Distance to VY Canis Majoris with VERA 3

arXiv:0808.0641v1 [astro-ph] 5 Aug 2008

∗ PASJ: ntne sdsusdi asy(03,Mse Olsen & Massey (2003), the Massey numbers, For in small uncertain. discussed still their as because are to instance, stars partly massive Due this evolved rare for of lifetime. extremely properties reasons short the are their of stars of One massive massive that poor. of still is stages is late evolution their of stars’ of spite understanding In our ISM. the prin- importance, in are elements they heavy inter- of Also, sources the 1982). cipal into (Abbott (ISM) energy medium evo- supernova mechanical stellar and their inject winds of they stellar stages explosions, strong late their the Through in lution. particularly Universe the Introduction 1. Yukitoshi 1 c onKyung Yoon 8 eateto srnm,Gaut colo cec,TeUn The Science, of School Graduate Astronomy, of Department 3 Kurayama Hiroshi rdaeSho fSineadEgneig aohm Univ Kagoshima Engineering, and Science of School Graduate eateto srnmclSine,Gaut University Graduate Sciences, Astronomical of Department asv tr lya motn oei h vlto of evolution the in role important an play stars Massive 08 srnmclSceyo Japan. of Society Astronomical 2018. u e ¨gl6,511Bn,Germany. Bonn, Radioastrono für 53121 Hügel 69, Max-Planck-Institut dem Auf is address Present 2 Takumi 7 iuaaVR bevtr,Ntoa srnmclObserva Astronomical National Observatory, VERA Mizusawa eateto srnm,Yne nvriy 3 Shinchon- 134 University, Yonsei Astronomy, of Department cetddsac.Wt u eut er-siaetelmnst o luminosity the ove re-estimate been we has result, CMa our VY With L of luminosity distance. The accepted measurement. parallax annual itneo 1.14 of distance o 3mnh,w ucsflymaue rgnmti aalxo parallax trigonometric a measured (VERA) successfully Astrometry we Radio months, of Exploration 13 VLBI for with out carried CMa) loal oe(..telf ieo h aah rc)ta previous than u track) permit Hayashi not the does still of diagram surface side stellar (HR) left the Hertzsprung-Russel of the the temperature effective on (i.e. CMa zone VY allowable of location makes ai aoi)–VERA – Majoris) Canis Tomoaki ul srn o.Jpn,1– , Japan Soc. Astron. Publ. 9 ⊙ eateto pc uvyadIfrainTcnlg,Ajo Technology, Information and Survey Space of Department 4 erpr srmti bevtoso H of observations astrometric report We e words: Key pc LIPoet ainlAtooia bevtr fJ of Observatory Astronomical National Project, VLBI Space sn h ooercflxitgae vrotcladI wavelength IR and optical over integrated flux bolometric the using Imai Kan-ya 10 Nagayama 6 oenVB ewr,KraAtooyadSaeSineInst Science Space and Astronomy Korea Network, VLBI Korean , 2 Choi aut fSine aohm nvriy –13 Korimot 1–21–35 University, Kagoshima Science, of Faculty , 6 Shibata Oyama Seiji Kenzaburo , , 7 1,2 Manabe Noriyuki − +0 5 srmty–msr (H masers – astrometry , ∗ 2 iuaaVR bevtr,Ntoa srnmclObserva Astronomical National Observatory, VERA Mizusawa itnet YCnsMjrswt VERA with Majoris Canis VY to Distance 0 , , 2,3,4 . . 8 Tomoya 11 09 Satoshi 3 hnhndn,Soamng,Sol1079 Republic 120–749, Seoul Seodaemun-gu, Shinchon-dong, 134 Akiharu p.Ti stems cuaedsac oV M n h rtoeb one first the and CMa VY to distance accurate most the is This kpc. Iwadate Yoshiaki , 3,5 Kawaguchi –2Hsig-k,Mzsw-u susi wt 023–0861 Iwate Oshu-shi, Mizusawa-ku, Hoshi-ga-oka, 2–12 ?? Sakai Hirota Kenta , Nakagawa Rcie 08Arl2;acpe 08Ags 5) August 2008 accepted 21; April 2008 (Received , Tamura 5 , Takaaki 5 Maruyama , Tetsuo 2,3 , 2,3,4 Mareki 2 [email protected] )–sas itne tr:sprins–sas niiul(VY individual stars: – supergiants stars: – distances stars: – O) , , 6 3,5 Masachika Jike Sasao Kayoko 2 aesaon h e uegatV ai aoi (VY Majoris Canis VY supergiant red the around masers O Miyuki , Honma 8 , Abstract mie, 5 Makoto Seiji , 9,10 o dacdSuis –11Oaa iaa oy 181–8588 Tokyo Mitaka, Osawa, 2–21–1 Studies, Advanced for Nakamura Tsushima Kijima riy –13 oioo aohm,Kgsia890–0065 Kagoshima Kagoshima, Korimoto, 1–21–35 ersity, h uioiyo (2–5) obtained un- al. (2007) of et also Jones luminosity Smith & is the (1999), Helton, diagram al. Humphreys, HR et and the (2001) Monnier on instance, For