First detections of 610 MHz radio emission from hot magnetic stars
P. Chandra, G. A. Wade, J. O. Sundqvist, D. Oberoi, J. H. Grunhut, A. ud-Doula, V. Petit, D. H. Cohen, M. E. Oksala, A. David-Uraz
aa r X i v : . [ a s t r o - ph . S R ] M a y Mon. Not. R. Astron. Soc. , 1– ?? (2000) Printed 16 October 2018 (MN L A TEX style file v2.2)
First detections of 610 MHz radio emission from hot magnetic stars
P. Chandra ⋆ , G. A. Wade , J. O. Sundqvist , D. Oberoi , J. H. Grunhut , A. ud-Doula ,V. Petit , D. H. Cohen , M. E. Oksala , and A. David-Uraz , National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, P.O. Box 3, Pune 411007, India Department of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, Ontario K7K 7B4, Canada Department of Physics & Astronomy, University of Delaware, Newark, DE 19716, USA European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany Penn State Worthington Scranton, 120 Ridge View Drive, Dunmore, PA 18512, USA Dept. of Physics & Space Sciences, Florida Institute of Technology, Olin Physical Science, 346, Melbourne, FL 32901, USA Department of Physics and Astronomy, Swarthmore College, Swarthmore, PA 19081, USA LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Universit´e Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France Department of Physics, Engineering Physics and Astronomy, Queen’s University, 99 University Avenue, Kingston, ON K7L 3N6, Canada
Submitted 8th May 2015
ABSTRACT
We have carried out a study of radio emission from a small sample of magnetic O- and B-type stars using the Giant Metrewave Radio Telescope, with the goal of investigating theirmagnetospheres at low frequencies. These are the lowest frequency radio measurements everobtained of hot magnetic stars. The observations were taken at random rotational phases inthe 1390 and the 610 MHz bands. Out of the 8 stars, we detect five B-type stars in boththe 1390 and the 610 MHz bands. The O-type stars were observed only in the 1390 MHzband, and no detections were obtained. We explain this result as a consequence of free-freeabsorption by the free-flowing stellar wind exterior to the closed magnetosphere. We alsostudy the variability of individual stars. One star - HD 133880 - exhibits remarkably strong andrapid variability of its low frequency flux density. We discuss the possibility of this emissionbeing coherent emission as reported for CU Vir by Trigilio et al. (2000).
Key words: radiation mechanisms:non-thermal — stars: massive — stars: individual: HD133880, etc. — stars: magnetic field — radio continuum: stars
The last decade has witnessed the identification and elaboration ofan important population of hot O- and B-type stars hosting strong,organized magnetic fields (e.g. Donati et al. 2002; Oksala et al.2010; Grunhut et al. 2012; Wade et al. 2012). Observational andtheoretical studies of the interaction of their intense radiation-driven winds with their magnetic fields (e.g. ud-Doula & Owocki2002; Townsend et al. 2005; Sundqvist et al. 2012) have revealedthat the stellar wind properties of magnetic hot stars are modifiedin important ways as compared to the winds of non-magnetic stars,introducing significant changes across the entire electromagneticspectrum.The presence of an organized magnetic field at the surface ofa hot star leads to channelling and confinement of its outflowingstellar wind, creating a magnetosphere, which can radiate in variouswavebands (Andre et al. 1988; Linsky et al. 1992).In regions close to the star, the magnetic pressure dominatesand causes the wind to follow dipolar field lines. At greater dis-tances from the stellar surface, the wind kinetic energy density ⋆ E-mail: [email protected] exceeds the magnetic pressure due to the stronger radial declineof magnetic field energy density than of wind kinetic energy den-sity. The radius at which the wind kinetic energy density becomesequal to the magnetic pressure is defined as the Alfv´en radius (e.g.ud-Doula & Owocki 2002). The region below the Alfv´en radius,i.e. interior to the largest closed magnetic loop, is defined as theinner magnetosphere (Trigilio et al. 2004), which is the site of gen-eration of X-ray and H α emission (e.g. Babel & Montmerle 1997;Gagn´e et al. 2005; Howarth et al. 2007; Sundqvist et al. 2012). Be-yond the Alfv´en radius, the wind opens the magnetic field lines,generating a current sheet in the magnetic equatorial plane. Thisregion is defined as the middle magnetosphere. At greater distancesfrom the star, the outer magnetospheric region is characterized bya radial magnetic topology following wind streamlines.Radio emission from non-magnetic hot stars is expected to bethermal free-free emission from the ionized wind in the circumstel-lar environment (Panagia & Felli 1975; Wright & Barlow 1975).However, in the presence of magnetic fields electrons can be accel-erated to relativistic energies, either by reconnection near the cur-rent sheet in the middle magnetosphere (Usov & Melrose 1992) orgenerated in strong, large-scale shocks in the inner magnetosphere(Babel & Montmerle 1997; ud-Doula et al. 2006, 2008). These en- c (cid:13) Chandra et al. ergetic electrons can give rise to gyrosynchrotron radio emission(Drake et al. 1987; Linsky et al. 1992; Trigilio et al. 2004).Various surveys of cooler magnetic A- and B-type stars havebeen carried out at 5–8 GHz radio frequencies, resulting in ≈ − R ⋆ , with higher frequency emission originating from plasmalocated