John E. Gizis

University of Delaware, Delaware, United States

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Publications (162)525.7 Total impact

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    ABSTRACT: Condensate clouds are present in brown dwarf atmospheres due to their low surface temperatures. As the coolest (Teff < 600 K) class of brown dwarfs currently known, Y dwarfs allow us to study the unique atmospheric physics that occur at these temperatures including the formation of sulfide, chloride, and water clouds. Dynamic inhomogeneities in cloud cover should manifest as photometric variabilities in the observed light curves of brown dwarfs. This phenomenon was originally documented in two brown-dwarfs by Morales-Calderón et al. (2006) at 4.5 microns, and in one brown dwarf by Heinze et al. (2013) at 3.6 microns. We describe our ongoing program to monitor fourteen Y dwarfs for photometric variability at 3.6 and 4.5 microns with the Spitzer Space Telescope and present initial results including the first detection of Y dwarf variability.
    01/2014;
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    ABSTRACT: We report on the results of fifteen months of monitoring the nearby field L1 dwarf WISEP J190648.47+401106.8 (W1906+40) with the Kepler mission. Supporting observations with the Karl G. Jansky Very Large Array and Gemini North telescope reveal that the L dwarf is magnetically active, with quiescent radio and variable H$\alpha$ emission. A preliminary trigonometric parallax shows that W1906+40 is at a distance of $16.35^{+0.36}_{-0.34}$ pc, and all observations are consistent with W1906+40 being an old disk star just above the hydrogen-burning limit. The star shows photometric variability with a period of 8.9 hours and an amplitude of 1.5%, with a consistent phase throughout the year. We infer a radius of $0.92 \pm 0.07 R_J$ and $\sin i > 0.57$ from the observed period, luminosity ($10^{-3.67 \pm 0.03} L_\odot$), effective temperature ($2300 \pm 75$K), and $v \sin i$ ($11.2 \pm 2.2$ km/s). The light curve may be modeled with a single large, high latitude dark spot. Unlike many L-type brown dwarfs, there is no evidence of other variations at the $\gtrsim 2%$ level, either non-periodic or transient periodic, that mask the underlying rotation period. We suggest that the long-lived surface features may be due to starspots, but the possibility of cloud variations cannot be ruled out without further multi-wavelength observations. During the Gemini spectroscopy, we observed the most powerful flare ever seen on an L dwarf, with an estimated energy of $\sim 1.6 \times 10^{32}$ ergs in white light emission. Using the Kepler data, we identify similar flares and estimate that white light flares with optical/ultraviolet energies of $10^{31}$ ergs or more occur on W1906+40 as often as 1-2 times per month.
    The Astrophysical Journal 10/2013; 779(2). · 6.73 Impact Factor
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    ABSTRACT: We discover four high proper motion L dwarfs by comparing the Wide-field Infrared Survey Explorer (WISE) to the Two Micron All Sky Survey (2MASS). WISE J140533.32+835030.5 is an L dwarf at the L/T transition with a proper motion of 0.85+/-0.02" yr^-1, previously overlooked due to its proximity to a bright star (V=12 mag). From optical spectroscopy we find a spectral type of L8, and from moderate-resolution J band spectroscopy we find a near-infrared spectral type of L9. We find WISE J140533.32+835030.5 to have a distance of 9.7+/-1.7 pc, bringing the number of L dwarfs at the L/T transition within 10 pc from six to seven. WISE J040137.21+284951.7, WISE J040418.01+412735.6, and WISE J062442.37+662625.6 are all early L dwarfs within 25 pc, and were classified using optical and low-resolution near-infrared spectra. WISE J040418.01+412735.6 is an L2 pec (red) dwarf, a member of the class of unusually red L dwarfs. We use follow-up optical and low-resolution near-infrared spectroscopy to classify a previously discovered (Castro & Gizis 2012) fifth object WISEP J060738.65+242953.4 as an (L8 Opt/L9 NIR), confirming it as an L dwarf at the L/T transition within 10 pc. WISEP J060738.65+242953.4 shows tentative CH_4 in the H band, possibly the result of unresolved binarity with an early T dwarf, a scenario not supported by binary spectral template fitting. If WISEP J060738.65+242953.4 is a single object, it represents the earliest onset of CH_4 in the H band of an L/T transition dwarf in the SpeX Library. As very late L dwarfs within 10 pc, WISE J140533.32+835030.5 and WISEP J060738.65+242953.4 will play a vital role in resolving outstanding issues at the L/T transition.
