1 from the stellar velocity. Based on this velocity shift, we suggest that the detected CI emission towards omi Cet potentially arises from a compact region near its hot binary companion. The velocity shift could, therefore, be the result of the orbital velocity of the binary companion around omi Cet. In this case, the CI column density is estimated to be 1.1 × 1019 cm−2. This would imply that strong UV radiation from the companion and/or accretion of matter between two stars is most likely the origin of the CI enhancement. However, this hypothesis can be confirmed by high-angular resolution observations.

Keywords
astrochemistry, stars: individual: omi Cet, V Hya, circumstellar matter, atomic processes, stars: abundances, stars: AGB and post-AGB
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-356493 (URN)10.1051/0004-6361/201833080 (DOI)000431823900002 ()
Funder
EU, European Research Council, 614264Swedish National Space Board
Available from: 2018-07-30 Created: 2018-07-30 Last updated: 2018-07-30Bibliographically approved
Bujarrabal, V., Alcolea, J., Mikolajewska, J., Castro-Carrizo, A. & Ramstedt, S. (2018). High-resolution observations of the symbiotic system R Aqr Direct imaging of the gravitational effects of the secondary on the stellar wind. Astronomy and Astrophysics, 616, Article ID L3.
Open this publication in new window or tab >>High-resolution observations of the symbiotic system R Aqr Direct imaging of the gravitational effects of the secondary on the stellar wind
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 616, article id L3Article in journal (Refereed) Published
Abstract [en]

We have observed the symbiotic stellar system R Aqr, aiming to describe the gravitational interaction between the white dwarf (WD) and the wind from the Mira star, the key phenomenon driving the symbiotic activity and the formation of nebulae in such systems. We present high-resolution ALMA maps of the (CO)-C-12 and (CO)-C-13 J = 3-2 lines, the 0.9 mm continuum distribution, and some high-excitation molecular lines in R Aqr. The maps, which have resolutions ranging between 40 mas and less than 20 mas probe the circumstellar regions at suborbital scales as the distance between the stars is similar to 40 mas. Our observations show the gravitational effects of the secondary on the stellar wind. The AGB star was identified in our maps from the continuum and molecular line data, and we estimated the probable position of the secondary from a new estimation of the orbital parameters. The (preliminary) comparison of our maps with theoretical predictions is surprisingly satisfactory and the main expected gravitational effects are directly mapped for the first time. We find a strong focusing in the equatorial plane of the resulting wind, which shows two plumes in opposite directions that have different velocities and very probably correspond to the expected double spiral due to the interaction. Our continuum maps show the very inner regions of the nascent bipolar jets, at scales of some AU. Continuum maps obtained with the highest resolution show the presence of a clump that very probably corresponds to the emission of the ionized surroundings of the WD and of a bridge of material joining both stars, which is likely material flowing from the AGB primary to the accretion disk around the WD secondary.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2018
Keywords
stars: AGB and post-AGB, circumstellar matter, binaries : close, binaries: symbiotic, stars: individual: R Aqr
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-362686 (URN)10.1051/0004-6361/201833633 (DOI)000441816900002 ()
Available from: 2018-11-12 Created: 2018-11-12 Last updated: 2018-11-12Bibliographically approved
Paladini, C., Baron, F., Jorissen, A., Le Bouquin, J.-B. -., Freytag, B., Van Eck, S., . . . Ramstedt, S. (2018). Large granulation cells on the surface of the giant star π1 Gruis. Nature, 553(7688), 310-+
Open this publication in new window or tab >>Large granulation cells on the surface of the giant star π1 Gruis
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2018 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 553, no 7688, p. 310-+Article in journal (Refereed) Published
Abstract [en]