closer to the star, and the lower frequency emission frommaterial further away.Leone & Umana (1993) monitored the 5 GHz radio emissionof two magnetic B-type stars, HD 37017 and σ Ori E, and ob-served variability in accordance with their respective rotational pe-riods. The coincidence of radio maxima with the extrema of thelongitudinal magnetic field in these stars led those authors to sug-gest that radio emitting regions are located above the magneticpoles. Radio variability was also measured from the magnetic B8pstar HD 133880 (HR Lup). The flux density and circular polariza-tion were observed to vary according to the stellar rotation period(Lim et al. 1996; Bailey et al. 2012), and was also interpreted asgyrosynchrotron emission (Bailey et al. 2012).Leone et al. (2004) argued that as per the standard radio emis-sion gyrosynchrotron models, emission at a given frequency shouldbe emitted in a well-localised torus above the magnetic poles. Theyexamined the radio spectral energy distributions (SEDs) of fivemagnetic B-type stars in the range from 1 GHz to 87.7-GHz. Theyconcluded that the observed slopes of the radio spectra and the ab-sence of millimetre emission are not generally compatible with thismodel, and suggested a cut-o ff frequency of the radio emission.Trigilio et al. (2000) detected rapid, intense, narrow-band andhighly polarized radio bursts from the late B-type star CU Vir,which they hypothesised to be Electron Cyclotron Maser Emission(ECME) at 1.4 GHz. Such emission is expected to occur principallyat low frequencies ( < . § § § § The GMRT observations of 8 magnetic O- and B-type stars weretaken between 2013 October 24 to 2014 January 24 under cycle25 in the 1390 (L-band) and 610 MHz bands. Table 1 gives detailsof the observed stars. Out of these, 5 B-type stars were observedin both the 1390 and 610 MHz bands, while the 3 O-type starswere observed only in the 1390 MHz band. Each observation wasof approximately 3 hours in duration and a total of 26-29 good an-tennae were used. For all observations, visibilities were recordedfor two circular polarizations (RR and LL) with a bandwidth 32MHz, divided into 256 frequency channels, and 16 s integrationtime. Calibrator sources were used to remove the e ff ect of varia-tion of the instrumental and other non-astronomical factors in themeasurements. 3C48, 3C286 and 3C147 were used as flux cali-brators in various observations. Flux calibrators were observed for10–15 minutes, either in the beginning or towards the end of eachobservation. Flux calibrators were also used for bandpass calibra-tion. Phase calibrators were chosen from the VLA calibrator man-ual such that they are located within 15 degrees of the target star.They were observed more frequently. This is important not only forthe tracking of instrumental phase and gain drifts and atmosphericand ionospheric gain and phase variations but also for monitoringthe quality and sensitivity of the data and for spotting occasionalgain and phase jumps. In the 1390 MHz band, the phase calibratorobservations were made approximately every 25–30 minutes for5 minutes duration, whereas in the 610 MHz band, the 6-minutephase calibrator scans were obtained every 35–40 minutes.A FLAGging and CALibration (FLAGCAL) software pipelinedeveloped for automatic flagging and calibration of the GMRTdata (Prasad & Chengalur 2012) was used to flag and calibrate theGMRT data. In 70% of the cases, the results were satisfactory andonly minor additional flagging was needed after the FLAGCAL.However, in some cases, the data needed more significant flaggingand recalibration. In such cases the flagging was done manually anda flag file was created. The FLAGCAL was then run again usingthe manual flag file. The calibrated data were inspected to deter-mine the presence of any spurious signature. The above step wasrepeated until satisfactory calibration was achieved. The flaggedand calibrated visibility data were used to make continuum imagesusing the standard tasks in the Astronomical Image Processing Sys-tem (AIPS). To avoid bandwidth smearing in the continuum im-age, the total bandwidth was divided into 6 sub-bands in the 1390MHz observations, and in 10 sub-bands in the 610 MHz observa-tions. Because of the large field of view (FoV) of the GMRT, thethree-dimensional (3D) imaging feature in the AIPS task ‘IMAGR’was used in which the entire FoV is divided into multiple subfields(facets) and each of which was imaged separately. For the 1390MHz image, the total FoV was divided in 19 subfields, and in the610 MHz band the FoV was divided into 37 subfields. The pres-ence of a large number of bright sources in the FoV of variousstars allowed us to carry out self-calibration to improve the com-plex gains. This reduces the errors from temporal variations in thesystem gain, and spatial and temporal variations in the ionosphericproperties. After 3 rounds of phase self-calibration, the clean com-ponent model was subtracted from the UV data to identify theresidual bad data. Some more flagging was performed and 2 morerounds of phase self-calibration were run. A final round of ampli-tude and phase self-calibration was also performed in each dataset.Since the phase variations occur on time-scales of a few minutes,the time interval for the phase self-calibration was chosen to be 1minute, and 5 minutes for the amplitude and phase self-calibration. c (cid:13) , 1– ?? adio emission from hot magnetic stars Table 1.