    08/2013;
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    ABSTRACT: We report on more than one year of monitoring the nearby field L1 dwarf WISEP J190648.47+401106.8 with the Kepler mission. Supporting ground-based observations show that it is magnetically active with quiescent radio and H alpha emission. The rotation rate is 8.9 hours with a long-lived large surface feature, possibly a starspot or cloud. We report new constraints on the surface features from simultaneous spectroscopy and photometry. Short-cadence Kepler observations detect white light flares with estimated energy 10^31 ergs that occur at least once a month; the flare rise times are a few minutes and the most powerful last for hours. We discuss simultaneous Gemini spectroscopy of these flares that confirm white light and atomic emission lines. Despite the low effective temperature 2300K) and size (<1 Jupiter radius) of this L dwarf, it has a flare rate comparable to the Sun.
    06/2013;
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    ABSTRACT: The launch of NASA's Wide-field Infrared Survey Explorer (WISE) in late 2009 ushered in an new era in the study of the solar neighborhood. Its mid-infrared capabilities have proven critical to the discovery of the bulk of the coolest (\teff < 1000 K) brown dwarfs, including the new Y dwarfs, and its all-sky coverage lends itself to searches for high proper motions stars using other infrared surveys such as the Two Micron All Sky Survey (2MASS), UKIRT Infrared Deep Sky Survey (UKIDSS), and Visible and Infrared Survey Telescope for Astronomy (VISTA) surveys. This splinter session of the 17th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun was organized to bring together researchers working with WISE data to both identify brown dwarfs and characterize their properties.
    Astronomische Nachrichten 02/2013; · 1.40 Impact Factor
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    ABSTRACT: The T Tauri phase of young stellar evolution is known to exist down to substellar masses. However, the intrinsic faintness of these objects and distances to star forming regions has limited detailed study of this critical phase. We propose monitoring observations of the nearest pair of low-mass T Tauri accretors, TWA 30A and B. Both of these 8 Myr sources exhibit spectroscopic signatures indicating actively accreting, nearly edge-on disks with jets and stellar outflows. However, their time-dependent behavior at optical and near-infrared wavelengths are distinct, suggesting differing geometries and differing sources for the observed emission. We propose to test these models through short-term (continuous over 6 hr) and medium-term (daily for 40 days) IRAC monitoring of both stars. These observations will allow us to simultaneously probe variations in the accretion on and warping of the outer disk of TWA 30A, and coherent scaleheight and opacity variations in the disk around TWA 30B in the region where planets may be forming. Combined with coincident ground-based follow-up, our program will provide the most detailed picture of disk evolution during the planet-building phase of the lowest-mass stars.
    Spitzer Proposal. 12/2012;
  • B. Riaz, J. E. Gizis
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    ABSTRACT: We present Herschel SPIRE observations of the TW Hydrae association (TWA) brown dwarf disks SSSPM J1102-3431 (SS1102) and 2MASSW J1207334-393254 (2M1207). Both disks are undetected in the SPIRE 200-500 μm bands. We analyzed the archival PACS data and found no detection of either source in the 160 μm band. On the basis of radiative transfer modeling, we estimate an upper limit to the disk mass for both sources of 0.1 MJup. The lack of detection in the SPIRE bands could be due to a paucity of millimeter-sized dust grains in the 2M1207 and SS1102 disks. We also report a non-detection for the brown dwarf 2MASS J1139511-315921 (2M1139) in the PACS 70 and 160 μm bands. We argue for the presence of a warm debris disk around 2M1139, based on an excess emission observed at 24 μm. The mid-infrared colors for 2M1139 are similar to the transition disks in the Taurus and Ophuichus regions. A comparison of the brown dwarf disk masses over a ~1-10 Myr age interval suggests that there is a decline in the disk mass with the age of the system. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
    Astronomy and Astrophysics 12/2012; · 5.08 Impact Factor
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    ABSTRACT: We propose Spitzer observations of the L1 dwarf WISEP J190648.47+401106.8. This chromospherically active very-low-mass star or brown dwarf happens to lie in the Kepler field, allowing a unique opportunity to monitor optical variations over the next several years. The first nine months of Kepler data establish that the object shows periodic variations that have remained in phase -- this is in contrast to many of the results reported for less sensitive ground-based campaigns with much poorer time coverage. There is evidence of magnetic activity, and Spitzer observations can distinguish between proposed starspot, cloud, and aurora models for the optical variations. The proposed observations will lead to a better understanding of warm substellar atmospheres.
