Time Variation in G24.78+0.08 A1: Evidence for an Accreting Hypercompact H II Region?
Roberto Galván-Madrid, Luis F. Rodríguez, Paul T. P. Ho, Eric Keto
aa r X i v : . [ a s t r o - ph ] J a n To appear in The Astrophysical Journal Letters
TIME VARIATION IN G24.78+0.08 A1: EVIDENCE FOR ANACCRETING HYPERCOMPACT H II REGION?
Roberto Galv´an-Madrid , , Luis F. Rodr´ıguez , Paul T. P. Ho , , and Eric Keto ABSTRACT
Over a timescale of a few years, an observed change in the optically thickradio continuum flux can indicate whether an unresolved H II region around anewly formed massive star is changing in size. In this Letter we report on a studyof archival VLA observations of the hypercompact H II region G24.78+0.08 A1that shows a decrease of ∼
45 % in the 6-cm flux over a five year period. Sucha decrease indicates a contraction of ∼
25 % in the ionized radius and could becaused by an increase in the ionized gas density if the size of the H II regionis determined by a balance between photoionization and recombination. Thisfinding is not compatible with continuous expansion of the H II region after theend of accretion onto the ionizing star, but is consistent with the hypothesis ofgravitational trapping and ionized accretion flows if the mass-accretion rate isnot steady. Subject headings: H II regions — ISM: individual (G24.78+0.08) — stars: for-mation
1. Introduction
The formation of massive stars (
M > M ⊙ ) by accretion presents a number of theoreti-cal difficulties; among them, that once a star attains a sufficient mass its surface temperature Centro de Radioastronom´ıa y Astrof´ısica, UNAM, Morelia 58090, M´exico;[email protected] Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA; rgalvan,[email protected] Academia Sinica Institute of Astronomy and Astrophysics, Taipei, Taiwan; [email protected] II region (see the reviews on ultracompact and hyper-compact H II regions by Churchwell 2002; Kurtz 2005; and Hoare et al. 2007). The thermalpressure differential between the hot ( ∼ K) ionized gas and the cold ( ∼
100 K) molecu-lar gas can potentially reverse the accretion flow of molecular gas and prevent the star fromever reaching a higher mass. However, recent models that include the effects of gravity (e.g.,Keto 2007) have shown that if the H II region is small enough (ionized diameter ≤ ≥ M ⊙ ), the gravitational attraction of the star(s) dominates thethermal pressure and the molecular accretion flow can cross the ionization boundary andproceed toward the star as an ionized accretion flow within the H II region. In this stage theH II region is said to be gravitationally trapped by the star.Previous studies have demonstrated a number of observational techniques bearing on theevolution of small H II regions around newly formed stars. Accretion flows onto and throughH II regions can be directly observed by mapping molecular and radio recombination lines(RRLs) at very high angular resolution (e.g., Sollins et al. 2005; Keto & Wood 2006). If theH II region is too small to be spatially resolved, the frequency dependence of the velocitiesand widths of RRLs can be used to infer steep density gradients and supersonic velocitieswithin the H II regions (Keto et al. 2007), as expected from the presence of ionized accretionflows or bipolar outflows. Radio continuum observations at very high angular resolution,made at two epochs some years apart, can also be used to directly observe changes in size ofsmall H II regions (Franco-Hernandez & Rodriguez 2004). Changes in size indicate whetherthe evolution of the H II region is consistent with pressure-driven expansion or gravitationaltrapping.In this Letter we demonstrate yet another technique – how a comparison of radio con-tinuum observations at different epochs can be used to infer size changes in H II regionseven if the observations do not spatially resolve them. At optically thick frequencies theradio continuum flux depends to first approximation only on the size of an H II region andis independent of its internal density structure. Therefore, even if the size is not known,a change in the flux over time still indicates a change in the size of the H II region. Thistechnique is particularly valuable because it can be used on the smallest and youngest H II regions, and because lower angular resolution radio continuum observations generally requireless observing time than spectral line and high angular resolution observations.For the study presented in this Letter we selected the hypercompact (HC) H II regionG24.78+0.08 A1 which lies at the center of a massive molecular accretion flow (Beltr´an et al.2004, 2006) and also has multi-epoch 6 cm radio continuum observations in the VLA archive.G24.78+0.08 was detected in the centimetric (cm) continuum by Becker et al. (1994), andlater resolved by Codella et al. (1997) into a compact (A) and an extended (B) component. 3 –A millimeter (mm) interferometric study (Beltr´an et al. 2004) revealed the presence of twomassive rotating toroids centered in respective dust cores (A1 and A2). The compact cmemission comes from the mm component A1 (hereafter G24 A1), and has recently beenresolved by Beltr´an et al. (2007). If G24 A1 is ionized by a single star, its spectral typeshould be earlier than O9 (Codella et al. 1997). Also, G24 A1 likely powers a massive COoutflow (Furuya et al. 2002).The infalling and rotating molecular gas and bipolar outflow all suggest ongoing accre-tion. However, based on proper motions of H O masers around the H II region, Beltr´an et al.(2007) and Moscadelli et al. (2007) proposed that at the present time, the H II region is ex-panding into the accretion flow. The suggested timescale for the expansion is short enoughthat we should be able to detect a corresponding increase in the optically thick radio con-tinuum flux within a few years.