Details (spectral type, e ff ective temperature, luminosity, radius, mass, rotational period, polar strength of the magnetic dipole at the stellar surface,Kepler co-rotation radius, Alfv´en radius, mass loss rate and wind terminal velocity) of magnetic O and B stars observed with the GMRT. R ff is the radius ofthe free-free absorption photosphere at 1390 MHz, discussed in Sect. 3.6.Star Spectral T e ff log L ⋆ R ⋆ M ⋆ Period B p R K R A log( ˙ M ) v ∞ R ff R ff / R A Type (kK) ( L ⊙ ) ( R ⊙ ) ( M ⊙ ) (d) (kG) ( R ∗ ) ( R ∗ ) ( M ⊙ yr − ) (km s − ) ( R ∗ )HD215441 B8-9p 15 ± ± − . ± > . > − . < . ± − . ± − . ± − . ± − . ± > > − . < . Hunger et al. 1989, Townsend et al. 2010, Oksala et al. 2012, Bolton et al. 1998, Bohlender et al. 1987, Petit et al. 2013, Leone et al. 2010, Bailey etal. 2012, Simon-Diaz et al. 2006, Stahl et al. 2008, Wade et al. 2006, Grunhut et al. 2009, Grunhut et al. 2012, Wade et al. 2012
Table 2.
GMRT observations of magnetic B and O type stars.
Star Obs Date Mean Mean Flux density rms Obs Date Mean Mean Flux density rms1.4 GHz HJD Phase F (mJy) µ Jy 0.6 GHz HJD Phase F (mJy) µ JyHD 215441 01-Nov-13 2456598.262 ± . ± .
10 46 24-Oct-13 2456590.168 ± . ± .
10 56HD 37479 31-Oct-13 2456597.433 ± . ± .
11 57 24-Oct-13 2456590.402 ± . ± .
28 51HD 37017 31-Oct-13 2456597.422 ± . ± .
12 53 24-Oct-13 2456590.430 ± . ± .
32 153HD 36485 27-Oct-13 2456593.378 ± . ± .
10 54 26-Oct-13 2456591.494 ± . ± .
17 88HD 133880 24-Jan-14 2456681.616 ± . ± .
11 49 14-Jan-14 2456671.590 ± . ± .
15 66HD 37022 31-Oct-13 2456597.476 0.4 < .
95 1650 ... ... ... ... ...
HD 57682 01-Nov-13 2456598.370 0.13 < .
16 52 ... ... ... ... ...
NGC 1624-2 02-Nov-13 2456598.487 1.0 < .
21 71 ... ... ... ... ...
The map resolutions for the 1390 MHz images were around 2”–3”,and around 5”-6” for the 610 MHz data. In Table 2, we report thevalues of the final flux density. For non-detections, we quote 3 σ upper limits. With our GMRT observations, we have detected 5 out of 8 stars.The detected stars, HD 215441, HD 37479, HD 37017, HD 36485and HD 133880, are B-type stars and have been detected in both the1390 and 610 MHz bands. The O-type stars, HD 37022, HD 57682and NGC 1624-2, were observed only in the 1390 MHz band, andnone of them are detected.Of the B stars, 4 stars (HD 37479, HD 37017, HD 36485 andHD 133880) are su ffi ciently rapid rotators that rotation contributessignificantly to the support of their magnetospheres (so-called ‘cen-trifugal magnetospheres’; Petit et al. 2013). On the other hand, theO-type stars, as well as HD 215441, rotate slowly, and their mag-netospheres receive no significant rotational support (so-called ‘dy-namical magnetospheres’; Petit et al. 2013).The observations are summarized in Table 2. In this section,we give details of the detected B-type stars, and explore potentialcauses of non-detections in O-type stars. HD 215441 (Babcock’s star) is a cool magnetic B8-9p star. Itexhibits no optical, UV or X-ray evidence of a magnetosphere,but was detected in the 5 GHz and 1.4 GHz bands (flux density ∼ = . + (9 . ± . · E . (1)Radio emission is clearly detected in both the 1390 and 610MHz bands at phases 0.93 and 0.08, respectively, with a flux den-sity somewhat larger than that reported at higher frequencies byLinsky et al. (1992, 1.2–1.3 mJy). In both bands, our observationscover less than 1% of the rotational period; hence we are unableto evaluate any variability. The flux is somewhat ( ∼ . ± .
