L. Valdivielso

Publications (7)4.9 Total impact

• Source

  Available from: ArXiv

  Article: Testing the fragmentation limit in the Upper Sco association

  N. Lodieu, N. C. Hambly, P. D. Dobbie, N. J. G. Cross, L. Christensen, E. L. Martin, L. Valdivielso
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  ABSTRACT: We present the results of a deep (J ~ 21 mag at 5 sigma) infrared photometric survey of a 0.95 square degree area in the central region of the Upper Sco association. The photometric observations consist of a deep (Y+J)-band images obtained with the WFCAM camera on the UKIRT InfraRed Telescope (UKIRT) with partly coverage in Z complemented by methane ON and OFF conducted with WIRCam on the Canada France Hawaii Telescope. We have selected five potential T-type objects belonging to the Upper Sco association on the basis of their blue methane colours and their J-CH4off colours. We have also identified a sample of 7-8 Upper Sco member candidates bridging the gap between known cluster M-types and our new T-type candidates. These candidates were selected based on their positions in various colour-magnitude diagrams and they follow the sequence of known Upper Sco members identified in the UKIRT Infrared Deep Sky Survey (UKIDSS) Galactic Clusters Survey (GCS). We present additional membership constraints using proper motion estimates from the multiple epochs available to us. We also present optical and near-infrared spectra obtained with the X--Shooter spectrograph on the Very Large Telescope for five L-type candidates covering the 0.6 to 2.5 micron wavelength range, none of them being confirmed as a young brown dwarf. We discuss the lack of detection of new candidate members as well as the possible turn down in the USco mass function as we are approaching the fragmentation limit.
  08/2011;
• Article: T Tauri candidates and accretion rates using IPHAS: method and application to IC 1396

  Geert Barentsen, Jorick S. Vink, J. E. Drew, R. Greimel, N. J. Wright, J. J. Drake, E. L. Martin, L. Valdivielso, R. L. M. Corradi
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  ABSTRACT: The Isaac Newton Telescope (INT) Photometric H-Alpha Survey (IPHAS) is a 1800 deg2 survey of the Northern Galactic Plane, reaching down to r′∼ 21. We demonstrate how the survey can be used to (1) reliably select classical T Tauri star candidates and (2) constrain the mass accretion rates with an estimated relative uncertainty of 0.6 dex. IPHAS is a necessary addition to spectroscopic surveys because it allows large and uniform samples of accretion rates to be obtained with a precise handle on the selection effects.We apply the method on a region of 7 deg2 towards the H ii region IC 1396 in Cepheus OB2 and identify 158 pre-main-sequence candidates with masses between 0.2 and 2.0 M⊙ and accretion rates between 10−9.2 and 10−7.0 M⊙ yr−1. We find a power-law dependency between the stellar mass and the accretion rates with a slope of α= 1.1 ± 0.2, which is less steep than indicated by previous studies. We discuss the influence of method-dependent systematic effects on the determination of this relationship.The majority of our sample consists of faint, previously unknown, low-mass T Tauri candidates (56 per cent between 0.2 and 0.5 M⊙). Many candidates are clustered in front of three bright-rimmed molecular clouds, which are being ionized by the massive star HD 206267 (O6.5V). We discover a spatio-temporal gradient of increasing accretion rates, increasing Spitzer infrared excess, and younger ages away from the ionizing star, providing a strong indication that the formation of these clusters has been sequentially triggered by HD 206267 during the last ∼1 Myr.
  Monthly Notices of the Royal Astronomical Society 05/2011; 415(1):103 - 132. · 4.90 Impact Factor
• Source

  Available from: ArXiv

  Article: T Tauri candidates and accretion rates using IPHAS: method and application to IC1396