Convection plays a major part in many astrophysical processes, including energy transport, pulsation, dynamos and winds on evolved stars, in dust clouds and on brown dwarfs1,2. Most of our knowledge about stellar convection has come from studying the Sun: about two million convective cells with typical sizes of around 2,000 kilometres across are present on the surface of the Sun3—a phenomenon known as granulation. But on the surfaces of giant and supergiant stars there should be only a few large (several tens of thousands of times larger than those on the Sun) convective cells3, owing to low surface gravity. Deriving the characteristic properties of convection (such as granule size and contrast) for the most evolved giant and supergiant stars is challenging because their photospheres are obscured by dust, which partially masks the convective patterns4. These properties can be inferred from geometric model fitting5,6,7, but this indirect method does not provide information about the physical origin of the convective cells5,6,7. Here we report interferometric images of the surface of the evolved giant star π1 Gruis, of spectral type8,9 S5,7. Our images show a nearly circular, dust-free atmosphere, which is very compact and only weakly affected by molecular opacity. We find that the stellar surface has a complex convective pattern with an average intensity contrast of 12 per cent, which increases towards shorter wavelengths. We derive a characteristic horizontal granule size of about 1.2 × 1011 metres, which corresponds to 27 per cent of the diameter of the star. Our measurements fall along the scaling relations between granule size, effective temperature and surface gravity that are predicted by simulations of stellar surface convection10,11,12.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-343860 (URN)10.1038/nature25001 (DOI)000423475100045 ()29258298 (PubMedID)
Funder
EU, Horizon 2020, 730890
Available from: 2018-03-02 Created: 2018-03-02 Last updated: 2018-03-02Bibliographically approved
Brunner, M., Danilovich, T., Ramstedt, S., Marti-Vidal, I., De Beck, E., Vlemmings, W. H., . . . Kerschbaum, F. (2018). Molecular line study of the S-type AGB star W Aquilae ALMA observations of CS, SiS, SiO and HCN. Astronomy and Astrophysics, 617, Article ID A23.
Open this publication in new window or tab >>Molecular line study of the S-type AGB star W Aquilae ALMA observations of CS, SiS, SiO and HCN
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 617, article id A23Article in journal (Refereed) Published
Abstract [en]

Context: With the outstanding spatial resolution and sensitivity of the Atacama Large Millimeter/sub-millimeter Array (ALMA), molecular gas other than the abundant CO can be observed and resolved in circumstellar envelopes (CSEs) around evolved stars, such as the binary S-type asymptotic giant branch (AGB) star W Aquilae.

Aims: We aim to constrain the chemical composition of the CSE and determine the radial abundance distribution, the photospheric peak abundance, and isotopic ratios of a selection of chemically important molecular species in the innermost CSE of W Aql. The derived parameters are put into the context of the chemical evolution of AGB stars and are compared with theoretical models.

Methods: We employ one-dimensional radiative transfer modeling - with the accelerated lambda iteration (ALI) radiative transfer code-of the radial abundance distribution of a total of five molecular species (CS, SiS, (SiS)-Si-30, (SiO)-Si-29 and (HCN)-C-13) and determine the best fitting model parameters based on high-resolution ALMA observations as well as archival single-dish observations. The additional advantage of the spatially resolved ALMA observations is that we can directly constrain the radial profile of the observed line transitions from the observations.

Results: We derive abundances and e-folding radii for CS, SiS, (SiS)-Si-30, (SiO)-Si-29 and (HCN)-C-13 and compare them to previous studies, which are based only on unresolved single-dish spectra. Our results are in line with previous results and are more accurate due to resolution of the emission regions.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2018
Keywords
stars: abundances, stars: AGB and post-AGB, circumstellar matter, stars: mass-loss, stars: winds, outflows, submillimeter: stars
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-365291 (URN)10.1051/0004-6361/201832724 (DOI)000444607000001 ()
Funder
Swedish National Space Board
Available from: 2018-11-14 Created: 2018-11-14 Last updated: 2018-11-14Bibliographically approved
Vlemmings, W. H., Khouri, T., De Beck, E., Olofsson, H., Garcia-Segura, G., Villaver, E., . . . Ramstedt, S. (2018). Rotation of the asymptotic giant branch star R Doradus [Letter to the editor]. Astronomy and Astrophysics, 613, Article ID L4.
Open this publication in new window or tab >>Rotation of the asymptotic giant branch star R Doradus
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 613, article id L4Article in journal, Letter (Refereed) Published
Abstract [en]

High-resolution observations of the extended atmospheres of asymptotic giant branch (AGB) stars can now directly be compared to the theories that describe stellar mass loss. Using Atacama Large Millimeter/submillimeter Array (ALMA) high angular resolution (30 x 42 mas) observations, we have for the first time resolved stellar rotation of an AGB star, R Dor. We measure an angular rotation velocity of omega(R) sin i = (3.5 +/- 0.3) x 10(-9) rad s(-1), which indicates a rotational velocity of vertical bar nu(rot) sin i vertical bar = 1.0 +/- 0.1 km s(-1) at the stellar surface (R-* = 31.2 mas at 214 GHz). The rotation axis projected on the plane of the sky has a position angle Phi = 7 +/- 6 degrees. We find that the rotation of R Dor is two orders of magnitude faster than expected for a solitary AGB star that will have lost most of its angular momentum. Its rotational velocity is consistent with angular momentum transfer from a close companion. As a companion has not been directly detected, we suggest R Dor has a low-mass, close-in companion. The rotational velocity approaches the critical velocity, set by the local sound speed in the extended envelope, and is thus expected to affect the mass-loss characteristics of R Dor.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2018
Keywords
stars: AGB and post-AGB, stars: rotation
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-357271 (URN)10.1051/0004-6361/201832929 (DOI)000432980000002 ()
Funder
EU, European Research Council, 614264Swedish Research Council
Available from: 2018-08-20 Created: 2018-08-20 Last updated: 2018-08-20
Montez, R. J., Ramstedt, S., Kastner, J. H., Vlemmings, W. & Sanchez, E. (2017). A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars. Astrophysical Journal, 841(1), Article ID 33.
Open this publication in new window or tab >>A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars
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2017 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 841, no 1, article id 33Article in journal (Refereed) Published
Abstract [en]