09 mJy at 1.4GHz on July 2, 1987. According to Eq. 1, this corresponds to phase0.08, i.e. the same phase as our 610 MHz observation (0 . ± . α = log( F / F )] / log(1390 / =+ . ± .
03) at this phase. This result appears inconsistent withthe α = − .
26 index derived by Leone et al. (2004) based on higherfrequency observations. This suggests that the SED flattens at lowfrequency. c (cid:13) , 1– ?? Chandra et al.
Phase F l ux d e n s it y ( m J y ) GMRT 1.4 GHzVLA 1.4 GHzGMRT 0.61 GHz
Figure 1.
GMRT and VLA 1.4 GHz measurements (black and red dia-monds, respectively) and GMRT 610 MHz measurements (blue squares) ofHD 37479 ( σ Ori E). The VLA measurements are taken from the archive.
HD 37479 ( σ Ori E) is the prototypical B2 Vp star hosting a cen-trifugal magnetosphere. It exhibits strong and variable H α emissionthat has been studied in some detail (e.g. Townsend et al. 2005;Oksala et al. 2012). It was detected by Linsky et al. (1992) at 15,5 and 1.4 GHz frequency bands using the VLA, and observed atseveral phases. The flux density was observed to vary by a factorof about 2, from 2.4-3.1 mJy at 15 GHz, 2.8-3.9 mJy at 5 GHz, and1.5-3.2 mJy at 1.4 GHz bands. They also detected variable circularpolarization at 15 GHz.This star was also observed by Leone & Umana (1993), whodetected 5 GHz emission varying with the rotational period, withmaxima coinciding with the magnetic extrema.We obtained multiple GMRT observations in both bands. Tophase the data, we use the ephemeris of Townsend et al. (2010),which takes into account the (slow) rotational braking of the star:JD = . + . · E + . × − · E . (2)Our observations of HD 37479 were obtained at phases 0.95,0.08, 0.13 and 0.22 in the 1390 MHz band, and phases 0.15 and0.21 in the 610 MHz band. We have supplemented our observa-tions with the four 1.4 GHz measurements obtained by Linsky et al.(1992). The phased flux variation is shown in Fig. 1.The combined 1390 MHz data suggest a roughly sinusoidalvariation peaking at phase 0.5, with a peak flux of ∼ . /
610 MHz GMRT measurements were ob-tained at similar phases. From these we infer an instantaneous ( φ = . − .
2) index of the SED at these frequencies of α = . ± . α = .
12 derived by Leone et al. (2004)based on higher frequency observations.
HD 37017 is an early-type magnetic B2 Vp star that also exhibitsH α emission due to the presence of a dense centrifugal magneto-sphere (Leone et al. 2010; Petit et al. 2013). This target is in facta double-lined spectroscopic binary (SB2) system with an 18.65dorbital period containing the magnetic B star in addition to a late Bdwarf (Leone et al. 2010).Leone & Umana (1993) detected variable radio emission fromHD 37017. Like σ Ori E, the radio emission was found to vary withthe rotational period, and magnetic extrema coincided with radiomaxima. It was detected by Linsky et al. (1992) at 15, 5 and 1.4GHz bands using the VLA, and observed at several phases. Theflux density was observed to vary by a factor of about 2, from 0.9–2.1 mJy at 15 GHz, 1.4–2.6 mJy at 5 GHz, and 1.5–2.4 mJy at 1.4GHz bands. Leone et al. (2004) studied HD 37017 in mm bandsand examined the properties of the radio spectrum in the 1.4–87.7GHz range. They found the flux density to increase up to a fre-quency of 22.5 GHz and then to decrease in the mm range. Thisindicates a possible cut-o ff frequency.We obtained GMRT observations in the 1390 and 610 MHzbands. We phased the data using the ephemeris of Bolton et al.(1998):JD = . + (0 . ± . · E (3)Our 1390 MHz observations were obtained at phases 0.07,0.80 and 0.98, while our 610 MHz observations were obtained atphases 0.15 and 0.21. We supplemented these measurements withthe 1420 MHz observations of Linsky et al. (1992). The phaseddata are illustrated in Fig. 2.Leone & Umana (1993) described the 5 GHz flux variation assinusoidal, with a maximum at phase 0.0 and a minimum at phase0.5. The variation shown in Fig. 2, although weak and coarselysampled, appears opposite to this description. This is entirely a con-sequence of the di ff erent ephemeris used by those authors and byus. The phasing relative to the longitudinal field variation is thesame.Again, our two 610 MHz observations exhibit lower flux thanthe lowest of the 7 measurements at 1390 MHz. One of the VLA 1.4GHz observations was obtained at a phase similar to the GMRT 610MHz data. From these we infer an instantaneous ( φ = . − . α = . ± .