  Geert Barentsen, Jorick S. Vink, J. E. Drew, R. Greimel, N. J. Wright, J. J. Drake, E. L. Martin, L. Valdivielso, R. L. M. Corradi
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  ABSTRACT: The INT Photometric H-Alpha Survey (IPHAS) is a 1800 deg^2 survey of the Northern Galactic Plane, reaching down to r'~21. We demonstrate how the survey can be used to (1) reliably select classical T Tauri star candidates and (2) constrain the mass accretion rates with an estimated relative uncertainty of 0.6 dex. IPHAS is a necessary addition to spectroscopic surveys because it allows large and uniform samples of accretion rates to be obtained with a precise handle on the selection effects. We apply the method on a region of 7 deg^2 towards the HII region IC1396 in Cepheus OB2 and identify 158 pre-main sequence candidates with masses between 0.2 and 2.0 Msun and accretion rates between 10^-9.2 and 10^-7.0 Msun yr^-1. We find a power-law dependency between the stellar mass and the accretion rates with a slope of \alpha = 1.1 +/- 0.2, which is less steep than indicated by previous studies. We discuss the influence of method-dependent systematic effects on the determination of this relationship. The majority of our sample consists of faint, previously unknown, low-mass T Tauri candidates (56 per cent between 0.2 and 0.5 Msun). Many candidates are clustered in front of three bright-rimmed molecular clouds, which are being ionized by the massive star HD206267 (O6.5V). We discover a spatio-temporal gradient of increasing accretion rates, increasing Spitzer infrared excess, and younger ages away from the ionizing star, providing a strong indication that the formation of these clusters has been sequentially triggered by HD206267 during the last ~1 Myr.
  03/2011;
• Source

  Available from: Herve Bouy

  Article: An IPHAS-based search for accreting very low-mass objects using VO tools

  L. Valdivielso, E. L. Martin, H. Bouy, E. Solano, J. E. Drew, R. Greimel, R Gutierrez, Y. C. Unruh, J. S. Vink
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  ABSTRACT: The main goal of this paper is to prove that accreting very low-mass stars and brown dwarfs can be identified in IPHAS, a Halpha emission survey of the northern Milky Way.Full exploitation of the IPHAS database and a future extension of it in the southern hemisphere will be useful to identify very low-mass accreting objects near and far well-known star forming regions. We have used Virtual Observatory tools to cross-match the IPHAS catalogue with the 2MASS catalogue. We defined photometric criteria to identify Halpha emission sources with near-infrared colours similar to known young very low-mass stars and brown dwarfs. 4000 candidates were identified that met our criteria over an area of 1600 square degrees. We present low-resolution optical spectra of 113 candidates. Spectral types have been derived for the 33 candidates that have spectroscopically confirmed Halpha emission, negligible reddening and M spectral class. We have also measured Halpha emission and investigated the NaI doublet (818.3 nm, 819.5 nm) in these 33 objects. We confirm that 33 IPHAS candidates have strong Halpha indicative of disk accretion for their spectral type. 23 of them have spectral class M4 or later, of which 10 have classes in the range M5.5-M7.0 and thus could be very young brown dwarfs. Also many objects have weak NaI doublet, an indication of low surface gravity.We conclude that IPHAS provides a very valuable database to identify accreting very low-mass stars and brown dwarfs, and that Virtual Observatory tools provide an efficient method for identifying these objects over large areas of the sky. Based on our success rate of 23 Halpha emission objects with spectral type in the range M4-M7 out of 113 candidates with spectroscopic follow-up, we estimate that there could be hundreds of such objects in the full IPHAS survey. Comment: 14 figures
  10/2008;
• Source

  Available from: David Montes

  Article: Low-resolution spectroscopy and spectral energy distributions of selected sources towards sigma Orionis

  J. A. Caballero, L. Valdivielso, E. L. Martin, D. Montes, S. Pascual, P. G. Perez-Gonzalez
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  ABSTRACT: Aims: We investigated in detail nine sources in the direction of the young sigma Orionis cluster, which is considered a unique site for studying stellar and substellar formation. The nine sources were selected because of some peculiar properties, such as extremely red infrared colours or too strong Halpha emission for their blue optical colours. Methods: We took high-quality, low-resolution spectroscopy (R ~ 500) of the nine targets with ALFOSC at the Nordic Optical Telescope. We also re-analyzed [24]-band photometry from MIPS/Spitzer and compiled the best photometry available at the ViJHKs passbands and the four IRAC/Spitzer channels for constructing accurate spectral energy distributions covering from 0.55 to 24 mum. Results: The nine targets were classified into: one Herbig Ae/Be star with a scatterer edge-on disc, two G-type stars, one X-ray flaring, early-M, young star with chromospheric Halpha emission, one very low-mass, accreting, young spectroscopic binary, two young objects at the brown dwarf boundary with the characteristics of classical T Tauri stars, and two emission-line galaxies, one undergoing star formation, and another one whose spectral energy distribution is dominated by an active galactic nucleus. Besides, we discover three infrared sources associated to overdensities in a cold cloud in the cluster centre. Conclusions: Low-resolution spectroscopy and spectral energy distributions are a vital tool for measuring the physical properties and the evolution of young stars and candidates in the sigma Orionis cluster. Comment: Accepted for publication in A&A
  09/2008;
• Source