CMa VY supergiants, certain. red of other location to the Similar supergiants. red studied luminous. too and cool appear too supergiants be red to observed stel- the models, with on evolutionary Compared lar stars, evolved diagram. discrep- high-mass (HR) loca- a supergiants, Hertzsprung-Russel predicted was red theoretically of there and tions (2005), observed al. between et ancy Levesque and (2003) htteaoeprmtr ol aeV M cooler CMa VY make would & suggested parameters Sidaner (2006) above (Le al. the et type Massey 2800– that spectral However, stellar of 1996). the Bertre temperature Le on effective based and K kpc 1978) 1.5 3000 Reid of distance & the (Lada assuming (SED) distribution ergy vriyo oy,731Hno ukok,Tko113–0033 Tokyo Bunkyo-ku, Hongo, 7–3–1 Tokyo, of iversity Katsuhisa Kameno , og edeu-u eu 2–4,Rpbi fKorea of Republic 120–749, Seoul Seodaemun-gu, dong, 2,3 YCnsMjrs(YCa soeo h otwell- most the of one is CMa) (VY Majoris Canis VY oyo aa,22– sw,Mtk,Tko181–8588 Tokyo Mitaka, Osawa, 2–21–1 Japan, of tory Matsui Hideyuki nvriy uo 4–4,Rpbi fKorea of Republic 443–749, Suwon University, u pn –11Oaa iaa oy 181–8588 Tokyo Mitaka, Osawa, 2–21–1 apan, , 2,3 , , 8 , ,Kgsia aohm 890–0065 Kagoshima Kagoshima, o, 8 6 iKyoung Mi omk nlconclusion. final a make to s , Sato n Kazuyoshi and 8 hn Sik Chung Osamu 0.88 f tt,POBx osiUniversity, Yonsei P.O.Box, itute, Naoko Kobayashi YCat e(3 be to CMa VY f , .Ti mrvdlmnst value luminosity improved This s. ae natoercmonitoring astrometric on Based . 5 ns huhucranyi the in uncertainty theoretically though the ones, to closer much Mayumi ± etmtddet previously a to due restimated Kameya Matsumoto oyo Japan, of tory .8ms orsodn oa to corresponding mas, 0.08 fKorea of Kim Oh × , , 1,2,4,5 , 10 1,2 Yamashita 1,2 Sato , 5 3,5 Toshihiro Seisuke L ⊙ Ryuichi Takeshi , , 1,2 8 rmteseta en- spectral the from Takeshi ± asnr M. Katsunori Kuji 2,3 0.5) sdo an on ased Omodaka Bushimata Kamohara , 5 Miyaji × Tomoharu 10 5 , 6 , 2,4 , , 2,4 2 2 Y. K. Choi et al. [Vol. , and more luminous than what current evolutionary mod- metric observations of H2O masers around VY CMa to els allow and would place it in the “forbidden zone” of measure an accurate parallax with VLBI Exploration of the HR diagram, which is on the right-hand side of the Radio Astrometry (VERA) and here we present the re- Hayashi track in the HR diagram (see figure 4). Massey sults. et al. (2006) reexamined the effective temperature and obtained a new value of 3650 ± 25 K based on optical spec- 2. Observations and Data Reduction trophotometry combined with a stellar atmosphere model and suggested that the luminosity of VY CMa is only 6.0 We have observed H2O masers (H2O616–523 transition 4 × 10 L⊙, which is probably a lower limit (Massey et al. at the rest frequency of 22.235080 GHz) in the red super- 2008). giant VY CMa with VERA at 10 epochs over 13 months. Levesque et al. (2005) determined the effective tempera- The epochs are April 24, May 24, September 2, October tures of 74 Galactic red supergiants based on optical spec- 30, November 27 in 2006, January 10, February 14, March trophotometry and stellar atmosphere models. They ob- 26, April 21 and May 27 in 2007 (day of year 114, 144, tained the effective temperatures of the red supergiants 245, 303, 331 in 2006, 010, 045, 085, 111 and 147 in 2007, with a precision of 50 K. Their new effective tempera- respectively). In each epoch, VY CMa and a position ref- tures are warmer than those in the literature. These new erence source J0725–2640 [α(J2000) = 07h25m24.413135s, effective temperature values seem to be consistent with δ(J2000) = –26d40’32.67907” in VCS 5 catalog, (Kovalev the theoretical stellar evolutionary tracks. However, the et al. 2007)] were observed simultaneously in dual-beam luminosities of the red supergiants still had large uncer- mode for about 7 hours. The separation angle between VY tainty due to possible errors in estimated distances. Since CMa and J0725–2640 is 1.059 degrees. The instrumental most of red supergiants are very far, it has been difficult phase difference between