    Spitzer Proposal. 12/2012;
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    ABSTRACT: Condensate clouds play a critical role in shaping the emergent spectra of both brown dwarfs and gas giant planets. Their impact on the appearance of these objects is perhaps most dramatically illustrated on Jupiter where large holes in the cloud decks allow radiation from deeper, hotter layers of the atmosphere to emerge, which gives rise to the so-called 5 micron 'hot spots' seen in thermal images of Jupiter. Gelino & Marley (2000) estimate that the disk-integrated 5 micron flux could vary by up to 20% due to the rotational modulation of these hot spots. With the recent discovery of Y dwarfs by NASA's Wide-field Infrared Survey Explorer (WISE), we have finally identified a population of ultracool brown dwarfs (Teff < 600 K) whose atmospheric properties are approaching that of Jupiter (Teff=128 K). There are strong theoretical reasons to expect that the condensate clouds in the Y dwarfs may also be nonuniform so we therefore propose to monitor the known sample of Y dwarfs for photometric variability using warm Spitzer. Our proposed observations will determine and characterize any mid-infrared variability exhibited by the Y dwarfs and determine whether the variability evolves over time scales of many months. These observations will not only improve our understand of the atmospheric dynamics of brown dwarf atmospheres but also directly inform the interpretation and characterization of cool gas giant exoplanets detected with the next generation of high-contrast imagers like the Gemini Planet Imager (GPI) and the SPHERE instrument for the VLT.
    Spitzer Proposal. 09/2012;
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    ABSTRACT: We report the discovery of an unusually red brown dwarf found in a search for high proper motion objects using WISE and 2MASS data. WISEP J004701.06+680352.1 is moving at 0.44$ arcsec/yr and lies relatively close to the Galactic Plane (b=5.2 degrees). Near-infrared photometry and spectroscopy reveals that this is one of the reddest (2MASS J-K_s = 2.55 +/- 0.08 mag) field L dwarfs yet detected, making this object an important member of the class of unusually red L dwarfs. We discuss evidence for thick condensate clouds and speculate on the age of the object. Although models by different research groups agree that thick clouds can explain the red spectrum, they predict dramatically different effective temperatures, ranging from 1100K to 1600K. This brown dwarf is well suited for additional studies of extremely dusty substellar atmospheres because it is relatively bright (K_s = 13.05 +/- 0.03 mag), which should also contribute to an improved understanding of young gas-giant planets and the transition between L and T brown dwarfs.
    The Astronomical Journal 07/2012; 144(4). · 4.97 Impact Factor
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    B. Riaz, J. E. Gizis
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    ABSTRACT: We present Herschel SPIRE observations for the TW Hydrae association (TWA) brown dwarf discs SSSPM J1102-3431 (SS1102) and 2MASSW J1207334-393254 (2M1207). Both discs are undetected in the SPIRE 200-500mu bands. We have also analyzed the archival PACS data and find no detection for either source in the 160mu band. Based on radiative transfer modeling, we estimate an upper limit to the disc mass for both sources of 0.1 M_Jup. The lack of detection in the SPIRE bands could be due to a paucity of millimeter sized dust grains in the 2M1207 and SS1102 discs. We also report a non-detection for the brown dwarf 2MASS J1139511-315921 (2M1139) in the PACS 70 and 160mu bands. We have argued for the presence of a warm debris disc around 2M1139, based on an excess emission observed at 24mu. The mid-infrared colors for 2M1139 are similar to the transition discs in the Taurus and Ophuichus regions. A comparison of the brown dwarf disc masses over a ~1-10 Myr age interval suggests a decline in the disc mass with the age of the system.
    06/2012;
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    ABSTRACT: We present Herschel/SPIRE observations for the 2MASS 1207334-393254 (2M1207) system. Based on radiative transfer modelling of near-infrared to submillimetre data, we estimate a disc mass of 3 ± 2 MJup and an outer disc radius of 50-100 au for the 2M1207A disc. The relative disc mass for 2M1207A is similar to the T Tauri star TW Hya, which indicates that massive discs are not underabundant around substellar objects. In probing the various formation mechanisms for this system, we find that core accretion is highly uncertain mainly due to the large separation between the primary and the companion. Disc fragmentation could be a likely scenario based on analytical models, and if the disc initially was more massive than its current estimate. Considering that the TW Hydrae Association (TWA) is sparsely populated, this system could have formed via one of the known binary formation mechanisms (e.g. turbulent fragmentation of a core) and survived disruption at an early stage.