We have performed a comprehensive study of the UV emission detected from asymptotic giant branch (AGB) stars by the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our sample, 316 were observed by GALEX. In the near-UV (NUV) bandpass (lambda(eff) similar to 2310 angstrom), 179 AGB stars were detected and 137 were not detected. Only 38 AGB stars were detected in the far-UV (FUV) bandpass (lambda(eff) similar to 1528 angstrom). We find that NUV emission is correlated with optical to near-infrared emission, leading to higher detection fractions among the brightest, and hence closest, AGB stars. Comparing the AGB time-variable visible phased light curves to corresponding GALEX NUV phased light curves, we find evidence that for some AGB stars the NUV emission varies in phase with the visible light curves. We also find evidence that the NUV emission and possibly the FUV emission are anticorrelated with the circumstellar envelope density. These results suggest that the origin of the GALEX-detected UV emission is an inherent characteristic of the AGB stars that can most likely be traced to a combination of photospheric and chromospheric emission. In most cases, UV detections of AGB stars are not likely to be indicative of the presence of binary companions.

Place, publisher, year, edition, pages
IOP PUBLISHING LTD, 2017
Keywords
general, stars: AGB and post-AGB, stars: chromospheres, ultraviolet: stars
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-326239 (URN)10.3847/1538-4357/aa704d (DOI)000402310500019 ()
Funder
EU, European Research Council, 614264
Available from: 2017-07-06 Created: 2017-07-06 Last updated: 2017-07-06Bibliographically approved
Wittkowski, M., Hofmann, K.-H. -., Höfner, S., Le Bouquin, J. B., Nowotny, W., Paladini, C., . . . Weigelt, G. (2017). Aperture synthesis imaging of the carbon AGB star R Sculptoris Detection of a complex structure and a dominating spot on the stellar disk. Astronomy and Astrophysics, 601, Article ID A3.
Open this publication in new window or tab >>Aperture synthesis imaging of the carbon AGB star R Sculptoris Detection of a complex structure and a dominating spot on the stellar disk
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2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 601, article id A3Article in journal (Refereed) Published
Abstract [en]

Aims. We present near-infrared interferometry of the carbon-rich asymptotic giant branch (AGB) star R Sculptoris (R Scl). Methods. We employ medium spectral resolution K-band interferometry obtained with the instrument AMBER at the Very Large Telescope Interferometer (VLTI) and H-band low spectral resolution interferometric imaging observations obtained with the VLTI instrument PIONIER. We compare our data to a recent grid of dynamic atmosphere and wind models. We compare derived fundamental parameters to stellar evolution models. Results. The visibility data indicate a broadly circular resolved stellar disk with a complex substructure. The observed AMBER squared visibility values show drops at the positions of CO and CN bands, indicating that these lines form in extended layers above the photosphere. The AMBER visibility values are best fit by a model without a wind. The PIONIER data are consistent with the same model. We obtain a Rosseland angular diameter of 8.9 +/- 0.3 mas, corresponding to a Rosseland radius of 355 +/- 55 R-Theta, an effective temperature of 2640 +/- 80 K, and a luminosity of log L/L-Theta = 3.74 +/- 0.18. These parameters match evolutionary tracks of initial mass 1.5 +/- 0.5 M-Theta and current mass 1.3 +/- 0.7 M-Theta. The reconstructed PIONIER images exhibit a complex structure within the stellar disk including a dominant bright spot located at the western part of the stellar disk. The spot has an H- band peak intensity of 40% to 60% above the average intensity of the limb-darkening-corrected stellar disk. The contrast between the minimum and maximum intensity on the stellar disk is about 1:2.5. Conclusions. Our observations are broadly consistent with predictions by dynamic atmosphere and wind models, although models with wind appear to have a circumstellar envelope that is too extended compared to our observations. The detected complex structure within the stellar disk is most likely caused by giant convection cells, resulting in large-scale shock fronts, and their effects on clumpy molecule and dust formation seen against the photosphere at distances of 2-3 stellar radii.