the two beams was measured at to apply the most reliable trigonometric parallax method each station during the observations based on the corre- to their distance measurements. lations of artificial noise sources (Kawaguchi et al. 2000; In the case of VY CMa, the currently accepted distance Honma et al. 2008). A bright continuum source (DA193 of 1.5 kpc (Lada & Reid 1978) is obtained by assuming at 1st–3rd epochs and J0530+1330 at other epochs) was that VY CMa is a member of the NGC 2362 star clus- observed every 80 minutes for bandpass and delay calibra- ter and that the distance of NGC 2362 is same as that tion at each beam. of VY CMa. The distance to the NGC 2362 was deter- Left-handed circularly polarized signals were sampled mined from the color-magnitude diagram (Johnson et al. with 2-bit quantization and filtered with the VERA digi- 1961) with an accuracy of no better than 30 %. Since the tal filter unit (Iguchi et al. 2005). The data were recorded luminosity depends on the square of the distance, the ac- onto magnetic tapes at a rate of 1024 Mbps, providing a curacy of luminosity estimation is worse than 50 %. The total bandwidth of 256 MHz which consists of 16 × 16 proper motions measured with H2O masers in previous MHz IF channels. One IF channel was assigned to the studies (Richards, Yates, & Cohen 1998; Marvel et al. H2O masers in VY CMa and the other 15 IF channels 1998) have not been contradictory to the above distance were assigned to J0725–2640, respectively. The correla- value (1.5 kpc). When the H2O maser region around a tion processing was carried out on the Mitaka FX correla- star is modeled with a symmetrically expanding spherical tor (Chikada et al. 1991). The spectral resolution for H2O shell, the distance to the star from the Sun can be inferred maser lines is 15.625 kHz, corresponding to the velocity by assuming that the observed mean proper motion mul- resolution of 0.21 km s−1. tiplied by the distance should be equal to the observed All data reduction was conducted using the NRAO mean radial velocity of the H2O masers. The proper mo- Astronomical Image Processing System (AIPS) package. tion velocities in Richards, Yates, & Cohen (1998) are well The amplitude and the bandpass calibration for target consistent with the H2O maser spectral line velocities at source (VY CMa) and reference source (J0725–2640) were the distance of 1.5 kpc. Also, Marvel et al. (1998) derived performed independently. The amplitude calibration of the distance of VY CMa to be 1.4 ± 0.2 kpc. However, each antenna was performed using system temperatures such a “statistical parallax” method depends on kinemat- measured during observation. The bandpass calibration ical models fitted to the observed relative proper motions. was applied using a bright continuum source (DA193 In fact, if we add rotation or anisotropic expansion to the at 1st–3rd epochs and J0530+1330 at other epochs). simple spherical expansion model, the inferred distance Doppler corrections were carried out to obtain the radial value could vary beyond quoted error ranges. An accurate motions of the H2O masers relative to the Local Standard distance determination without an assumption is crucial of Rest (LSR). for obtaining the right location of VY CMa on the HR Then, we calibrated the clock parameters using the diagram and for determining fundamental parameters of residual delay of a bright continuum calibrator, which is the star. also used for the bandpass calibration. The fringe fitting Thanks to the recent progress in the VLBI technique, was made on the position reference source J0725–2640 the distance measurements with trigonometric parallaxes with integration time of 2 minutes and time interval of have become possible even beyond 5 kpc (e.g., Honma et 12 seconds to obtain residual delays, rates, and phases. al. 2007). Since VY CMa has strong maser emission in These phase solutions were applied to the target source its circumstellar envelope, we have conducted an astro- VY CMa and we also applied the dual-beam phase cali- No. ] Distance to VY Canis Majoris with VERA 3