    Monthly Notices of the Royal Astronomical Society 05/2012; 422(1):L6-. · 5.52 Impact Factor
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    ABSTRACT: A special set of observations that utilized exposure times six times longer than the main 2MASS survey measurements were conducted in the final year of 2MASS observatory operations. The 2MASS "6x" observations achieved sensitivities ~1 mag deeper than the main 2MASS survey, and covered approximately 590 deg2 of sky in 30 discrete regions. The 2MASS 6x Point Source Working Databases (6x-PSWDB and 6x-XSWDB) contain all detections extracted from the raw 6x imaging data during pipeline data reduction. The 6x WDB entries include reliable detections of astrophysical sources, as well as spurious detections of noise excursions, image artifacts and transient events such as meteor trails, cosmic rays and hot pixels. In addition, the WDBs may contain multiple, independent detections of objects scanned more than once during the 6x observations. The 2MASS 6x Point Source Catalogs (6x-PSC and 6x-XSC) is a subset of extractions in the 6x-PSWDB that have been identified to be high reliability source detections, with only one measurement of sources detected multiple times for uniformity. (1 data file).
    VizieR Online Data Catalog. 02/2012;
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    ABSTRACT: We have revised our analysis of the SPIRE observations of 2MASSW J1207334-393254 (2M1207). Recent PACS observations show a bright source located ~25" east of 2M1207. There are issues in terms of the detection/non-detection of the bright source when comparing the Spitzer, WISE, and PACS observations. It is apparently inconsistent, perhaps due to variability or low signal-to-noise of the data. We have looked into the possible misidentification of the target, and have revised the measured SPIRE fluxes and the disc parameters for 2M1207. We have also reviewed which among the various formation mechanisms of this system would still be valid.
    01/2012;
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    Philip J. Castro, John E. Gizis
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    ABSTRACT: We discover a late-type L dwarf, WISEP J060738.65+242953.4 (W0607+2429), by comparing the Wide-field Infrared Survey Explorer (WISE) preliminary data release to the Two Micron All Sky Survey (2MASS) in search of high proper motion objects (>0.3" yr^-1). W0607+2429 was found to have a proper motion of 0.57 +/- 0.02" yr^-1. Based on colors and color-color diagrams using 2MASS and Sloan Digital Sky Survey (SDSS) photometry, we estimate the spectral type (optical) to be L8 within a spectral sub-type. Based on the spectral type estimated we find W0607+2429 to have a distance of 7.8^(+1.4)_(-1.2) pc, making it one of only four very-late L dwarfs within 10 pc, and the third closest L dwarf overall. This close L/T transition dwarf will play a pivotal role in resolving outstanding issues of condensate clouds of low temperature atmospheres.
    The Astrophysical Journal 10/2011; 746(1). · 6.73 Impact Factor
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    ABSTRACT: We report a nearby L7.5 dwarf discovered using the Preliminary Data Release of the Wide-field Infrared Survey Explorer (WISE) and the Two Micron All-Sky Survey (2MASS). WISEP J180026.60+013453.1 has a motion of 0.42 arcsec/yr and an estimated distance of 8.8 \pm 1.0 pc. With this distance, it currently ranks as the sixth closest known L dwarf, although a trigonometric parallax is needed to confirm this distance. It was previously overlooked because it lies near the Galactic Plane (b=12). As a relatively bright and nearby late L dwarf with normal near-infrared colors, W1800+0134 will serve as a benchmark for studies of cloud-related phenomena in cool substellar atmospheres.
    The Astronomical Journal 08/2011; 142. · 4.97 Impact Factor
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    ABSTRACT: We report two nearby high proper motion dwarfs of special interest identified using the Preliminary Data Release of the Wide-field Infrared Survey Explorer (WISE) and the Two Micron All-Sky Survey (2MASS). WISEP J191239.91-361516.4 has a motion of 2.1 arcseconds per year. Photometry identifies it as a mid-M dwarf. WISEP J190648.47+401106.8 is a spectroscopically confirmed L1 dwarf in the Kepler Mission field with a motion of 0.48 arcseconds per year. The estimated distance is 17 parsecs. Both lie at relatively low galactic latitudes and demonstrate the possibility of discovering proper motion stars independently of the historic photographic sky surveys.
    The Astrophysical Journal Letters 06/2011; 736(2). · 6.35 Impact Factor
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    Philip J. Castro, John E. Gizis, Marc Gagné
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    ABSTRACT: We report on a sequence of Chandra X-ray Observatory observations of the TW Hydrae brown dwarf (BD) 2MASSW J1139511-315921 (2M1139). In the combined 31 ks ACIS-S exposure, 2M1139 is detected at the 3-sigma confidence level. We find an X-ray luminosity of L_X = 1.4^(+2.7)_(-1.0) x 10^26 ergs s^-1 or log(L_X/L_bol) = -4.8 +/- 0.3. This object is similar to another TW Hydrae BD member, CD-33 7795B (TWA 5B): both have H-alpha emission, both show no signatures of accretion, and both have comparable ages and spectral types. TWA 5B was previously detected in X-rays with a luminosity of L_X = 4 x 10^27 ergs s^-1 or log(L_X/L_bol) = -3.4, an order of magnitude more luminous in X-rays than 2M1139. We find that the discrepancy between the X-ray luminosity of 2M1139 and TWA 5B is consistent with the spread in X-ray luminosity in the Orion Nebula Cluster (ONC) for BDs of similar spectral types. Though rotation may play a role in the X-ray activity of ultracool dwarfs like 2M1139 and TWA 5B, the discrepancy cannot be explained by rotation alone. We also examine two X-ray bright objects in the FOV of our Chandra observations and find one to be of spectral type K0IV and identify it as a possible RS Canum Venaticorum (RS CVn), and another X-ray bright object whose light-curve clearly shows the decay phase of an X-ray flare.