2. Observations
We searched the VLA archive for multi-epoch observations centered in the G24 A1region at optically thick frequencies (below 23 GHz for G24 A1). Since in the optically thickpart of the spectrum the flux density of the source scales as the angular size squared, fluxdensity variations corresponding to size variations can be detected even in observations ofmodest angular resolution. We chose three data sets of 6-cm observations in the C configu-ration, two from 1984 and one from 1989 (see Table 1).The observations were made in both circular polarizations with an effective bandwidthof 100 MHz. The amplitude scale was derived from observations of the absolute amplitudecalibrator 3C286. This scale was transferred to the phase calibrator and then to the source.We estimate an error not greater than 10 % for the flux densities of the sources.We edited and calibrated each epoch separately following the standard VLA proceduresusing the reduction software AIPS. Precession to J2000 coordinates was performed runningthe task UVFIX in the ( u, v ) data. After self-calibration, we made CLEANed images withuniform weighting and cutting the short spacings (up to 10 K λ ) to minimize the presence ofextended emission at scales larger than ∼ ′′ .Before subtraction, we made the images as similar as possible. We restored the CLEANcomponents with an identical Gaussian beam HPBW 4. ′′ × ′′
38, PA= − ◦ . We applied The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under cooper-ative agreement with the National Science Foundation. ≃ . ′′ ) shifts in positionas well as a scaling of ≃
10 % in amplitude. This was done in order to minimize the rmsresiduals of the difference image in the region of interest. A similar procedure was used byFranco-Hern´andez & Rodr´ıguez (2004) to detect a variation in the lobes of the bipolar UC H II region NGC 7538 IRS1. The individual maps, as well as the final difference image between1984 and 1989 are shown in Figure 1.
3. Discussion3.1. The Expected Variation Trend
Beltr´an et al. (2007) suggested that the 7 mm and 1.3 cm continuum morphologies areconsistent with limb-brightening from a thin, ionized-shell structure. Based on H O maserproper motions (see Fig. 3 of Beltr´an et al. 2007, or Fig. 4 of Moscadelli et al. 2007) theyalso suggested an expansion speed of ∼
40 km s − . The increase in flux corresponding tothe increase in size due to expansion ought to be detectable in a few years. If G24 A1 is the ionized inner portion of the star forming accretion flow, the long-term growth of the H II region due to the increasing ionizing flux of the star should beimperceptible. However, the H II region could change in size over an observable timescale ifthe gas density in the accretion flow is time variable. Because the mass of ionized gas withinthe H II region is very small compared to the mass of the accretion flow, even a small changein the flow density could affect the size of the H II region.For example, we can obtain a lower limit for the mass of G24 A1 assuming that it isspherical and homogeneous. From the equations of Mezger & Henderson (1967) we havethat: h M HII M ⊙ i = 3 . × − h S ν mJy i . h T e K i . × h ν . i . h D kpc i . h θ s arcsec i . , where the flux density S ν has been measured at a frequency ν in which the H II region isoptically thin, T e is the electron temperature, D is the distance to the region, and θ s is itsFWHP. Taking S ν = 101 mJy, D = 7 . θ s = 0. ′′
17 at 7 mm (Beltr´an et al. 2007),and assuming T e = 10 K, the ionized mass in G24 A1 would be ∼ × − M ⊙ . A higherlimit to the mass can be obtained considering that HC H II regions should have densitygradients ( n ∝ r α , with α = − . − .