Place, publisher, year, edition, pages
EDP Sciences, 2017
Keywords
techniques: interferometric, stars: AGB and post-AGB, stars: atmospheres, stars: fundamental parameters, stars: mass-loss, stars: individual: R Scl
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-327053 (URN)10.1051/0004-6361/201630214 (DOI)000402313500003 ()
Available from: 2017-08-03 Created: 2017-08-03 Last updated: 2017-08-03Bibliographically approved
Olofsson, H., Vlemmings, W. H., Bergman, P., Humphreys, E. M., Lindqvist, M., Maercker, M., . . . Tafoya, D. (2017). First detection of methanol towards a post-AGB object, HD 101584 [Letter to the editor]. Astronomy and Astrophysics, 603, Article ID L2.
Open this publication in new window or tab >>First detection of methanol towards a post-AGB object, HD 101584
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2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 603, article id L2Article in journal, Letter (Other academic) Published
Abstract [en]

The circumstellar environments of objects on the asymptotic giant branch and beyond are rich in molecular species. Nevertheless, methanol has never been detected in such an object, and is therefore often taken as a clear signpost for a young stellar object. However, we report the first detection of CH3OH in a post-AGB object, HD 101584, using ALMA. Its emission, together with emissions from CO, SiO, SO, CS, and H2CO, comes from two extreme velocity spots on either side of the object where a high-velocity outflow appears to interact with the surrounding medium. We have derived molecular abundances, and propose that the detected molecular species are the effect of a post-shock chemistry where circumstellar grains play a role. We further provide evidence that HD 101584 was a low-mass, M-type AGB star.

Keywords
circumstellar matter, stars: individual: HD 101584, stars: AGB and post-AGB, radio lines: stars
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-361225 (URN)10.1051/0004-6361/201731170 (DOI)000406619100148 ()
Funder
Swedish Research CouncilEU, European Research Council, 614264
Available from: 2018-09-25 Created: 2018-09-25 Last updated: 2018-09-25Bibliographically approved
De Nutte, R., Decin, L., Olofsson, H., Lombaert, R., de Koter, A., Karakas, A., . . . Van de Sande, M. (2017). Nucleosynthesis in AGB stars traced by oxygen isotopic ratios I. Determining the stellar initial mass by means of the O-17/O-18 ratio. Astronomy and Astrophysics, 600, Article ID A71.
Open this publication in new window or tab >>Nucleosynthesis in AGB stars traced by oxygen isotopic ratios I. Determining the stellar initial mass by means of the O-17/O-18 ratio
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2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, article id A71Article in journal (Refereed) Published
Abstract [en]

Aims. We seek to investigate the O-17/O-18 ratio for a sample of AGB stars containing M-, S-, and C-type stars. These ratios are evaluated in relation to fundamental stellar evolution parameters: the stellar initial mass and pulsation period.

Methods. Circumstellar (CO)-C-13-O-16, (CO)-C-12-O-17, and (CO)-C-12-O-18 line observations were obtained for a sample of nine stars with various single-dish long-wavelength facilities. Line intensity ratios are shown to relate directly to the surface O-17/O-18 abundance ratio.

Results. Stellar evolution models predict the O-17/O-18 ratio to be a sensitive function of initial mass and to remain constant throughout the entire TP-AGB phase for stars initially less massive than 5 M-circle dot. This makes the measured ratio a probe of the initial stellar mass.

Conclusions. Observed O-17/O-18 ratios are found to be well in the range predicted by stellar evolution models that do not consider convective overshooting. From this, accurate initial mass estimates are calculated for seven sources. For the remaining two sources, there are two mass solutions, although there is a larger probability that the low-mass solution is correct. Finally, we present hints at a possible separation between M/S- and C-type stars when comparing the O-17/O-18 ratio to the stellar pulsation period.

Place, publisher, year, edition, pages
EDP SCIENCES S A, 2017
Keywords
stars: AGB and post-AGB, stars: evolution, stars: fundamental parameters, circumstellar matter
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-324337 (URN)10.1051/0004-6361/201629195 (DOI)000400754000041 ()
Funder
EU, FP7, Seventh Framework ProgrammeEU, European Research Council, 646758 AEROSOLSwedish Research Council
Available from: 2017-06-15 Created: 2017-06-15 Last updated: 2017-06-15Bibliographically approved
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