2000 strength, this velocity channel is most suitable for astrometric measurements. Analyses for the other H O maser 1800 2 features will be reported in a forthcoming paper. 1600 Figure 3 shows the position measurements of the 1400 −1 0.55 km s H2O maser component for 13 months. 1200 The position oﬀsets are with respect to α(J2000.0) 1000 = 07h22m58s.32906, δ(J2000.0) = –25◦46’03”.1410. Flux [Jy] 800 Assuming that the movements of maser features are com- 600 posed of a linear motion and the annual parallax, we ob- 400 tained a proper motion and an annual parallax by the

200 least-square analysis. The declination data have too large

0 errors to be suitable for an annual parallax and a proper -5 0 5 10 15 20 25 30 35 motion measurement. Therefore, we obtained the paral- -1 VLSR [km s ] lax of VY CMa to be 0.88 ± 0.08 mas, corresponding to +0.11 a distance of 1.14 −0.09 kpc using only the data in right Fig. 1. The total power spectrum of the H2O masers in VY ascension. This is the first distance measurement of VY CMa obtained at Iriki station on April 24, 2006. CMa based on an annual parallax measurement with the highest precision. We estimated the positional uncertain- bration (Honma et al. 2008) to correct the instrumental ties from the least-square analysis, and the values of the delay difference between the two beams. We calibrated errors are 0.17 mas in right ascension and 0.68 mas in the tropospheric zenith delay offset by using the GPS declination making reduced χ2 to be 1. measurements of the tropospheric zenith delay made at We also determined the absolute proper motion in right each station (Honma, Tamura, & Reid 2008). After these ascension and declination. The absolute proper motion is calibrations, synthesized clean images were obtained. A –2.09 ± 0.16 mas yr−1 in right ascension and 1.02 ± 0.61 typical synthesized beam size (FWHM) was 2 mas × 1 −1 ◦ mas yr in declination. Compared with the proper mo- mas with a position angle of –23 . We measured positions tion of VY CMa obtained with Hipparcos, 9.84 ± 3.26 mas of the H2O maser features relative to the extragalactic yr−1 in right ascension and 0.75 ± 1.47 mas yr−1 in dec- source J0725–2640 with two-dimensional Gaussian fitting. lination (Perryman et al. 1997), there is a discrepancy in the proper motion in right ascension. Nearly 12 mas yr−1 3. Results difference (more than 65 km s−1 at 1.14 kpc) seems too large even when we take into account possible relative mo- The autocorrelation spectrum of the H2O masers tion of the 0.55 km s−1 maser feature with respect to the around VY CMa is shown in figure 1. The spectrum shows star itself. The difference may be due to the fact that cir- rich maser emission over LSR velocities ranging from –5 −1 cumstellar envelope of VY CMa has complex small-scale to 35km s . Though there are variations in flux density, structure at optical wavelengths. This might seriously af- overall structures of maser spectrum are common to all fect proper motion measurements with Hipparcos. the epochs, indicating that maser spots are surviving over The quantitative estimation of the individual error the observing period of 13 months. sources in the VLBI astrometry is difficult as previously To reveal the distribution of the H2O masers, we mentioned in Hachisuka et al. (2006), Honma et al. (2007), mapped the H2O maser features in VY CMa at the first and Hirota et al. (2007). Therefore, we estimated errors epoch (April 24, 2006). We detected 55 maser features from the standard deviations of the least-square analysis that have signal-to-noise ratio larger than 7 in two adja- to be 0.17 mas in right ascension and 0.68 mas in dec- cent channels. The distribution of the H2O maser features lination. When we consider that the statistical errors of is shown in figure 2. The most red-shifted component and the position, 0.02–0.09 mas, are estimated from residuals the most blue-shifted component lie near the apparent of the Gaussian fitting, the large values of the standard center of the distribution and the moderately red-shifted deviations suggest that some systematic errors affect the components on a NE-SW line of 150 mas. This distribu- result of astrometry. In the following, we discuss causes tion well agrees with the previous observational results by of these astrometric errors, although we cannot separate Marvel (1996). each factor of error. Among the H2O maser features in VY CMa, one maser −1 First, we consider the errors originating from the refer- feature at a LSR velocity of about 0.55 km s is ana- ence source. The positional errors of the reference source lyzed to detect an annual parallax. This feature, the most J0725–2640 affect those of the target source. The position blue-shifted discrete component, is stably detected at all of J0725–2640 was determined with an accuracy of 0.34 epochs. It is the third brightest maser component in the mas in right ascension and 0.94 mas in declination, re- total-power spectrum, but in fact it is the second brightest spectively (Kovalev et al. 2007). Because these offsets are component in the map. We note that while the brightest constant at all epochs, it dose not affect the parallax mea- channel has multiple components, there is only one com- surements. Also, when the reference source is not a point ponent in the channel at the LSR velocity of about 0.55 −1 source, the positional errors of the target source could oc- km s . Because of the simple structure as well as its cur due to the structure and its variation of the reference 4 Y. K. Choi et al. [Vol. ,

Fig. 2. The distribution of the H2O masers in VY CMa obtained on April 24, 2006. Color, as shown on the scale on the right-hand side, represents the LSR velocities of the maser features.

2 3

1 2

0 1

-1 0 RA offset[mas] DEC offset[mas]

-2 -1

-3 -2 0 100 200 300 400 500 600 0 100 200 300 400 500 600 DOY [days from January 1, 2006] DOY [days from January 1, 2006]