5) rather than being homogeneous (Keto 2007). Inthis case we obtain ionized masses between 1 × − M ⊙ and 3 × − M ⊙ , a factor of 2 − Figure 1 shows the individual maps of the 1984 May 11 + 14 (1984.36) and 1989 June23 (1989.48) epochs, as well as the difference image. Sources A1 and B (the former labeledA by Codella et al. 1997) are unresolved ( θ s ≤ ′′ ), and the total emission is dominated bycomponent B. However, the flux decrease is centered at the position of A1. We performedGaussian fits to both components using the task JMFIT in AIPS, and the results are sum-marized in Table 2. We have checked the reliability of the fits to the image by making directfits to the (u,v) data. The values obtained from both techniques are entirely consistent butsuggest that the errors given by the tasks used in the image (JMFIT) and (u,v) (UVFIT)fittings are underestimated by a factor of 2. The errors given in Table 2 have been correctedby this factor.The decrease in the flux density of G24 A1 between 1989.48 and 1984.36 is 5 . ± . ±
10 %. Component B shows no evidence of time variability, as expected for amore evolved H II region not associated with signs of current star-forming activity such asH O or OH masers (Codella et al. 1997). We set an upper limit of 2 % to the circularly-polarized emission of the sources, which indicates that we are not dealing with variablegyrosynchrotron emission from an active stellar magnetosphere.
The ∼
45 % flux decrease (i.e., a contraction of ∼
25 % in the ionized radius) we havedetected at 6 cm toward G24 A1 is not consistent with the hypothesis that this H II region 6 –is expanding rapidly into the molecular accretion flow. Assuming a radius R ∼
500 AU andan expansion velocity v ∼
40 km s − (Beltr´an et al. 2007), a flux increase of ∼
20 % shouldhave been observed between the compared epochs.We attribute the contraction of the H II region to an increase in its density produced bythe enhancement of accretion, either caused by an isotropic increment in the mass-accretionrate or by the sudden accretion of a localized clump in the neutral inflow (for example, clumpshave been observed in the accretion flow onto the UC H II region G10.6–0.4; Sollins and Ho2005; Keto & Wood 2006). Our data do not allow us to distinguish among these two possi-bilities.The additional mass required to increase the density can be estimated. The ionizedradius r s scales roughly with the density n as r s ∝ n − / (if the radius of the H II region is setby a balance between photoionization and recombination) and at optically thick frequenciesthe flux density scales as S ν ∝ r . The ionized mass within G24 A1 is ∼ × − M ⊙ ( § ∼ × − M ⊙ into the H II region would sufficeto explain the observed 45 % flux decrease. The molecular accretion flow around this H II region has a mass of ∼ M ⊙ (Beltr´an et al. 2004). Thus, the required variation, 50 % ofthe mass of the H II region, is only 0.001 % of the total mass of the accretion flow.Finally, we shall mention that even when the observations here reported are not con-sistent with a simple, continuous expansion of G24 A1, they do not rule out other morecomplex, non-steady evolutionary scenarios.