1. This isconsistent with α = .
15 derived by Leone et al. (2004) based onhigher frequency observations.
HD 36485 ( δ Ori C) is a another hot B3 Vp star hosting a centrifu-gal magnetosphere. The H α emission is variable on a timescale ofa few hours. Notwithstanding its sharp spectral lines, the star is arapid rotator with a period of about 1.5d, implying that it is viewedclose to the rotational pole (Bohlender et al. 1991; Leone et al.2010). δ Ori C was detected with the VLA at 5 GHz with a meanflux density of 0.95 mJy and showed a negative spectral index( α < − . f ν ∝ ν α ) between 5 and 15 GHz bands accordingto Drake et al. (1987), who concluded a non-thermal origin of theradio emission. Their 3 measurements at this wavelength did notshow any significant variability, with a measurement uncertainty of ± . = . + (1 . ± . · E . (4) c (cid:13) , 1– ?? adio emission from hot magnetic stars Phase F l ux d e n s it y ( m J y ) GMRT 1.4 GHzVLA 1.4 GHzGMRT 0.61 GHz
Figure 2.
GMRT and VLA 1.4 MHz measurements (black and red dia-monds, respectively) and GMRT 610 MHz measurements (blue squares) ofHD 37017. The VLA measurements are taken from the archive.
In our GMRT observations, we detected this star in both the1390 and 610 MHz bands. In our 3 hour observation at 1390 MHzcovering phases 0.91-0.96, the flux density varied between 0 . ± .
24 to 0 . ± .
14 mJy. Although these measurements are formallyconsistent with a constant flux density, the flux did monotonicallyincrease during the observation, suggesting a real change. In the610 MHz band, the flux density varied significantly, from 1 . ± .
16 to 0 . ± .
13 mJy between phases 0.62 to 0.71.
HD 133880 (HR Lup) is a cooler, rapidly rotating ( P = .
88 d;Bailey et al. 2012) Bp star hosting one of the strongest known stel-lar magnetic fields. However, unlike most Ap / Bp stars, HD 133880has a magnetic field topology that appears to be predominantlyquadrupolar as opposed to dipolar (Landstreet 1990; Bailey et al.2012). HD 133880 was previously observed with the AustraliaTelescope Compact Array (ATCA) at 5 and 8.5 GHz frequencybands simultaneously on 1995 February 12, 14 and 16, for around10 hours each day. Lim et al. (1996) had demonstrated that radioflux and circular polarization of HD 133880 at both frequenciesvary significantly and coherently according to the rotational pe-riod. They reported that the emission shows broad peaks near thephases of the longitudinal field extrema. The reanalysis of the databy Bailey et al. (2012) yielded the same result. They found fluxvariation to be complex, characterized by strong, broad maxima atphases 0.0 and 0.5 (i.e. the extrema of the longitudinal field), andsharper, somewhat weaker secondary extrema at quadrature phases(i.e. 0.25 and 0.75).George & Stevens (2012) observed this star with the GMRT indual mode in the 610 and 240 MHz bands on December 5 and 7th,2009. They reported non-detections in all the observations. How-ever, we extracted the data from the GMRT archive and reanalysedit, and find detections on both days in the 610 MHz band. Further-more, we also find variability in each observation in this band. Tore-confirm our detections of their data, we have analysed the databoth in AIPS, as well as in Common Astronomy Software Appli-cations (CASA), separating out LL and RR Stokes, and we findconsistent results. However, in the 240 MHz band we obtained nodetection in either observation, consistent with George & Stevens(2012). The 3 − σ upper limits in this band at the two epochs were1.5 and 3.0 mJy, respectively. We also located three data sets at 1420 MHz in the VLA archive, obtained between 15–17 February1995. We reanalyse these data and tabulate them in Table 3, alongwith the flux density in various time intervals in both our new ob-servations, as well as the reanalysed observations.To phase the data, we used the ephemeris of Bailey et al.(2012):JD = . + (0 . ± . · E . (5)The GMRT observations were made in the 1390 and 610 MHzbands. In Fig. 3, we plot the phased data in both bands. The fluxvariation of HD 133880 at both 1390 and 610 MHz is extraordi-nary. Unlike the other stars observed in this program (Table 4) thatexhibit a flux variation of about a factor of 2, the 610 MHz flux ofHD 133880 varies by a factor of 16 in our observations (and morethan a factor of 12 at 1390 MHz). Even with the sparse phase cover-age of our observations, it is clear that the maxima of both the 610and 1390 MHz data occur at phases ∼ .