−1 Fig. 3. Results of the measured positions of the H2O maser spot at the LSR velocity of 0.55 km s in VY CMa using J0725–2640 as a position reference source. The position offsets are with respect to α(J2000.0) = 07h22m58s.32906, δ(J2000.0) = –25◦46’03”.1410. The left panel shows the movements of the maser spot in right ascension as a function of time (day of year). The right panel is the same as the left panel in declination. Solid lines represent the best fit model with an annual parallax and a linear proper motion for the maser spot. Dotted lines represent the linear proper motion (–2.09 ± 0.16 mas yr−1 in right ascension and 1.02 ± 0.61 mas yr−1 in declination) and points represent observed positions of maser spot with error bars indicating the positional uncertainties in systematic errors (0.17 mas in right ascension and 0.68 mas in declination). No. ] Distance to VY Canis Majoris with VERA 5 source. However, since the reference source for our mea- ground based images in Smith et al. (2001) and IRAS surements is point-like and shows no structural variations fluxes from 25 to 100 µm. The Fbol is obtained by inte- between epochs, this is not likely to be the main source of grating the observed fluxes. The estimated luminosity of 5 the positional errors. VY CMa with our distance is (3 ± 0.5) × 10 L⊙. Secondly, we consider the baseline errors originated We re-estimate the luminosities of VY CMa in the pre- from the positional errors of each VLBI station. The posi- vious studies with our distance. Le Sidaner & Le Bertre tions of VERA stations are determined with an accuracy (1996) obtained a luminosity of VY CMa to be 9 × 105 of 3 mm by the geodetic observations at 2 and 8 GHz every L⊙ from the SED at a kinematic distance of 2.1 kpc. With 2 weeks. The positional errors derived from the baseline the distance of 1.14 kpc, the luminosity of Le Sidaner & Le 5 errors are 11 µas at a baseline of 1000 km with the base- Bertre (1996) became 2.6 × 10 L⊙. Smith et al. (2001) line error of 3 mm. This error is much smaller than our also estimated a luminosity of VY CMa from the SED and 5 astrometric errors. their luminosity is 5 × 10 L⊙ at a distance of 1.5 kpc, 5 Thirdly, a variation of the structure of the maser fea- which is is revised to be 3.0 × 10 L⊙ using our distance. ture could be one of the error sources. Hirota et al. (2007), These luminosities are well consistent with each other. Imai et al. (2007), and Hirota et al. (2008) proposed the Massey et al. (2006) estimated an effective temperature maser structure effect as main sources in their trigonomet- of VY CMa to be 3650 K, which fitted the MARCS at- ric parallax measurements for nearby star forming regions mosphere model to the observed spectrophotometric data. Orion KL, IRAS 16293–2422 in ρ Oph East, and SVS 13 They estimated an absolute V magnitude MV using the in NGC 1333, correspondingly. However, the effect does observed V magnitude, currently known distance of 1.5 not seem dominating in our case because, (1) the 0.55 km kpc and a visible extinction AV . Since the MARCS model s−1 maser feature showed a stable structure in the closure presents bolometric corrections as a function of effective phase, spectrum and map at all epochs of our observation, temperature, they obtained the luminosity of VY CMa 4 (2) this effect is inversely proportional to the distance of to be 6.0 × 10 L⊙, much lower than our value derived the target source and hence should be more than twice above. When they adopt our distance, their luminosity less significant for VY CMa as those in the