  Available from: Franck Marchis

  Article: Follow-up observations of binary ultra-cool dwarfs

  H. Bouy, E. L. Martin, W. Brandner, T. Forveille, X. Delfosse, N. Huelamo, G. Basri, J. Girard, M. R. Zapatero Osorio, M. Stumpf, A. Ghez, L. Valdivielso, F. Marchis, A. J. Burgasser, K. Cruz
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  ABSTRACT: Astrometric observations of resolved binaries provide estimates of orbital periods and will eventually lead to measurement of dynamical masses. Only a few very low mass star and brown dwarf masses have been measured to date, and the mass-luminosity relation still needs to be calibrated. We have monitored 14 very low mass multiple systems for several years to confirm their multiplicity and, for those with a short period, derive accurate orbital parameters and dynamical mass estimates. We have used high spatial resolution images obtained at the Paranal, Lick and HST observatories to obtain astrometric and photometric measurements of the multiple systems at several epochs. The targets have periods ranging from 5 to 200 years, and spectral types in the range M7.5 - T5.5. All of our 14 multiple systems are confirmed as common proper motion pairs. One system (2MASSW J0920122+351742) is not resolved in our new images, probably because the discovery images were taken near maximum elongation. Six systems have periods short enough to allow dynamical mass measurements within the next 15 to 20years. We estimate that only 8% of the ultracool dwarfs in the solar neighborhood are binaries with separations large enough to be resolved, and yet periods short enough to derive astrometric orbital fits over a reasonable time frame with current instrumentation. A survey that doubles the number of ultracool dwarfs observed with high angular resolution is called for to discover enough binaries for a first attempt to derive the mass-luminosity relationship for very low-mass stars and brown dwarfs. Comment: 13 pages, 6 Tables, 4 Figures, Accepted for A&A, reference pb corrected
  01/2008;
• Source

  Available from: R. L. Smart

  Article: A very cool brown dwarf in UKIDSS DR1

  S. J. Warren, D. J. Mortlock, S. K. Leggett, D. J. Pinfield, D. Homeier, S. Dye, R. F. Jameson, N. Lodieu, P. W. Lucas, A. J. Adamson, [......], N. C. Hambly, P. C. Hewett, P. Hirst, M. J. Irwin, A. Lawrence, M. C. Liu, E. L. Martin, R. L. Smart, L. Valdivielso, B. P. Venemans
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  ABSTRACT: (abridged) We report the discovery of a very cool brown dwarf, ULAS J003402.77-005206.7 (ULAS J0034-00), identified in UKIDSS DR1. We provide optical, near-infrared, and mid-infrared photometry of the source, and two near-infrared spectra. Comparing the spectral energy distribution of ULAS J0034-00 to that of the T8 brown dwarf 2MASS J0415-09, the latest-type and coolest well-studied brown dwarf to date, with Teff~750 K, we find evidence that ULAS J0034-00 is significantly cooler. First, the measured values of the near-infrared absorption spectral indices imply a later classification, of T8.5. Second, the H-[4.49] colour provides an empirical estimate of the temperature of 540<Teff<660 K (+/-2sig range). Third, the J- and H-band peaks are somewhat narrower in ULAS J0034-00, and detailed comparison against spectral models calibrated to 2MASS J0415-09 yields an estimated temperature lower by 60-120 K relative to 2MASS J0415-09 i.e. 630<Teff<690 K (+/-2sig), and lower gravity or higher metallicity according to the degenerate combination -0.5<delta(log g-2[m/H])<-0.25 (+/-2sig). Combining these estimates, and considering systematics, it is likely the temperature lies in the range 600<Teff<700 K. Despite the low inferred Teff we find no evidence for strong absorption by NH3 over the wavelength range 1.51-1.56 um. Evolutionary models imply that the mass and age are in the ranges 15-36 M(Jup) and 0.5-8 Gyr, respectively. The measured proper motion, of (0.37+/-0.07)arcsec/yr, combined with the photometrically estimated distance of 14-22 pc, implies a tangential velocity of ~30 km/s. ULAS J0034-00 is significantly bluer than 2MASS J0415-09 in Y-J, so future searches should allow for the possibility that cooler T dwarfs are bluer still.
  09/2007;