25 and 0.75, coincidentwith the phases of the minor maxima at 5 and 8 GHz described byBailey et al. (2012).Considering the important di ff erences in the phase variationsof the GMRT data and the ATCA data, we speculate that HD133880 may exhibit maser emission in a manner similar to CU Vir(Trigilio et al. 2000). In CU Vir, while no coherent emission wasfound at 5 GHz, the 1.4 GHz emission was identified as ECMEwith a basal flux of 2–3 mJy and then very large increments in theflux density at around phases 0.35-0.45 and 0.75-0.85. While, wedo not yet have complete phase coverage for HD 133880, the ob-served features are highly reminiscent of those of CU Vir (see Fig.3). Schnerr et al. (2007) carried out 5 and 1.4 GHz observations of 5(non-magnetic) O stars with the Westerbork Synthesis Radio Tele-scope (WSRT). They detected 3 stars, for the first time at 1.4 GHzband, with one star ( ζ Per) inferred to show non-thermal radio emis-sion while two other ( α Cam and λ Cep) were inferred to show ther-mal radio emission. In all cases, the observed flux was lower thantheir predicted thermal flux, based on the mass loss rates inferredfrom the H α line profiles. They explained this discrepancy as dueto a higher influence of wind clumping on the formation of the H α line. In our program we also observed 3 magnetic O-type stars at1390 MHz. None of these targets were detected (see Table 2). Forone of these stars (HD 37022) our upper limit is quite poor becauseit resides in a region with extended radio emission in Orion.The high mass loss rates of O stars can make the wind op-tically thick due to free-free absorption out to a very large “radiophotosphere”. In such a case radio observations are unlikely to di-rectly see the magnetospheric emission. On the other hand, theymay probe the global mass loss, which is predicted to be quiteheavily reduced for O stars with dynamical magnetospheres, dueto quenching of the mass loss by the field and the resultant plasmain-fall (ud-Doula & Owocki 2002; ud-Doula et al. 2008).We can estimate the radius of the free-free radio photosphere,under the assumption of a spherically-symmetric, non-magneticwind, using Eq. (4) of Torres (2011): τ ff = × ˙ M v − ∞ f − ν − T − / R − ff (6)where τ ff is the free-free optical depth, ˙ M is the mass-loss rate inunits of 10 − M ⊙ / yr, v ∞ is the wind terminal velocity in units of10 cm / s, f is the clump volume filling factor, ν is the frequency of c (cid:13) , 1– ?? Chandra et al.
Table 3.
Variability of HD 133880 .Freq Telescope Date Mean Phase Flux density rmsGHz HJD mJy µ Jy1.388 GMRT 24-Jan-14 2456681.572 ± . ± .
14 10824-Jan-14 2456681.604 ± . ± .
12 12324-Jan-14 2456681.636 ± . ± .
12 11724-Jan-14 2456681.663 ± . ± .
14 1771.425 VLA 15-Feb-95 2449764.040 ± . ± .
40 39615-Feb-95 2449764.098 ± . ± .
33 33116-Feb-95 2449765.013 ± . ± .
24 23917-Feb-95 2449766.010 ± . ± .
27 2670.608 GMRT 14-Jan-14 2456671.552 ± . ± .
23 7614-Jan-14 2456671.579 ± . ± .
24 6814-Jan-14 2456671.607 ± . ± .
24 6514-Jan-14 2456671.631 ± . ± .
14 750.606 GMRT 05-Dec-09 2455170.712 ± . ± .
31 31105-Dec-09 2455170.776 ± . ± .
33 3280.607 GMRT 07-Dec-09 2455172.663 ± . ± .
32 21807-Dec-09 2455172.726 ± . ± .
40 28707-Dec-09 2455172.787 ± . ± .
37 347
Table 4.
Variability of other stars in our sample.Star Tel. Freq Date Mean Phase Flux density rms Ref.GHz HJD mJy µ JyHD215441 GMRT 1.388 01 Nov 13 2456598.223 ± . ± .
18 76 This paper01 Nov 13 2456598.255 ± . ± .
15 7001 Nov 13 2456598.283 ± . ± .
18 7401 Nov 13 2456598.306 ± . ± .
19 92VLA 1.4 02 Jul 87 2446979.250 0.078 1 . ± . . . . HD215441 GMRT 0.608 24 Oct 13 2456590.136 ± . ± .
13 71 This paper24 Oct 13 2456590.168 ± . ± .
13 6924 Oct 13 2456590.201 ± . ± .
22 108HD37479 GMRT 1.388 31 Oct 13 2456597.273 ± . ± .
17 87 This paper31 Oct 13 2456597.430 ± . ± .
15 8401 Nov 13 2456597.514 ± . ± .
17 8601 Nov 13 2456597.595 ± . ± .
19 106VLA 1.4 11 Mar 85 2446136.467 0.585 3 . ± . . . . Linsky et al. (1992)12 Mar 85 2446136.685 0.768 2 . ± . . . .
16 Mar 85 2446141.480 0.795 2 . ± . . . .
17 Mar 85 2446141.699 0.979 1 . ± . . . . HD37479 GMRT 0.608 24 Oct 13 2456590.370 ± . ± .
24 134 This paper24 Oct 13 2456590.434 ± . ± .