above cases would be increased. The accurate distance is essential to for a given size of the structure variation, (3) it is difficult estimate the accurate luminosity. to explain the large difference between astrometric errors To place VY CMa on the HR diagram, we adopted an in right ascension and declination by this effect. effective temperature from the literatures. As we already Finally, we have to consider the errors by the zenith de- mentioned, Massey et al. (2006) estimated an effective lay residual due to tropospheric water vapor. These errors temperature of VY CMa to be 3650 K based on the ob- originate from the difference of path length through the served spectrophotometric data. With the previously ac- atmosphere between the target and the reference sources, cepted spectral type of M4-M5, Le Sidaner & Le Bertre and generally larger in declination than in right ascen- (1996) obtained an effective temperature of 2800 K and sion. According to the result of the simulation in Honma, Smith et al. (2001) also adopted an effective temperature Tamura, & Reid (2008), the positional error by the tro- of 3000 K. Since the effective temperature does not depend pospheric delay is 678 µas in declination when the atmo- on the distance, our measurements cannot judge which ef- spheric zenith residual is 3 cm, the declination is –30 de- fective temperature is correct. grees, separation angle (SA) is 1 degree, and the position Although we cannot estimate the effective temperature angle (PA) is 0 degrees. This is well consistent with our of VY CMa with our measurements, when we adopt the measurements. Therefore, the atmospheric zenith delay effective temperature of 3650 K from the MARCS atmo- residual is likely to be the major source of the astrometric sphere model (Massey et al. 2006), the location of VY errors. CMa on the HR diagram is determined as the filled square in figure 4. For comparison, we also show VY CMa’s loca- 4. The Location on the HR Diagram tions on the HR diagram obtained in the previous studies as filled circles. Our results suggest that the location of We successfully detected a trigonometric parallax of VY CMa on the HR diagram is now consistent with the ± +0.11 0.88 0.08 mas, corresponding to a distance of 1.14 −0.09 evolutionary track of an evolved star with an initial mass kpc to VY CMa. Compared with the previously accepted of 25 M⊙. Also, re-scaled luminosity values of Le Sidaner distance 1.5 kpc (Lada & Reid 1978), the distance to VY & Le Bertre (1996) and Smith et al. (2001) imply much CMa became 76 %. Since the luminosity depends on the closer locations to the 25 M⊙ track than their original square of the distance, the luminosity should become 58 ones which were deeply inside the “forbidden zone”. On % of previous estimates. Hence, here we re-estimate the the other hand, the lower limit of luminosity of 6.0 × 104 luminosity of VY CMa with the most accurate distance. L⊙ in Massey et al. (2006) would be consistent with 15 M⊙ The luminosity can be estimated as follows: initial mass of VY CMa. According to Hirschi, Meynet, 2 & Maeder (2004), there is an order of magnitude differ- L =4πd Fbol, (1) ence in lifetimes between 15 M⊙ and 25 M⊙ in the initial where L is luminosity, d is distance and Fbol is the bolo- mass. The improvement in mass and age values should af- metric flux. To obtain Fbol, we used the SED of VY CMa. fect statistical studies on evolution of massive stars such The data are based on HST optical images and near-IR as the initial mass function (Salpeter 1955). 6 Y. K. Choi et al. [Vol. ,