4. Conclusions
Our analysis of archival VLA observations of the H II region G24.78+0.08 A1 indicatesa contraction of its radius between 1984.36 and 1989.48. This finding is consistent with thehypothesis that this HC H II is the inner, ionized part of the larger scale accretion flowseen in the molecular line observations of Beltr´an et al. (2004, 2006). Future high-resolutionRRL observations could unambiguously resolve the ionized-gas kinematics and confirm thishypothesis. REFERENCES
Becker, R. H., White, R. L., Helfand, D. J., & Zoonematkermani, S. 1994, ApJ, 91, 347Beltr´an, M. T., Cesaroni, R., Codella, C., Testi, L., Furuya, R. S., & Olmi, L. 2006, Nature, 7 –443, 427Beltr´an, M. T., Cesaroni, R., Moscadelli, L., & Codella C. 2007, A&A, 471, L13Beltr´an, M. T., Cesaroni, R., Neri, R., Codella, C., Furuya, R. S., Testi, L., & Olmi, L. 2004,ApJ, 601 L187Codella, C., Testi, L., & Cesaroni, R. 1997, A&A, 325, 282Churchwell, Ed. 2002, ARA&A, 40, 27Franco-Hern´andez, R., & Rodr´ıguez, L. F. 2004, ApJ, 604, L105Furuya, R. S., Cesaroni, R., Codella, C., Testi, L., Bachiller, R., & Tafalla, M. 2002, A&A,390, L1Hoare, M. G., Kurtz, S. E., Lizano, S., Keto, E., & Hofner, P. 2007, in Protostars andPlanets V, ed. B. Reipurth, D. Jewitt, & K. Keil (Tucson: Univ. Arizona Press), 181Keto, E. 2007, ApJ, 666, 976Keto, E., & Wood, K. 2006, ApJ, 637, 850Keto, E., Zhang, Q., & Kurtz, S. 2007, preprint (arXiv:0708.3388)Kurtz, S. 2005, in IAU Symp. 227, Massive Star Birth: A Crossroad of Astrophysics, ed. R.Cesaroni et al. (Cambridge: Cambridge Univ. Press), 111Mezger, P. G., & Henderson, A. P. 1967, ApJ, 147, 471Moscadelli, L., Goddi, C., Cesaroni, R., Beltr´an, M. T., & Furuya, R. S. 2007, A&A, 472,867Sollins, P. K., & Ho, P. T. P. 2005, ApJ, 630, 987Sollins, P. K., Zhang, Q., Keto, E., & Ho, P. T. P. 2005, ApJ, 624, L49
This preprint was prepared with the AAS L A TEX macros v5.2.
Epoch Phase Center a Amplitude Phase Bootstrapped Flux Beam α (J2000) δ (J2000) Calibrator Calibrator Density (Jy) (arcsec × arcsec; deg)1984 May 11 18 36 12.145 −
07 11 28.17 3C286 1743 −
038 2 . ± .
005 5 . × . − −
07 11 28.17 3C286 1743 −
038 2 . ± .
007 4 . × .
34; +031989 Jun 23 18 36 10.682 −
07 11 19.87 3C286 1834 −
126 0 . ± .
001 4 . × . − a Units of right ascension are hours, minutes, and seconds. Units of declination are degrees, arcminutes, and arcseconds.
Epoch Component Position a ,b Flux Density α (J2000) δ (J2000) (mJy)1984.36 A1 18 36 12.545 −
07 12 10.87 11 . ± . −
07 12 15.37 31 . ± . −
07 12 10.87 06 . ± . −
07 12 15.37 32 . ± . a Units of right ascension are hours, minutes, and seconds. Units of decli-nation are degrees, arcminutes, and arcseconds. b For the 1989.48 epoch, the centers of the Gaussians were fixed to thoseobtained for the 1984.36 epoch.
10 – D E C L I NA T I O N ( J2000 ) RIGHT ASCENSION (J2000)18 36 13.4 13.2 13.0 12.8 12.6 12.4 12.2 12.0-07 12 000510152025 D E C L I NA T I O N ( J2000 ) RIGHT ASCENSION (J2000)18 36 13.4 13.2 13.0 12.8 12.6 12.4 12.2 12.0-07 12 000510152025 D E C L I NA T I O N ( J2000 ) RIGHT ASCENSION (J2000)18 36 13.4 13.2 13.0 12.8 12.6 12.4 12.2 12.0-07 12 000510152025
Fig. 1.— VLA images of G24.78+0.08 for 1984.36 ( top ), 1989.48 ( middle ), and the differenceof 1989.48 - 1984.36 ( bottom ). The contours are -10, -8, -6, -5, -4, 4, 5, 6, 8, 10, 12, 15, 20,30, 40, and 60 times 0.57 mJy beam − . The half power contour of the synthesized beam(4. ′′ × ′′
38 with a position angle of − ◦ ) is shown in the bottom left corner of the images.The crosses indicate the positions of the components A1 and B from our Gaussian fits to the1984.36 image. The negative residuals observed in the difference image indicate a decreaseof ∼∼