33 158HD37017 GMRT 1.388 31 Oct 13 2456597.301 ± . ± .
15 74 This paper31 Oct 13 2456597.458 ± . ± .
13 6601 Nov 13 2456597.542 ± . ± .
16 78VLA 1.4 11 Mar 85 2456136.488 0.459 2 . ± . . . . Linsky et al. (1992)12 Mar 85 2456136.623 0.609 1 . ± . . . .
16 Mar 85 2456141.419 0.931 1 . ± . . . .
17 Mar 85 2456141.637 0.173 1 . ± . . . . HD37017 GMRT 0.608 24 Oct 13 2456590.402 ± . ± .
37 74 This paper24 Oct 13 2456590.462 ± . ± .
24 66HD36485 GMRT 1.388 27 Oct 13 2456593.338 ± . ± .
17 9027 Oct 13 2456593.365 ± . ± .
33 8527 Oct 13 2456593.393 ± . ± .
14 9227 Oct 13 2456593.419 ± . ± .
17 78VLA 1.4 07 Jul 86 2446619.810 0.221 < . ± . ± .
16 108 This paper25 Oct 13 2456591.461 ± . ± .
18 21826 Oct 13 2456591.526 ± . ± .
19 8726 Oct 13 2456591.558 ± . ± .
13 97 c (cid:13) , 1– ?? adio emission from hot magnetic stars Phase F l ux d e n s it y ( m J y ) GMRT L-bandVLA L-bandGMRT 610 MHz (our)GMRT 610 MHz (GS12)
Phase -4000-200002000 L ong it ud i n a l f i e l d ( G ) Figure 3.
Upper frame -
Phase versus flux density plot of HD 133880 at 1.4GHz (red color) and 610 MHz (blue color). Here red squares denote GMRT1390 MHz measurements, red diamonds denote VLA 1.4 GHz measure-ments, blue squares denote GMRT 610 MHz observations made by us in2014 January, whereas the blue diamonds are the 2009 December data ofGeorge & Stevens (2012) reanalyzed by us. Note that unlike in Figs. 1 and2, the vertical scale in this figure is logarithmic.
Lower frame -
Longitudi-nal magnetic field variation of HD 133880, from Bailey et al. (2012). Thevertical lines in both plots indicate phases 0.25 and 0.75. observation in GHz, T is the wind temperature in units of 10 K,which we here for simplicity approximate with the e ff ective tem-perature of the star, and R ff is the distance from the star, in units of3 × cm.We adopt a volume filling factor f =
1, i.e. a smooth, un-clumped radio emitting region, since it has been demonstrated(Puls et al. 2006) that f = τ ff =
1, we have computed the position ofthe radio photosphere ( D , in units of R ∗ ) at 1390 MHz, using thewind parameters (computed using the Vink scaling) and physicalparameters reported by Petit et al. (2013), except for HD 133880,for which we use values reported by Bailey et al. (2012). The re-sults are summarized in Table 1 and illustrated in Fig. 4.If R ff < R A we expect the magnetospheric emission to escapethe wind and to be detectable (in principle). However, if R ff > R A ,the magnetospheric emission will be hidden within the (thermal)radio photosphere of the wind.In our sample, B stars show ratios of R ff / R A which are clus-tered around values 0.1–1.5, whereas all the O-type stars have thisratio clustered between 10–100. This is illustrated in Fig. 4. Thisconvincingly demonstrates that the free-free absorption of the mag-netospheric radio emission of the B stars likely is negligible, and that the principal cause of the null results for O stars is free-freescattering in the free streaming wind.Current models of the magnetically confined winds of slowlyrotating O stars imply that they host “dynamical magnetospheres”.In this scenario, wind streamlines inside R A in the magnetic equa-torial region are constrained to follow closed field loops. After thewind stream reach the respective loop top and shock, the coolingplasma returns to the stellar surface on a relatively short (dynami-cal) timescale. Simulations (ud-Doula & Owocki 2002) predict thatas a consequence of this confinement and fall-back, the global massloss of O stars with dynamical magnetospheres should be signif-icantly reduced, by up to a factor of 10–20 compared to similarnon-magnetic stars.In the context of free-free absorption, this reduction in massloss has important implications for the location of the radio photo-sphere. The expected mass-loss rate can be estimated using equa-tion 23 of ud-Doula et al. (2008), leading to a reduction of ˙ M by afactor of more than 3 for HD 37022, 5 for HD 57682, and 15 forNGC 1624-2. If we reduce the mass loss rate in Eq. (6) by thesefactors, for HD 37022 and HD 57682 the ratio R ff / R A is reduced toroughly 40, i.e. still well outside the Alfv´en radius. However, the ra-tio for NGC 1624-2 is reduced to ∼
2. At shorter wavelengths (3-6cm), this value becomes <
1. Hence multiwavelength radio obser-vations spanning the radio spectrum have the potential to providenovel constraints on predictions of mass-loss quenching of stronglymagnetic O stars like NGC 1624-2.