On the other hand, there is an argument against the Hirota, T. et al. 2008, PASJ, 60, 37 effective temperature in Massey et al. (2006). Humphreys Hirschi, R., Meynet, G., & Maeder, A. 2004, A&A, 425, 649 (2006) suggested that the spectrum in Massey et al. (2006) Honma, M. et al. 2007, PASJ, 59, 889 are more like their M4-type reference spectrum than M2- Honma, M., Tamura, R., & Reid, M. J. 2008, PASJ, submitted type reference spectrum. Humphreys (2006) also pointed Honma, M. et al. 2008, PASJ, submitted out that the modeling applicable for “standard” stars Humphreys, R. M. 2006, ArXiv Astrophysics e-prints, arXiv:astro-ph/0610433 without mass-loss (MARCS) is simply not valid for an Humphreys, R. M., Helton, L. A., & Jones, T. J. 2007, AJ, object like VY CMa. When we adopt the previously 133, 2716 accepted spectral type, instead of M2.5I (Massey et al. Iguchi, S., Kurayama, T., Kawaguchi, N., & Kawakami, K. 2006), the effective temperature is 3000 K (Smith et al. 2005, PASJ, 57, 259 2001). The location of VY CMa on the HR diagram is Imai, H. et al. 2007, PASJ, 59, 1107 shown as the open square in figure 4. In this case, the po- Johnson, H. L., Hoag, A. A., Iriarte, B., Mitchell, R. I., and sition on the HR diagram is still not consistent with the Hallan, K. L. 1961, Lowell Obs. Bull., 5, 133 theoretical evolutionary track. Table 1 summarized our Kawaguchi, N., Sasao, T., & Manabe, S. 2000, in Proc. SPIE, adopted parameters for VY CMa. 4015, Radio Telescopes, ed. H. R. Butcher (Washington: The accurate distance measurements of red supergiants, SPIE), 544 Kovalev, Y. Y., Petrov, L., Fomalont, E., & Gordon, D. 2007, which provide true luminosities, will greatly contribute AJ, 133, 1236 to the understanding of massive star evolution, though Lada, C. J., & Reid, M. J. 1978, ApJ, 219, 95 there is still uncertainty in the effective temperature on Le Sidaner, P., & Le Bertre, T. 1996, A&A, 314, 896 the stellar surface. Levesque, E. M., Massey, P., Olsen, K. A. G., Plez, B., Jpsselin, E. Maeder, A., & Meynet, G. 2005, ApJ, 628, 973 5. Conclusion Massey, P. 2003, ARAA, 41, 15 Massey, P., & Olsen, K. A. G. 2003, AJ, 126, 2867 We have observed the H2O masers around the red su- Massey, P., Leveque, E. M., & Plez, B. 2006, ApJ, 646, 1203 pergiant VY CMa with VERA during 10 epochs spread Massey, P., Levesque, E. M., Plez, B., & Olsen, K. A. G. 2008, in Massive Stars as Cosmic Engines, IAU Symp 250, over 13 months. Simultaneous observations for both H2O masers around VY CMa and the position reference source ed. F. Bresolin, P. A. Crowther, & J. Puls (Cambridge: Cambridge University Press), 97 J0725–2640 were carried out. We measured a trigono- Marvel, K. B. 1996, PhD Thesis, New Mexico State University metric parallax of 0.88 ± 0.08 mas, corresponding to a +0.11 Marvel, K. B., Diamond, P. J., and Kemball, A. J. 1998, in distance of 1.14 −0.09 kpc from the Sun. It is the first Cool Stars, Stellar Systems and the Sun, ASP Conference result that the distance of VY CMa is determined with Series, Vol. 154, ed. Donahue, R. A. and Bookbinder, J.A., an annual parallax measurement. There had been over- CD-1621 estimation of the luminosities in previous studies due to Meynet, G., & Maeder, A. 2003, A&A, 404, 975 the previously accepted distance. Using the most accu- Monnier, J. D., Tuthill, P. G., Lopez, B., Cruzalebes, Danchi, rate distance based on the trigonometric parallax mea- W. C., & Haniff, C. A. 1999, ApJ, 512, 351 surements and the bolometric flux from the observed SED, Monnier, J. D., et al. 2004, ApJ, 605, 436 we estimated the luminosity of VY CMa to be (3 ± 0.5) Perryman, M.A.C. et al. 1997, A&A, 323, L49 Richards, A. M. S., Yates, J. A., & Cohen, R. J. 1998, MNRAS, × 105 L . The accurate distance measurements provided ⊙ 299, 319 the improved luminosity. The location of VY CMa on Salpeter, E. E. 1955, ApJ, 121, 161 the HR diagram became much close to the theoretically Smith, N., Humphreys, R. M., Davidson, K., Gehrz, R. D. allowable region, though there is still uncertainty in the Schuster, M. T., & Krautter, J. 2001, ApJ, 121, 1111 effective temperature.