    The Astrophysical Journal 05/2011; 736. · 6.73 Impact Factor
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    ABSTRACT: We present a detailed examination of the brown dwarf multiples 2MASS J08503593+1057156 and 2MASS J17281150+3948593, both suspected of harboring components that straddle the L dwarf/T dwarf transition. Resolved photometry from Hubble Space Telescope/NICMOS show opposite trends in the relative colors of the components, with the secondary of 2MASS J0850+1057 being redder than its primary, while that of 2MASS J1728+3948 is bluer. We determine near-infrared component types by matching combined-light, near-infrared spectral data to binary templates, with component spectra scaled to resolved NICMOS and Kp photometry. Combinations of L7 + L6 for 2MASS J0850+1057 and L5 + L6.5 for 2MASS J1728+3948 are inferred. Remarkably, the primary of 2MASS J0850+1057 appears to have a later-type classification compared to its secondary, despite being 0.8-1.2 mag brighter in the near-infrared, while the primary of 2MASS J1728+3948 is unusually early for its combined-light optical classification. Comparison to absolute magnitude/spectral type trends also distinguishes these components, with 2MASS J0850+1057A being ~1 mag brighter and 2MASS J1728+3948A ~0.5 mag fainter than equivalently-classified field counterparts. We deduce that thick condensate clouds are likely responsible for the unusual properties of 2MASS J1728+3948A, while 2MASS J0850+1057A is either an inflated young brown dwarf or a tight unresolved binary, making it potentially part of a wide, low-mass, hierarchical quintuple system. Comment: 17 pages, accepted for publication to AJ; spectral data are available at http://www.browndwarfs.org/spexprism
    The Astronomical Journal 11/2010; · 4.97 Impact Factor
  • John D. Shaw, J. E. Gizis
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    ABSTRACT: Several nearby associations have been identified as having members with the similar absolute motions with a small velocity dispersion. The surveys performed to date have concentrated on relatively bright members with spectral types earlier than M5, though a number of candidate members have been identified serendipitously (Cruz et al. 2009, 2009 AJ,137,3345.) Large samples of brown dwarfs are known at the very young ages ( < 5 Myr) in star-forming regions and at large age (100 Myr - 8Gyr) in the field. We need samples of brown dwarfs with ages of 10 - 50 Myr to study planet-formation and evolutionary effects. To that end, a sky survey of 435 square degrees ( 1.0% of the sky) covering areas with known members of four nearby associations aged 8Myr - 50Myr (Zuckerman and Song 2004) was completed in October and November of 2006. From this, astrometric positions of point sources were then computed and compared with the Two-Micron All Sky Survey (2MASS: 1998-2001) point source catalog. Many known high proper motion objects were recovered as well as previously unpublished objects with J band magnitudes [10, 16] have been identified. Spectroscopy (0.6 - 0.95 micron) of 215 objects (J < 14) was carried out at the CTIO 1.5m telescope from October 2008 through February 2009. We present the proper motion and spectroscopy results including more than two dozen previously unidentified late M dwarfs and subdwarfs.
    12/2009;

Publication Stats

4k Citations
525.70 Total Impact Points

Institutions

  • 2001–2012
    • University of Delaware
      • Department of Physics and Astronomy
      Delaware, United States
  • 1998–2009
    • University of Massachusetts Amherst
      • Department of Astronomy
      Amherst Center, Massachusetts, United States
  • 1998–2008
    • California Institute of Technology
      • • Infrared Processing and Analysis Center
      • • Spitzer Science Center
      Pasadena, CA, United States
  • 2005
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 2003
    • The Catholic University of America
      Washington, Washington, D.C., United States
  • 2000
    • The University of Arizona
      • Department of Astronomy
      Tucson, AZ, United States
    • University of Pennsylvania
      • Department of Physics and Astronomy
      Philadelphia, PA, United States