We have studied 8 O- and B-type stars with the GMRT in the 1390and 610 MHz bands. Our 610 MHz detections of the 5 B type starsare the first detections of this class of objects at such low frequen-cies. This indicates that free-free absorption of their winds is notable to shadow their low frequency emission. We also reanalysedarchival observations of HD 133880 obtained in December 2009,for which George & Stevens (2012) claimed non-detections. Sur-prisingly, we do detect the radio emission in both their observationsallowing us to diagnose variability.For HD 37017 and HD 37479, we confirm weak variability ofthe 1390 MHz flux density, as reported by Leone & Umana (1993)in 5 GHz band. The variability appears to be in phase with the vari-ation of the longitudinal magnetic field. We do not find evidence ofa double-wave 1390 MHz variation of HD 37479 (as reported byLeone & Umana 1993, at 5 GHz), although better phase coverageis necessary to robustly confirm this conclusion.For the B star HD 36485, our observations suggest a relativelyrapid change in the flux density in both bands in narrow phaseranges. This result needs to be confirmed by further observations.The cooler B star HD 133880 shows remarkable variability inboth the 1390 and 610 MHz bands. Even with our limited phasecoverage, the initial results suggest that the fluxes in both bandspeak at phases coinciding with the minor peaks obtained fromATCA data (Bailey et al. 2012). We propose that this behaviourmay be a consequence of ECME, first observed from the B8p starCU Vir by Trigilio et al. (2000). The phenomenon, only observableat low radio frequencies, strongly motivates immediate monitoringof this target.There were no detections obtained for any of the three mag-netic O stars in the 1390 MHz band. Using the scaling law of Torres(2011), we are able to conclude that this null result is in gen-eral agreement with the expected free-free optical depth of dense c (cid:13) , 1– ?? Chandra et al.
15 20 25 30 35 40
Effective temperature (kK) R ff / R A B-starsO- stars
Figure 4.
Ratio of R ff to R A versus e ff ective temperature for the observedsample (excluding HD 215441, for which R A is unavailable). Here red cir-cles and blue circles are for B- and O- type stars. The inverted triangleindicate upper limits as listed in Table 1. winds of O-type stars. We also conclude that radio observations athigher frequencies will be capable of testing predictions of massloss quenching of strongly magnetic O stars like NGC 1624-2.Leone et al. (2004) commented that the radio spectra of HD142301 and HD 215441 (the two stars in their sample with thestrongest magnetic fields) exhibited a sharp drop in flux densitybetween 15 and 22.5 GHz. They concluded that this suggests thereis a cut-o ff frequency in this range. On the other hand, the mag-netic chemically peculiar stars with relatively weak magnetic fieldsshowed flux densities that certainly decrease from the cm to the mmrange. In our sample, we were able to measure spectral indices be-tween 1390 and 610 MHz for 3 stars in common with the sample ofLeone et al. (2004). For two stars (HD 37017 and HD 37479), theindices were similar to those obtained at higher frequencies, sug-gesting a smooth decrement of the flux density. On the other hand,for HD 215441 (the nondegenerate star with the strongest knownmagnetic field), we obtained a spectral index that di ff ered stronglyfrom that derived by Leone et al. (2004). This implies an equallysignificant change in the spectral index at low frequencies, and de-serves further investigation.The results presented in this paper represent a first feasibilitystudy of the detection of low frequency emission from magneticO- and B-type stars. Having demonstrated the general propertiesof a range of objects, we are now carrying out a larger survey toobserve all known magnetic OB stars in radio bands from low tohigh frequencies. We are also performing monitoring of individualmagnetic stars, in order to better understand the rotational variationof their fluxes and spectral energy distributions. ACKNOWLEDGMENTS
We thank the sta ff of the GMRT that made these observations pos-sible. GMRT is run by the National Centre for Radio Astrophysicsof the Tata Institute of Fundamental Research. AIPS is producedand maintained by the National Radio Astronomy Observatory, afacility of the National Science Foundation operated under coop-erative agreement by Associated Universities, Inc. GAW acknowl-edges Discovery Grant support from the Natural Science and En-gineering Research Council (NSERC) of Canada. AuD acknowl-edges support by NASA through Chandra Award number TM4-15001A and 16200111 and DHC for TM4-15001B issued by the Chandra X-ray Observatory Center which is operated by the Smith-sonian Astrophysical Observatory for and behalf of NASA undercontract NAS8- 03060. AuD also acknowledges support for Pro-gram number HST-GO-13629.008-A provided by NASA througha grant from the Space Telescope Science Institute, which is oper-ated by the Association of Universities for Research in Astronomy,Incorporated, under NASA contract NAS5-26555. REFERENCES
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