We are grateful to the referee, Dr. Philip Massey, for helpful comments on the manuscript. We would like to thank Prof. Dr. Karl M. Menten for his invaluable comments and for his help improving the manuscript. The authors also would like to thank all the supporting staﬀs at Mizusawa VERA observatory for their assistance in observations.

References

Abbott, D. C., 1982, ApJ, 263, 723 Chikada, Y., et al. 1991, in Frontiers of VLBI, ed. H. Hirabayashi, M. Inoue, & H. Kobayashi (Tokyo: Universal Academy Press), 79 Hachisuka, K. et al. 2006, ApJ, 645, 337 Hirota, T. et al. 2007, PASJ, 59, 897 No. ] Distance to VY Canis Majoris with VERA 7

Table 1. Adopted parameters for VY CMa

Parameter Value Note Parallax 0.88 ± 0.08 mas +0.11 Distance 1.14 −0.09 kpc 5 Luminosity (3 ± 0.5) × 10 L⊙ Mass 25M⊙ Meynet & Maeder (2003) Temperature 3650 ± 25K Masseyetal.(2006) 3000K Smithetal.(2001)

Fig. 4. The various locations of VY CMa on the HR diagram. The filled and the open squares represent our results. The luminosity of our result is calculated using the distance based on the trigonometric parallax measurements and the bolometric flux from the SED. The filled square adopted the effective temperature of 3650 K (Massey et al. 2006) and the open square adopted 3000 K(Smith et al. 2001). The circles “1,” “2” and “3” represent the results of Le Sidaner & Le Bertre (1996), Smith et al. (2001) and Massey et al. (2006), respectively. The evolutionary tracks are from Meynet & Maeder (2003).